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MCP6N11 and MCP6V2x Wheatstone Bridge Reference

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1. External Signals Internal Signals Label Tes t Description Label Description Point 5 0V TP1 Power Supply Vdd1 Filtered lab power supply analog supply PCB s power plane Vrtd Connection to Vdd2 Filtered USB power digital power VL 4 Connection to Vdd3 USB power negative output of bridge Vad1 TP6 INA 1 s signal output VR Positive output of bridge Vad2 TP7 INA 1 s ref output Vpwm PWM output from PIC MCU Vad3 TP13 INA 2 s signal output Vref Buffered 1 8V reference Vad4 TP14 INA 2 5 ref output E C 1 signal EN CAL sent by PIC MCU Vo1 TP5 U2 s output Vo2 TP9 U3 Side A s output Vo3 TP10 U3 Side B s output Vo4 TP11 U4 s output Vpwmx TP16 External PWM input 2012 Microchip Technology Inc DS52031A page 21 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design 2 5 4 Connection and Configuration Points on the PCB Figure 2 1 shows the points on the PCB where equipment is connected and where the configuration is altered T Couple a Rtd2 2 ICSP RTD T u2 giso Ji e WB R12 LI 22 HE C6 e R5e R11 mq CP un I eis Gia nCalSu DONT a N e ea N Vadi e 4 AN TP7 Ut oski e MICROCHIP 2 R32 c23 e R340 im e INA 2 e C26 C25 C24
2. 2 2 2 Bench Setup Measurements on the bench focus on analog performance they only use the PIC microcontroller to generate the 25 kHz interference signal The USB is not connected e MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design Lab Power Supply with single output Generates 5 0 TP1 and ground TP2 Voltmeter Signal Analyzer oscilloscope network analyzer spectrum analyzer High input impedance e g 1 MO 10 pF CAUTION When using the bench setup 5V supply at TP1 5 0V and TP2 GND do not connect a USB cable to the demo board This avoids contention with the lab supply 2012 Microchip Technology Inc DS52031A page 17 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design 2 3 BASIC CONFIGURATIONS The following sections discuss various configurations supported by this board 2 3 1 Out of the Box Setup The setup for these boards when they are shipped is as follows R2 RTD is mounted on the PCB JP1 is at position 1 the PWM signal capacitively couples to the bridge JP2 is at position 1 PWM PIC the PIC MCU generates the PWM interference signal The user must enable the PWM output see 26 in Section 5 9 2 Additional Configuration and RTD Temp Tabs USB cable connects the PCB and PC and provides power to the PCB No external power supply is connected to TP1 and TP2 Thermal Management Utility GUI needs to be running before
3. Shadow Color Quick Styles Graph Attributes C Graph Foreground Bitmap Gradient Styles C Graph Background _ Light Medium Dark Table Foreground E _ Inset Inset Inset Table Background Q Shadow C Shadow C Shadow Line C Line C Line No Border No Border No Border FIGURE 5 10 Data Acquisition Customization Color Tab DS52031A page 46 2012 Microchip Technology Inc Software GUI 5 9 4 7 DATA ACQUISITION CUSTOMIZATION STYLE TAB Figure 5 11 shows that the Data Acquisition dialog box with the Style tab selected To open this box double click inside the Strip Chart area 5 The Strip Chart s curve col ors are changed here n General Plot Subsets Points Font Color Style RTD1 Temperature RTD2 Temperature VE ERR VAD2 VAD3 e 11 Point Type LSolid Circle Line Type FIGURE 5 11 Data Acquisition Customization Style Tab 5 9 48 EXPORTING DATA ACQUISITION BOX Figure 5 12 shows the Exporting Data Aquisition box To open this dialog double click inside the Strip Chart area 5 then press the Export button on the bottom right e g see Figure 5 11 When the data is exported as a picture its destination and size c
4. Calculations Save Results MCP6V26 Communications Communications FIGURE 1 1 Overall Block Diagram This board uses an RTD temperature sensor in a Wheatstone bridge to measure the board s temperature The bridge s output is a small Differential mode DM signal with a large Common mode CM interference signal superimposed The CM signal is a PWM waveform generated by the microcontroller which simulates real world interference VREF not a fixed reference outputs a voltage level proportional to the analog supply voltage keeping the INAs outputs in range The two INAs process the bridge s output rejecting the CM interference and providing large gains for the DM signal There are R C low pass filters at the input and output of each INA A pulse width modulation PWM signal at 25 kHz is an optional output of the microcon troller It shows how the two INAs perform with one type of real world interference This signal can represent for instance interference from an H bridge motor controller Test points simplify bench setups The power supply can be provided by the USB or by a lab power supply A 12 bit ADC inside the microcontroller converts analog signals to digital data The microcontroller s firmware averages the data to achieve noise and data rate reduc tion It then subtracts the VREF voltage from the INA outputs correcting any slow VREF errors calculates the voltages and temperatures calibrates the temperature a
5. Analog for the Digital Age Application Maestro chipKIT chipKIT logo CodeGuard dsPICDEM dsPICDEM net dsPlCworks dsSPEAK ECAN ECONOMONITOR FanSense HI TIDE In Circuit Serial Programming ICSP Mindi MiWi MPASM MPLAB Certified logo MPLIB MPLINK mTouch Omniscient Code Generation PICC PICC 18 PICDEM PICDEM net PICtail REAL ICE rfLAB Select Mode Total Endurance TSHARC UniWinDriver WiperLock and ZENA are trademarks of Microchip Technology Incorporated in the U S A and other countries SQTP is a service mark of Microchip Technology Incorporated in the U S A All other trademarks mentioned herein are property of their respective companies 2012 Microchip Technology Incorporated Printed in the U S A All Rights Reserved LI Printed on recycled paper ISBN ISBN 978 1 61341 925 0 Microchip received ISO TS 16949 2009 certification for its worldwide headquarters design and wafer fabrication facilities in Chandler and Tempe Arizona Gresham Oregon and design centers in California and India The Company s quality system processes and procedures are for its PIC MCUs and dsPIC DSCs KEELOQ code hopping devices Serial EEPROMs microperipherals nonvolatile memory and analog products In addition Microchip s quality system for the design and manufacture of development systems is ISO 9001 2000 certified DS52031A page 2 2012 Microchip Technology Inc e MCP6N11 AND MCP6V2X WH
6. The GUI is Microchip s Thermal Management Utility software It downloads data from the USB PIC MCU calculates one point calibration coefficients for TRTD1 and TRTD2 sends them to the PIC MCU sends an mCa1 event trigger signal sends the number of averages displays the data on a strip chart and exports data in different file formats including text 5 2 PLATFORM REQUIREMENTS The Thermal Management Utility software used for many of our thermal demo boards runs on Microsoft Windows XP SP3 Vista and 7 64 and 32 bit It also needs the NET2 framework package the installer package will install it if it is not on your machine 5 3 USB COMMUNICATIONS The GUI sends configuration information to the firmware via the USB This includes temperature calibration coefficient number of averages and a strobe signal to trigger a Vog calibration event nCa1 in INA 1 The GUI collects the following averaged data from the firmware Vap4 Vap2 VADA TRTD4 TRTD2 RRTD1 and Rrtp2 This data is collected at adjustable time intervals with a default value of 200 ms 5 SPS per variable 2012 Microchip Technology Inc DS52031A page 37 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design 5 4 DISPLAY DATA The GUl uses a standard interface to display the temperature calibration coefficient and number of averages Displays the four voltages Vap4 through Vapa on a strip chart Also displays the RTD temperatu
7. 11 a Wheatstone Bridge z amp Reference Design Bros T Em ARD00354 TP10 TPS TP11 TP12 TP13 o o a D E E 3 5 gt R21 C14 ICSP connector for re programming the PIC MCU s firmware USB connector bottom layer for the USB cable from the PC when TP1 and TP2 are not connected see item 5 3 Test Points for evaluating INA 2 s performance on the bench a Auto zeroed op amp output voltages Vo2 Vo3 and Vo4 b Filtered output voltages sent to the ADC Vad3 and Vad4 4 PWM signal configuration a Test Points for inputting an external PWM signal b Jumper for selecting between PWM sources PIC microcontroller and external 5 Test Points for inputting power 5V from a lab supply when the USB is not connected see item 2 6 Test Points for evaluating INA 1 s performance on the bench a INA output voltage Vo1 b Filtered output voltages sent to the ADC Vad1 and Vad2 7 Manually input a calibration for INA 1 s nCa1 event 8 Jumper for configuring the PWM signal s coupling capacitive only series R C or no coupling open FIGURE 2 1 Equipment Connection Scheme 0 52031 22 2012 Microchip Technology Inc MCP6N11 AND MCP6V2X WHEATSTONE BRIDGE REFERENCE DESIGN USER S GUIDE MICROCHIP Chapter 3 Analog Circuit 3 1 INTRODUCTION This chapter discusses the performance of the analog circuitry Items discuss
8. 500 C Vertical io C Horizontal Slanted CO O1 4 CO mud mel mud AB FIGURE 5 8 Data Acquisition Customization Points Tab 2012 Microchip Technology Inc DS52031A page 45 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design 5 9 4 5 DATA ACQUISITION CUSTOMIZATION FONT TAB Figure 5 9 shows that the Data Acquisition dialog box with the Font tab selected To open this box double click inside the Strip Chart area 5 The Strip Chart s font types and sizes are changed here General Plot Subsets Points Font Color Style Main Title Microsoft Sans Serif IM bold italic underline Sub Title Times New Roman zj Ebola italic underline Subset Point Axis Labels Arial bold italic underline Table Data Arial sample FIGURE 5 9 Data Acquisition Customization Font Tab 5 9 4 6 DATA ACQUISITION CUSTOMIZATION COLOR TAB Figure 5 10 shows that the Data Acquisition dialog box with the Color tab selected To open this box double click inside the Strip Chart area 5 The font and object colors are changed here 5 Data Acquisition Customization eo General Plot Subsets Points Font Style Desk Background E
9. Data Acquisition 1 1 INA 2 RTDi Temperature em wmm RTD2 Temperature 150 RTD 1 RTD2 RTD Resistance 11022 9 110 27 9 RTD Temperature 26326 2639 C 400 Vam Reference Pts V 2 108 1 654 2 113 1 657 Temperature Simplified Circuit Diagram 50 VADI em VAD VAD uum VAD4 RTD 45 JE t 40 T Brid ridge 5 30 gu L 20 WAA AA AA AA 15 1 0 05 Chart Update Speed RS Sampling Interval 100 ms Fastest 0 50 100 150 200 250 300 350 400 450 Data Buffer FIGURE 5 15 Zoom area in Strip Chart NENNEN DS52031A page 50 2012 Microchip Technology Inc MICROCHIP MCP6N11 AND MCP6V2X WHEATSTONE BRIDGE REFERENCE DESIGN USER S GUIDE Appendix A Schematics and Layouts A 1 INTRODUCTION This appendix contains the following schematics and layouts for the MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design Board Schematic Board Top Silk Screen and Pads Board Top Metal Layer Board Ground Plane Second layer Board Power Plane Third layer Board Bottom Metal Layer top view Board Bottom Silk and Pads top view The Gerber files for this board are available on the Microchip website www microchip com and are contained in the zip file 00354R3 Gerbers zip This is
10. 81 45 471 6166 Fax 81 45 471 6122 Korea Daegu Tel 82 53 744 4301 Fax 82 53 744 4302 Korea Seoul Tel 82 2 554 7200 Fax 82 2 558 5932 or 82 2 558 5934 Malaysia Kuala Lumpur Tel 60 3 6201 9857 Fax 60 3 6201 9859 Malaysia Penang Tel 60 4 227 8870 Fax 60 4 227 4068 Philippines Manila Tel 63 2 634 9065 Fax 63 2 634 9069 Singapore Tel 65 6334 8870 Fax 65 6334 8850 Taiwan Hsin Chu Tel 886 3 5778 366 Fax 886 3 5770 955 Taiwan Kaohsiung Tel 886 7 536 4818 Fax 886 7 330 9305 Taiwan Taipei Tel 886 2 2500 6610 Fax 886 2 2508 0102 Thailand Bangkok Tel 66 2 694 1351 Fax 66 2 694 1350 EUROPE Austria Wels Tel 43 7242 2244 39 Fax 43 7242 2244 393 Denmark Copenhagen Tel 45 4450 2828 Fax 45 4485 2829 France Paris Tel 33 1 69 53 63 20 Fax 33 1 69 30 90 79 Germany Munich Tel 49 89 627 144 0 Fax 49 89 627 144 44 Italy Milan Tel 39 0331 742611 Fax 39 0331 466781 Netherlands Drunen Tel 31 416 690399 Fax 31 416 690340 Spain Madrid Tel 34 91 708 08 90 Fax 34 91 708 08 91 UK Wokingham Tel 44 118 921 5869 Fax 44 118 921 5820 11 29 11 DS52031A page 70 2012 Microchip Technology Inc
11. EQUATION Tsi q Rgrp Rso Where Ts4 256 Rso 1200 Rs1 640 The RTD s resistance is estimated with this polynomial approximation with scaling EQUATION Where Ag 0 31251 A4 1 56353 5 0 05946 ER tg q p Aj p Ay q Rgj 0 0053 Q 0 013 len The RTD s temperature is estimated with this polynomial approximation with scaling EQUATION P Bo q B q B2 q B3 q B9 Trrp PTs Where Bg 0 201424 B 0 649547 B5 0 016232 0 000768 B4 0 000163 Ts4 Er qRRTD QTRTD P 0 013 C 0 0049 2012 Microchip Technology Inc DS52031A page 65 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design C 3 INA VOLTAGE TO TEMPERATURE Preliminary work was done in an Excel spreadsheet to fit polynomials to the Te rp using Vina see Table C 1 Different order approximations were produced for different purposes hand estimates firmware algorithm and spreadsheet calculations To accommodate fixed point arithmetic is scaled as before and Vina is scaled based on its binary representation EQUATION v Vinga Vpp 2 W Ts Where Vna Vani Mago for INAH for INA
12. 1 Data View Tab Figure 5 2 shows the default screen On the left the Data View tab 9 is selected and the Strip Chart is displayed J UU 2 M D 2 2012 Microchip Technology Inc DS52031A page 39 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design Select GUIs 9 Help gre Version 11 0 0 2 Thermg pnagement Ute Mee and MACP6V2x Wheatfone Bridge ference Design gt K ie Q Q Data View Additional Configuration Data Acquisition RTD Temperature m m 2 Temperature INA 1 RTD 1 109 91 Q RTD Temperature 25 46 25 46 1004 RTD Resistance Van Vas Reference Pts V 2 093 1 653 2 095 1654 Temperature Simplified Circuit Diagram VAD VAD VA Chart Update Speed Sampling Interval 100ms Fastest FIGURE 5 2 Data View Tab with Strip Chart The indicated features for Figure 5 2 are 1 Data Collection buttons a Data Acquisition Buttons standard data collection Start Acquisition Stop Acquisition Reset Chart Buffer b Record Acquisition Buttons data sent to a file with user selected length Record Acquisition Stop Record Ac
13. 1 is a DM and CM low pass filter The CM pole is at 8 Hz for further rejection of CM interference The DM pole is at 2 7 Hz it is faster than the RTD but slow enough to limit the output noise C12 is large enough to cause minimal gain error it causes a voltage division as it interacts with the ADC s input sampling capacitor Choun 25 pF R19 and R20 are small enough to have good EMI performance C11 C13 are sized for the desired poles mCa1 1 triggers an internal calibration event in U1 MCP6N11 which corrects its Vos the pole is set for SW1 s maximum bounce time 10 ms R16 makes the EN CAL pin of INA 1 act as a wired OR logic input an E C signal can be sent from the PIC MCU independent of mCalSw s state mCal 1 R13 mCalSw AN VV a m 100 1 sw1 C10 100nF 50V 1096 R15 RIG n VVV VW SW EVQ P2R02M 100K 5 10K 5 d x c9 SGND XY E 2N 0 or High Z 100nF 50V Input Filter 1 30 MS Output Filter 1 10uF 25V 10 1206 50V 10 ei Sb WW ave 100K 0 1 a HF SN WX n Es 1 0uF 25V 10 CT 1206 R11 R18 lt 499 01 200K 1 na AAA 1 et 20 0K 1 10nF 50 10 x 1 0uF 25V 10 WZ 1206 SGND NA SGND FIGURE 3 3 First INA 2012 Microchip Technology Inc DS52031A page 25 MCP6N11 an
14. 2 Vpp 2 2 5V Tg 256 The reason Vina subtracts Veer Vap2 Vapa is to correct the VREF s inaccuracy about 0 03 Since the PIC MCU s ADC uses the supply as its reference voltage v is easy to pro duce in firmware Simply use the most significant bit MSb as the sign bit and the other bits to produce the magnitude The fitted polynomials are based on the average of the two nominal INA gains Gina 201 2 V V the resulting nominal error is only 0 1 which is well within the circuit s error budget EQUATION w Ko v K v Ky v K3 v Kg ey Trrp Ts ep ew Ts Where several polynomials are of use Polynomials Parameters n 1 n 2 n 3 n 4 Ko 0 01453 0 00130 0 00002 0 00001 K4 0 55695 0 50628 0 50417 0 50465 K2 0 0 08652 0 07463 0 07452 K3 0 0 01370 0 01116 K4 0 0 00221 r C 6 1 0 35 0 021 0 0013 Note 1 2 are useful for hand calculations n 3 for the firm ware algorithm and n 4 for spreadsheet design calculations 2 Other RTDs will use different coefficients DS52031A page 66 2012 Microchip Technology Inc Y MCP6N11 AND MCP6V2X WHEATSTONE BRIDGE MICROCHIP REFERENCE DESIGN USER S GUIDE Appendix D Board Validation Summary This appendix summarizes the data collected during validation of two Rev 1 boards D 1 THERMAL STEADY STATE RESPONSE The boards bias voltage
15. Amp Precision Design PCB Layout Techniques for more information on this topic Second INA Traditional Three Op Amp Implementation Input Filter 2 INA 2 R26 Output Filter 2 100 1 SGND C21 C20 AN seu gt sam 100nF 50V 10 Our 25 10 CA U3A R24 A sji DA er ine 25V 10 200K 1 px 08 R35 TON 1 ANN Lam 2 1 T i 20K 196 erts 100K arm Sear 7 10nF 50V 10 SGND C25 SGND 4 10uF 25V 10 C18 R28 1206 100nF 50V 10 biis R29 R36 499 0 196 200K 1 c19 1 10uF 25V 10 1206 em 50V 10 SGND Lr NNV 10 0K 0 1 10 0K 0 1 SGND 20 0K 1 FIGURE 3 5 Second INA 2012 Microchip Technology Inc DS52031A page 27 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design 3 4 TEST POINTS Figure 3 6 shows the test points available to the user See Table 2 1 for a list of their functions 1 5 R37 TP9 R38 TP13 50V 597 Vol AA N Nol o Vo2 AAA Vad3 1 0K 1 m 1 0K 1 TP2 TP6 TP10 R39 14 9 GND gt SGND 1 adi Vo3 AAA Q4 1 0K 196 TP3 TP7 11 R40 15 lt ev a 970 AAA lt G SND Nsan 1 0K 1 TP4 TPS TP12 TP16 G R2 lt S82 sero O SX2 sono O Pu FIGURE 3 6 Test Poi
16. Description What Does this Kit Contain 1 2 PURPOSE This board demonstrates the performance of Microchip s MCP6N11 instrumentation amplifier INA and a traditional three op amp INA using the MCP6V26 and MCP6V27 auto zeroed op amps from Microchip The input signal comes from an RTD tempera ture sensor in a Wheatstone bridge Real world interference is added to the bridge s output to provide realistic performance comparisons Data is gathered and displayed on a PC for ease of use The USB PIC microcontroller and included Graphical User Interface GUI show how to implement these functions 2012 Microchip Technology Inc DS52031A page 11 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design 1 3 DESCRIPTION 1 3 1 Block Diagram Figure 1 1 shows the overall functionality of this demo board Detailed information is available in Appendix A Schematics and Layouts and Appendix B Bill Of Materials BOM Sensor Interference Miscellaneous PWM at 25 kHz Test Points Bridge Power Supply INA 1 Mixed Signal e __ Filters USB PIC MCU dom e INA with nCal gt PIC18F2553 MCP6N11 100 PWM output 7 USE 12 bit ADC Display VREF USB Ee INA 2 ISSP Filters Firmware gt Traditional INA Averaging Satie share MCP6V27
17. OL asz 3n04 T ZA 1 nes E OL tha Y p Ea NN vao anos eit 193 NAINO 4913 3ndu anos angs aps anos 901 NOL 901 S y00 lt cai sio 00 HOL z VJ TPPA TPA a AUS Ajddng anps anot 01 NOS 3001 so PIA eA 66 gt ae 3OIOS I INS4OIN AGE OL PA uo f dno gt MIN a y 01 NOS 0 oct IA E ez anor V WNI ands 193113 MAU WANI a Vi joya dur OL NOS 40001 SZ Guo m 902 5 3 XL M b ai shi 9 090 AE wo INS OOF S OL TPPA MSIe2ul TR ny Sleg b egw 9604 AOS 24001 S ECLOS I0004DIN 5 89 ma Y Ta m la angs NOS duoi 9 ooz TPPA Tp J3HA s aos Soe uuo 00 ro 0 gt XA aA m3 DTI ya DIA ERE SuMOQ0L gt sy rest oi zee Xusdy o Sze EET d 9 pr WMd ajdo buijdno5 WMd HTS uy Tp 2012 M
18. Technology DS39632 Provides information on the USB PIC MCU family PIC18F2458 2553 4458 4553 Data Sheet 28 40 44 High Performance Enhanced Flash USB Microcontrollers with 12 Bit A D and nanoWatt Technology DS39887 Provides additional information on the USB PIC MCU devices with 12 bit ADCs AN1258 Application Note Op Amp Precision Design PCB Layout Techniques DS01258 Discusses methods to minimize thermo junction voltage effects in a PCB design THE MICROCHIP WEB SITE Microchip provides online support via our web site at www microchip com This web site is used as a means to make files and information easily available to customers Accessible by using your favorite Internet browser the web site contains the following information e e Product Support Data sheets and errata application notes and sample pro grams design resources user s guides and hardware support documents latest software releases and archived software General Technical Support Frequently Asked Questions FAQs technical support requests online discussion groups Microchip consultant program member listing Business of Microchip Product selector and ordering guides latest Microchip press releases listing of seminars and events listings of Microchip sales offices distributors and factory representatives DS52031A page 8 2012 Microchip Technology Inc Preface CUSTOMER SUPPORT Users of Micro
19. This avoids contention with the USB When using the bench setup 5V supply at TP1 5 0V and TP2 GND do not connect USB to the demo board This avoids contention with the lab supply NENNEN 0 52031 26 2012 Microchip Technology Inc Analog Circuit 3 3 4 Figure 3 5 shows INA 2 which provides a means of comparing the MCP6N11 to traditional INA Input Filter 2 is the same as Input Filter 1 for ease of comparison R24 and R25 pro vide a separate signal path so that U3 s input bias currents and U2 s input bias currents don t interact INA 2 U3 U4 is set to a DM gain of 201 V V and has its output shifted up by 1 80V Vref producing an output that uses most of the ADC s input range INA 2 s CMRR at 25 kHz is excellent e g 96 dB INA 2 s PSRR rejects mains interference e g harmonics of 50 Hz or 60 Hz R27 R30 are 0 196 resistors to minimize gain error they can be lower precision if the user modifies the firmware to calibrate offset and gain errors R31 R34 are 0 1 resistors to minimize gain error and maximize CMRR To minimize thermo junction voltage effects at INA 2 s input R24 and R25 are close together and in parallel The gain resistors for U3 R27 R30 are also close together and in parallel R28 and R29 have not been combined into one resistor to make this possible The gain resistors for U4 R31 R34 are also close together and in parallel See AN1258 Op
20. USB PIC device The user presses SW mCalSw The output filter provides a low pass function for both CM and DM signals out of the INA It is slow to minimize noise and interference 2012 Microchip Technology Inc DS52031A page 13 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design 1 3 6 INA 2 The input and output filters are the same as for the INA 1 This input filter is isolated from INA 1 s input filter by the input resistors The INA is a traditional three op amp instrumentation amplifier It uses Microchip s auto zeroed MCP6V27 dual and MCP6V26 single Its performance can be com pared to INA 1 it is also set at a high DM gain 200 V V It has good CMRR at DC 2100 dB Its CMRR at 25 kHz e g 96 dB is also good 1 3 7 Mixed Signal The USB PIC microcontroller performs the following functions Interfaces with the analog circuitry 12 bit ADC PWM output for interference Does calculations in firmware Averages data from the ADC Converts voltages to temperature Corrects the RTD temperature using stored calibration coefficients Calculates the RTD resistance from temperature Communicates with the PC Uses the USB port to upload data averaged voltages temperatures and resistances Uses the USB port to download configuration options calibrate INA 1 num ber of averages temperature calibration coefficient Provides easy programing for user designs if desired CSP in
21. is that the resistance to temperature conversion is a non linear relationship The RTD s resistance R2 appears in the denominator of the equation describing the bridge s differential output voltage VR VL as a function of Vdd1 and the resistors R1 to R6 This means that the relationship between R2 and VR VL is also non linear Fortunately as will be discussed later the firmware does the necessary calculations to overcome these non linear relationships 2012 Microchip Technology Inc DS52031A page 23 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design 3 3 SIGNAL PROCESSING CIRCUITRY This circuitry includes two INAs a VREF and corresponding filters The MCP6N11 s performance is contrasted with a three op amp INA based on the MCP6V27 and MCP6V26 auto zeroed op amps 3 3 1 Voltage Reference Figure 3 2 shows the VREF block not a fixed reference which produces a buffered ratiometric proportional to Vpp voltage Vref Vref keeps the INAs within their range even when Vpp changes Errors in Vref are subtracted out later in firmware The pole set at VREF s input matches Input Filter 1 and 2 s Differential Mode DM poles so that power supply disturbances will be treated the same Its output is 1 80V which gives good output headroom for both INAs The filter on the supply helps keep Vref quiet R8 and R9 are 1 resistors to minimize circuit cost the error in Vref is corrected in firmware s
22. presses the mCalSw switch SW1 5 7 CONFIGURATION OPTIONS The GUI provides a means for sending the following configuration information to the firmware via the USB bus Temperature calibration coefficient Number of averages n Strobe signal to trigger a Vos calibration event mCa1 in INA 1 NENNEN DS52031A page 38 2012 Microchip Technology Inc Software GUI 5 8 EXPORT DATA TO A FILE Export all of the data 2 Vapa 2 RRTD1 and RRTD2 to user selected text file The GUI either sends the displayed data limited to 500 time points to the file or sends recorded data over a selected time period which has no time point limit Export the strip chart to a user selected file in one of several formats 5 9 SCREEN CAPTURES All of the figures in this section have circled numbers that point to important features These numbers correspond to the associated numbered list Cross references to these numbers are displayed in the text as follows 1 Some useful features that may be hard to find include Resize Right Edge of Information Box 4 Export Strip Chart See Section 5 9 4 8 Exporting Data Acquisition Box Data File Graphic File Maximize Strip Chart See Section 5 9 4 9 Maximized Strip Chart Display Maximizes Strip Chart only Chart Menu more chart options See Section 5 9 4 10 Right Mouse Click inside the Strip Chart Area 5 9
23. that Act Information contained in this publication regarding device applications and the like is provided only for your convenience and may be superseded by updates It is your responsibility to ensure that your application meets with your specifications MICROCHIP MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED WRITTEN OR ORAL STATUTORY OR OTHERWISE RELATED TO THE INFORMATION INCLUDING BUT NOT LIMITED TO ITS CONDITION QUALITY PERFORMANCE MERCHANTABILITY OR FITNESS FOR PURPOSE Microchip disclaims all liability arising from this information and its use Use of Microchip devices in life support and or safety applications is entirely at the buyer s risk and the buyer agrees to defend indemnify and hold harmless Microchip from any and all damages claims Suits or expenses resulting from such use No licenses are conveyed implicitly or otherwise under any Microchip intellectual property rights QUALITY MANAGEMENT SYSTEM CERTIFIED BY DNV ISO TS 16949 2009 Trademarks The Microchip name and logo the Microchip logo dsPIC KEELOQ KEELOQ logo MPLAB PIC PICmicro PICSTART PIC logo rfPIC and UNI O are registered trademarks of Microchip Technology Incorporated in the U S A and other countries FilterLab Hampshire HI TECH C Linear Active Thermistor MXDEV MXLAB SEEVAL and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U S A
24. the MCP6N11 Temperature Calibration PWM Output USB Communications 4 2 ANALOG SIGNALS AT THE ADC INPUTS As discussed in Section 3 2 Bridge With PWM Interference the relationship between the RTD temperature and RTD resistance is non linear as is the relationship between the RTD resistance and the bridge s differential output voltage These non linearities are corrected in firmware because that minimizes the design cost Section 3 3 Signal Processing Circuitry mentions that interference from the 25 kHz PWM signal and the mains 50 Hz or 60 Hz is reduced significantly but not totally eliminated at the ADC inputs The firmware uses averaging to reduce the effect of these interfering signals The 12 bit ADC supports a temperature resolution that could be as good as 0 05 C Since the analog inputs do not go rail to rail for a more robust solution the resolution is reduced somewhat The non linear relationships mentioned above make the resolu tion change depending on the RTD temperature The firmware solution gives a reso lution between 0 06 C and 0 09 C Obviously the ADC is not ideal so the temperature error can be somewhat larger 2012 Microchip Technology Inc DS52031A page 31 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design 4 3 DATA CONDITIONING 4 3 1 Basic Digital Data The firmware on the PIC microcontroller inputs data for key analog DC voltages Vad1 to Vad4 then converts them to digital V
25. 271 0 470 30 111 67 0 00279 2 362 0 561 35 113 61 0 00324 2 452 0 651 40 115 54 0 00368 2 541 0 740 45 117 47 0 00412 2 629 0 828 50 119 40 0 00455 2 716 0 915 55 121 32 0 00498 2 802 1 001 60 123 24 0 00540 2 887 1 086 65 125 16 0 00582 2 971 1 171 70 127 08 0 00623 3 055 1 254 75 128 99 0 00664 3 137 1 336 80 130 90 0 00705 3 219 1 418 85 132 80 0 00745 3 299 1 499 90 134 71 0 00785 3 379 1 579 95 136 61 0 00824 3 458 1 658 2012 Microchip Technology Inc DS52031A page 63 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design TABLE C 1 NOMINAL VALUES CONTINUED RRTD VBRIDGE VINA OUT VINA CC V V V 100 138 51 0 00863 3 537 1 736 105 140 40 0 00901 3 614 1 813 110 142 29 0 00939 3 691 1 890 115 144 18 0 00977 3 767 1 966 120 146 07 0 01014 3 842 2 041 125 147 95 0 01051 3 916 2 115 130 149 83 0 01088 3 990 2 189 135 151 71 0 01124 4 063 2 262 140 153 58 0 01160 4 135 2 334 145 155 46 0 01196 4 206 2 405 150 157 33 0 01231 4 277 2 476 155 159 19 0 01266 4 347 2 546 H rn Jn aq DS52031A page 64 2012 Microchip Technology Inc Conversion Polynomials 2 RTD TEMPERATURE AND RESISTANCE The RTD s temperature error is Class F0 3 EQUATION 0 30 0 005 ITRTDU Class F0 3 To accommodate fixed point arithmetic Rrrp and TRTD are scaled as follows
26. 44 Fax 408 961 6445 Toronto Mississauga Ontario Canada Tel 905 673 0699 Fax 905 673 6509 ASIA PACIFIC Asia Pacific Office Suites 3707 14 37th Floor Tower 6 The Gateway Harbour City Kowloon Hong Kong Tel 852 2401 1200 Fax 852 2401 3431 Australia Sydney Tel 61 2 9868 6733 Fax 61 2 9868 6755 China Beijing Tel 86 10 8569 7000 Fax 86 10 8528 2104 China Chengdu Tel 86 28 8665 5511 Fax 86 28 8665 7889 China Chongqing Tel 86 23 8980 9588 Fax 86 23 8980 9500 China Hangzhou Tel 86 571 2819 3187 Fax 86 571 2819 3189 China Hong Kong SAR Tel 852 2401 1200 Fax 852 2401 3431 China Nanjing Tel 86 25 8473 2460 Fax 86 25 8473 2470 China Qingdao Tel 86 532 8502 7355 Fax 86 532 8502 7205 China Shanghai Tel 86 21 5407 5533 Fax 86 21 5407 5066 China Shenyang Tel 86 24 2334 2829 Fax 86 24 2334 2393 China Shenzhen Tel 86 755 8203 2660 Fax 86 755 8203 1760 China Wuhan Tel 86 27 5980 5300 Fax 86 27 5980 5118 China Xian Tel 86 29 8833 7252 Fax 86 29 8833 7256 China Xiamen Tel 86 592 2388138 Fax 86 592 2388130 China Zhuhai Tel 86 756 3210040 Fax 86 756 3210049 ASIA PACIFIC India Bangalore Tel 91 80 3090 4444 Fax 91 80 3090 4123 India New Delhi Tel 91 11 4160 8631 Fax 91 11 4160 8632 India Pune Tel 91 20 2566 1512 Fax 91 20 2566 1513 Japan Osaka Tel 81 66 152 7160 Fax 81 66 152 9310 Japan Yokohama Tel
27. 5 1000 0603 SMD 0 1 1 10W SUSUMU RG1608P 101 B T5 1 R7 30 1 kO 0603 SMD 0 196 1 10W SUSUMU RG1608P 3012 B T5 1 R8 121 0603 SMD 1 1 10W Yageo RCO603FR 07121KL 1 R9 68 1 kO 0603 SMD 1 1 10W Yageo RCO603FR 0768K1L 1 R10 2000 0603 SMD 5 1 10W Yageo RC0603JR 07200RL 8 R11 R12 20 0 0603 SMD 1 1 10W Yageo RCO0603FR 0720KL R19 R20 R24 R25 R35 R36 2 R13 R26 10 00 0603 SMD 1 1 10W Yageo RCO603FR 0710RL 2 R14 R16 10 0603 SMD 5 1 10W Yageo RC0603JR 0710KL 2 R15 R23 100 0603 SMD 5 1 10W Yageo RC0603JR 07100KL 3 R17 R27 100 kQ 0603 SMD 0 1 1 10W SUSUMU RG1608P 104 B T5 R30 3 R18 R28 4990 0603 SMD 0 1 1 10W SUSUMU RG1608P 4990 B T5 R29 2 R21 R22 100 0603 SMD 5 1 10W Yageo RC0603JR 0710KL Note 1 The components listed in this Bill of Materials are representative of the PCB assembly The released BOM used in manufacturing uses all RoHS compliant components 2012 Microchip Technology Inc DS52031A page 59 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design TABLE B 1 BILL OF MATERIALS ASSEMBLED PCB CONTINUED Qty aaa Description Manufacturer Part Number 4 R31 R34 10 0 0603 SMD 0 1 1 10W SUSUMU RG1608P 103 B T5 4 R37 R40 1 00 kO 0603 SMD 1 1 10W Yageo RC0603FR 071KL 2 SH1 SH2 Shunt 100 mil Thru Hole 2 Pos 30 um Au Tyco Electronics Corp 881545 2 1 SW1 SMD Switch Push
28. 7 SOIC 8 Dual Auto Zeroed Op Amp Microchip Technology MCP6V07 E SN Inc MCP6V02 SOIC 8 Dual Auto Zeroed Op Amp Microchip Technology MCP6V02 E SN Inc 1 U4 MCP6V06 SOIC 8 Single Auto Zeroed Op Amp Microchip Technology MCP6V06 E SN Inc MCP6V01 SOIC 8 Single Auto Zeroed Op Amp Microchip Technology MCP6V01 E SN Inc Note 1 The components listed in this Bill of Materials are representative of the alternate components used for modifications The released BOM used in manufacturing uses all RoHS compliant components DS52031A page 60 2012 Microchip Technology Inc Bill Of Materials BOM TABLE B 4 BILL OF MATERIALS OPTIONAL PARTS Reference ris Qty Designator Description Manufacturer Part Number 1 01 Header 1x6 100 mil Molex 22 28 4060 16 TP1 TP16 SMD Test Point Keystone Electronics 5016 4 for PCB mounting Hemispherical Bumpon Standoff 3M SJ 5003 BLACK 0 44 in x 0 20 in 4 for PCB mounting Stand off Hex 0 500 4 x 40 Thread Keystone 1902C Nylon Electronics 0 285 max O D 4 for PCB mounting Machine Screw Phillips 4 x 40 Thread Building Fasteners NY PMS 440 0025 PH 1 4 long Nylon 2 for External 24 AWG wires twisted strands 2 m n a n a Note 1 The components listed in this Bill of Materials are representative of the optional parts that the user may wish to acquire and install The released BOM used in manufacturing us
29. 9 TP10 TP11 TP13 Vad3 Vad3 A Filtered signal output of INA 2 TP14 Vad4 Vad4 A Filtered reference output of INA 2 TP15 GND A for TP13 TP14 TP16 TP16 Vpwms Vpwmx D External PWM signal couples onto bridge Note 1 Use this test point for only one of two purposes to measure power supplied by the USB orto provide 5V power when the USB is not connected Do not provide power to the USB and this test point at the same time 2 5 2 Jumper and Switch Settings Table 2 2 gives the jumper settings TABLE 2 2 JUMPER SETTINGS Jumper Identity Position Comente Ref Label Label Des JP1 Couple 1 Uses capacitive coupling between PWM source and bridge 2 RC Uses resistive coupling between PWM source and bridge a DC blocking capacitor avoids DC bias shifts and has low step response droop 3 Open No coupling of PWM source to bridge JP2 PWM 1 PIC Use PIC MCU as source for PWM signal 2 EXT Use external source via TP16 for PWM signal Note 1 Changing JP2 to a vertical connection between pins 1 and 3 allows the PIC microcontroller s PWM signal to be sent off board via TP16 Vpwmx This is useful for connecting to other PCBs DS52031A page 20 2012 Microchip Technology Inc Installation and Operation 2 5 3 Schematic Connectors Table 2 3 shows the connector labels used in the schematic TABLE 2 3 CONNECTOR LABELS
30. AND PADS TOP VIEW IX tJ 855 ao QI IEEE TI esa E E 5 DS52031A page 58 2012 Microchip Technology Inc MICROCHIP MCP6N11 AND MCP6V2X WHEATSTONE BRIDGE REFERENCE DESIGN USER S GUIDE Appendix B Bill Of Materials BOM TABLE B 1 BILL OF MATERIALS ASSEMBLED PCB Reference ota Qty Designator Description Manufacturer Part Number 1 1 220 pF 0603 SMD Ceramic NPO 50V 5 AVX Corporation 06035A221JAT2A 10 C2 CA4 100 nF 0603 SMD Ceramic X7R 50V 10 AVX Corporation 06035C104KAT2A C9 C10 C18 C21 C23 C28 4 C5 C7 10 nF 0603 SMD Ceramic X7R 50V 10 AVX Corporation 06035C103KAT2A C17 C19 11 C8 1 0 pF 1206 SMD Ceramic X7R 25V 10 AVX Corporation 12063C105KAT2A C11 C14 C20 C22 C24 C27 2 C15 C16 10 uF 1206 SMD Ceramic X7R 16V 10 AVX Corporation 1206YC106KAT2A 1 02 Header Mini USB Tyco Electronics 1734035 2 1 JP1 Header 3x2 100 mil Molex9 10 89 7062 1 JP2 Header 2x2 100 mil Molex9 10 89 7042 1 L1 10 uH 100 mA 0805 SMD 20 Murata Electronics LQM21FN100M70L 1 PCB MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design 4 layer PCB 2 60 in x 2 60 in 2 R1 R6 2 49 0603 SMD 0 1 1 10W SUSUMU RG1608P 2491 B T5 1 R2 1000 RTD 0603 SMD 196 Vishay Beyschlag PTS060301B100RP100 3 R3 R
31. Ap4 to Vap4 with a 12 bit representation EQUATION Vap1 INA 1 s signal Vad1 Vapo INA 1 s VREF Vad2 Vap3 INA 2 s signal Vad3 Vapa INA 2 s VREF Vad4 4 3 2 Data Sampling The DC analog voltages are sampled in this order VAp4 VAD2 Vap3 and VAp4 Each voltage is sampled every 50 ms a 20 SPS rate Since the ADC samples all four voltages it takes a total of 80 samples per second 4 3 3 Data Averaging and Output Data Rate The GUI tells the microcontroller the number of averages n or whether averaging is turned off The GUI collects the averaged data from the microcontroller at adjustable time intervals When averaging is off the firmware makes the latest sample available to the GUI The samples are updated every 50 ms When averaging is on the firmware calculates an exponential moving average which is updated every 50 ms These samples are made available for collection by the GUI A higher number of averages n reduces noise more but has a slower response The following equation shows how Vap 1 VAp2 Vans and VAD4 shown as are averaged shown as EQUATION 1 _ Where n 2 4 8 16 32 64 or 128 a 2 1 current sample _1 previous average current average 0552031 32 2012 Microchip Technology Inc Firmware 4 44 TEMPERATURE CALCULATIONS It would be possible to convert the measur
32. Button 1 Pos Panasonic ECG EVQ P2R02M SPST NO 1 01 MCP6001 SOT 23 5 Single Op Amp Microchip Technology Inc MCP6001T E OT 1 02 MCP6N11 SOIC 8 Single INA GMIN 100 Microchip Technology Inc MCP6N11 100U SN 1 U3 MCP6V27 SOIC 8 Dual Auto Zeroed Op Microchip Technology Inc MCP6V27 E SN Amp 1 U4 MCP6V26 SOIC 8 Single Auto Zeroed Op Microchip Technology Inc MCP6V26 E SN Amp 1 05 PIC18F2553 SOIC 28 300 mil USB PIC Microchip Technology Inc PIC18F2553 I SO MCU 1 1 20 MHz Ceramic Resonator SMT E temp Murata Electronics CSTCE20M0V53Z RO Note 1 The components listed in this Bill of Materials are representative of the PCB assembly The released BOM used in manufacturing uses all ROHS compliant components TABLE B 2 BILL OF MATERIALS OTHER PARTS IN KIT Reference Qty Designator Description Manufacturer Part Number 1 USB 2 0 Cable A to Mini B 1 m 88732 8600 1 The components listed in this Bill of Materials are representative of the other parts in the kit The released BOM used in manufacturing uses all RoHS compliant components TABLE B 3 BILL OF MATERIALS ALTERNATE COMPONENTS Qty Deseo Description Manufacturer Part Number 1 C27 470 nF 1206 SMD Ceramic X7R 25V 1096 AVX Corporation 12063C474KAT2A 4 R31 R34 10 0 0603 SMD 0 0196 1 16W Stackpole Electronics RNCFO603TKY10KO Inc 1 U3 MCP6VO
33. Curve Points BestFitLine Axis 4 Points BestFitCurve Il Points BestFitLine ll Points BestFitLine Points Line Axis 5 Points BestFitLine Il Points Spline Points Line Spline Axis 6 3 5 Points Spline Spline 3D Q Shadow 03D E Mark Data Points FIGURE 5 6 Data Acquisition Customization Plot Tab DS52031A page 44 2012 Microchip Technology Inc Software GUI 5 9 4 3 DATA ACQUISITION CUSTOMIZATION SUBSETS TAB Figure 5 7 shows that the Data Acquisition Dialog box with the Subsets tab selected To open this box double click inside the Strip Chart area 5 The Strip Chart curves can be selected or de selected here General Plot Subsets Points Font Color Style Subsets to Graph RTD1 Temperature RTD2 Temperature Scrolling Subsets VADI VAD2 0 VAD3 VAD4 4 FIGURE 5 7 Data Acquisition Customization Subsets Tab 5 9 4 4 DATA ACQUISITION CUSTOMIZATION POINTS TAB Figure 5 8 shows that the Data Acquisition dialog box with the Points tab selected To open this box double click inside the Strip Chart area 5 The number of points displayed on the Strip Chart s x axis is changed here 7 General Plot Subsets Points Font Color Style Points to Graph Point Label Orientation 8 Sequential Selected Q Auto
34. DC Inputs RN 31 4 3 Data GonditloniTig uere E orte REI de er up 32 4 4 Temperature Calculations www mawaa imam 33 4 5 Resistance Calculations wwwwwwmmemamamwmw muna mamaa 34 4 6 Calibrating the MCP6N11 wana wwa wanawa nwa 34 4 7 Temperature Calibration mwa wa wamama awana wanawa eren 35 4 6 PWM QUIPUl IAA Sa ahi ee 35 4 9 USB Communications nan nnn nns 35 2012 Microchip Technology Inc DS52031A page 3 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design Chapter 5 Software GUI Appendix A Appendix B 5 VIM OAUCUON per 37 5 2 Platform Requirements pp 37 5 3 USB Communications 4 37 5 4 Display BM E E MEAN 38 5 5 Calibrate Temperature 38 5 6 Calibrate the MCPOINT uc oett eii E bete Coe ru neto a deiecit ds 38 5 7 Contigurauorr DOE cns Lea Nia idee 38 5 8 Export Data to a File 4 39 5 9 Screen Captures saote ton sie e ed Ped ER E et e ed rhet Lp EORR 39 Schematics and Layouts AA Introduction 2 ee rre e rr e mae ere rade OR d er eed a 51 A 2 Board Schemat n c aet sieut e ers Pese esie aure tears 52 Board Top Silk Screen and Pads RN 53 AA Board Top Metal Layer etie nee neni ere pin ER d Poen 54 A 5 Board Ground Plane Second laye pp 55 A 6 Board Power Plane Third la
35. EATSTONE BRIDGE REFERENCE DESIGN USER S GUIDE MICROCHIP Table of Contents Eu EE 5 Ss 5 Document Layout nti eerte ters dede ira ead ert Pe reped nies 6 Conventions Used in this Guide pt 7 Recommended Reading valet dede eia lr eee ae 8 The Microchip Web Site oid t ape tet EP I x 8 C stomer SUP POM 9 Document Revision HIStory Nt 9 Chapter 1 Product Overview 1 4 Introd etlon t bet D e ee Ee petris 11 1 2 EE ads 11 1 3 Description EE 12 1 4 What Does this Kit Contain 15 Chapter 2 Installation and Operation 2 1 Introduction ces 17 2 2 Required Tools ee iet EA WA STA IAEA WA TA KAMUA 17 2 3 Basic Configurations nennen nennen 18 2 4 Configurations with Modifications RN 19 2 5 Configuring the IAA nenne 20 Chapter 3 Analog Circuit 3 1 Introd ctloh dit ntt e tite eda us 23 3 2 Bridge With PWM Interference esseseseeeeeeeeee enne 23 3 3 Signal Processing Circuitry DE sues 24 3 4 Test Polnts ed eene IR tis 28 3 5 PIC Microcontroller 28 Chapter 4 Firmware 4 1 Introduction cote ete eed ee cee ede ee i tent hes 31 4 2 Analog Signals at the A
36. MCP6N11 sub tab 30 are selected and the Strip Chart is displayed Figure 5 2 has more information Fox View Select GUIs Help gt Q oh x Firmware Version V1 0 0 Data View Additional Configuration Strip Chart E Calibration Acquisition _ Temp MCPENTT 2 um RTD1 Temperature RTD2 Temperature i TriggermCal Calibration Result N A MCal Function Description This function will trigger an intemal calibration specifically hal designed in the the stand alone INA amp 1 to recalibrate the o offset 5 50 5 Temperature Units E Celsius C Fahrenheit F Kelvin K 5 Number of Averages OFF PWM PWM Enable a 50 VADT m a VAN VAD VAD4 Duty 50 Set Duty 45 Freg Adjustment 0 40 Est Frequency 25 KHz 35 30 2 5 20 15 10 0 5 t Chart Update Speed 5 Sampling Interval 100 ms Fastest x 78 50 400 150 200 250 300 350 400 450 Data Buffer ES FIGURE 5 4 Additional Configuration and MCP6N11 Tabs with Strip Chart The indicated features for Figure 5 4 are 28 Additional Configuration tab is selected 29 MCP6N11 sub tab is selected 30 Trigger mCal button sends a trigger to INA 1 to recalibrate its offset 31 Sampling In
37. MICROCHIP MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design User s Guide 0 52031 Note the following details of the code protection feature on Microchip devices Microchip products meet the specification contained in their particular Microchip Data Sheet Microchip believes that its family of products is one of the most secure families of its kind on the market today when used in the intended manner and under normal conditions There are dishonest and possibly illegal methods used to breach the code protection feature All of these methods to our knowledge require using the Microchip products in a manner outside the operating specifications contained in Microchip s Data Sheets Most likely the person doing so is engaged in theft of intellectual property Microchip is willing to work with the customer who is concerned about the integrity of their code Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code Code protection does not mean that we are guaranteeing the product as unbreakable Code protection is constantly evolving We at Microchip are committed to continuously improving the code protection features of our products Attempts to break Microchip s code protection feature may be a violation of the Digital Millennium Copyright Act If such acts allow unauthorized access to your software or other copyrighted work you may have a right to sue for relief under
38. a four layer PCB 2012 Microchip Technology Inc DS52031A page 51 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design BOARD SCHEMATIC A 2 auos ININ gSn NNOO dr zolOSASszd81DId sama L era is py g asa i und alil Ly ups 9 Eu m E 7 gy H oN xL amp anos auos Ir E 3O1SO YOLVNOSTY 40 ADS 24001 oa SA Panos 829 sva L WPA p T asn B spes ig E Img HG ay en ON ON 589 INV GON 5 D OPE NY HL E ea xon z ir T gt Panos Y DO 8Sn T 5 E NU 4 UND avos q dND no sai ru OL avsqQ UND gw OR TA TA PA Ove uil m Pe oR TA ae ecu pu a OL a0 ms TA a TA Aos ful zai En WL Sjuiod 1581 anos 985 asz n01 go 7 S WA 80108 66y sai nos Go pid ead 810
39. an be selected When the data is exported as text only its destination can be selected m OOOO o C Text Data Only Export Destination ClipBoard Eile Printer Object Size No Specific Size Millimeters Inches Points Width 1000 676 Units FIGURE 5 12 Exporting Data Acquisition Box 2012 Microchip Technology Inc DS52031A page 47 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design 5 9 4 9 MAXIMIZED STRIP CHART DISPLAY Figure 5 13 shows the maximized strip chart display To open this window on the PC s main display double click inside the Strip Chart area 5 then press the Maximize button on the bottom right e g see Figure 5 11 To exit this screen click on the top banner or press the Esc key on your keyboard G n UH t du 8 Temperature Voltage V DE E 100 180 E 280 300 E 400 450 Data Buffer FIGURE 5 13 Maximized Strip Chart Display under the Strip Chart Dialog Box NENNEN DS52031A page 48 2012 Microchip Technology Inc Software GUI 5 9 4 10 RIGHT MOUSE CLICK INSIDE THE STRIP CHART AREA Figure 5 14 shows a chart menu with more chart options This window is opened on the PC s main display by right clicking inside the Strip Chart area 5 Vi
40. chip products can receive assistance through several channels Distributor or Representative Local Sales Office Field Application Engineer FAE Technical Support Development Systems Information Line Customers can contact their distributor representative or field application engineer for support Local sales offices are also available to help customers A listing of sales offices and locations is included in the back of this document Technical support is available on the web site at http www microchip com support DOCUMENT REVISION HISTORY Revision A January 2012 The initial release of this document UU ci 2012 Microchip Technology Inc DS52031A page 9 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design NOTES DS52031A page 10 2012 Microchip Technology Inc MCP6N11 AND MCP6V2X WHEATSTONE BRIDGE REFERENCE DESIGN USER S GUIDE MICROCHIP Chapter 1 Product Overview 1 4 INTRODUCTION The MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design is described by the following Assembly 114 00354 R2 Order ADM00354 Name MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design This board uses the following Microchip ICs MCP6001 op amp MCP6N11 100 INA MCP6V27 dual auto zeroed op amp MCP6V26 single auto zeroed op amp PIC18F2553 USB Pic microcontroller Items discussed in this chapter include Purpose
41. corner The part number can be found on the label on the outside of the box see the example below Example MCP4725 SOT 23 6 Eval Board LIED The Order Number can 55 shown in this example Tha Mercen PRO USA and ctor Decks p DS52031A page 15 2012 Microchip Technology Inc MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design NOTES DS52031A page 16 2012 Microchip Technology Inc e MCP6N11 AND MCP6V2X WHEATSTONE BRIDGE REFERENCE DESIGN USER S GUIDE MICROCHIP Chapter 2 Installation and Operation 2 1 INTRODUCTION This chapter shows how to set up and operate the MCP6N11 and MCP6V2x Wheat stone Bridge Reference Design Items discussed in this chapter include Required Tools Basic Configurations Configurations with Modifications Configuring the PCB 2 2 REQUIRED TOOLS 2 2 1 Setup With the PC The default setup uses the USB bus to connect to the PC The following is required MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design USB 2 0 Cable A to Mini B e PC Microsoft Windows XP SP3 Vista and 7 64 and 32 bit NET2 Framework Thermal Management Utility Microchip s GUI for thermal demo boards Version 1 4 0 0 or later CAUTION When using the PC and USB do not connect lab supplies to test points TP1 5 0V TP2 GND This avoids contention with the USB
42. d MCP6V2x Wheatstone Bridge Reference Design 3 3 3 Power Supply Figure 3 4 shows the power supply connections and filters Power is provided at either the Lab Power Supply or by the USB connection The R C low pass filters have a pole at 0 80 kHz which reduces ripple and crosstalk from the PWM interference they are symmetrically placed so that applying power at either end works well L1 isolates the analog and digital supplies to prevent digital signals from interfering with the analog signals it produces an L C pole at 23 kHz a compromise between performance and cost R23 is a bleed off resistor that discharges the bypass capacitors when no power supply voltage is present its power drain is not significant The signal connectors shown are 5 0V An external connection to the analog power supply voltage When the USB powers the board no lab supply connected it provides a convenient point to measure the USB s 5V power When a lab supply powers the board USB is not connected it is connected here at TP1 Vdd1 Analog power supply voltage Vdd2 Digital power supply voltage Vdd3 USB 5V power Power Supply vddl 10uH 2096 0805 C14 C15 C16 R23 gt 100K 5 1 0uF 25V 1096 10uF 16V 10 10uF 16V 1096 1206 1206 1206 SGND SGND SGND SGND FIGURE 3 4 Power Supply CAUTION When using the PC and USB do not connect lab supplies to test points TP1 5 0V and TP2 GND
43. date Speed 0 0 27 Sampling Interval 100 ms Fastest 0 50 100 150 200 250 300 350 400 450 P Data Buffer i FIGURE 5 3 Additional Configuration and RTD Temp Tabs with Strip Chart 19 Calibration input boxes a Calibration Temperature press the Calibrate button 20 when ready b INA1 Calibration Offset input temperature correction for INA1 INA2 Calibration Offset input temperature correction for INA2 20 21 22 23 24 25 26 Calibrate button enter Calibration Temperature 19 then press when ready Reset button resets Calibration inputs 19 to 0 Additional Configuration tab is selected RTD Temp sub tab is selected Temperature Units selection radio buttons Number of Averages drop down menu PWM inputs a o Enabled check box turns on PWM output from PIC microcontroller and acti vates other features in the PWM box Duty input box inputs duty cycle Set Duty button sends updated value to the microcontroller 27 Freguency Adjustment slider changes the PWM freguency Approximate Frequency input box adjusts PWM frequency 25 kHz is preferred Sampling Interval drop down menu shown at 100 ms per sample o 2 aat DS52031A page 42 2012 Microchip Technology Inc Software GUI 5 9 3 Additional Configuration and MCP6N11 Tabs Figure 5 4 shows that the Additional Configuration tab 29 and
44. ed in this chapter include Bridge With PWM Interference Signal Processing Circuitry PIC Microcontroller 3 2 BRIDGE WITH PWM INTERFERENCE Figure 3 1 shows the Wheatstone bridge with one RTD element R2 and three other resistors R3 R4 and R5 R1 and R6 set the current which is ratiometric with the 5 0V supply R1 and R3 R6 are 0 196 resistors to minimize the gain error they can be lower precision if the user modifies the firmware to calibrate offset and gain errors Vpwm is a 5 Vp p PWM signal generated by the PIC MCU U3 which is set at 25 kHz and 50 duty cycle JP1 selects a capacitive feedthrough C1 a resistive feedthrough R7 with C2 for DC blocking or no feedthrough open The first two methods produce roughly 0 2 Vp p at the top of the bridge Vrtd while the last produces no PWM signal at Inputting an external PWM signal JP2 position 2 will give different results Cars R1 Bridge 249K 0 196 ci 220pF 50V 5 R7 c2 RC Wad gt w 100 RTD 1 78 32 to 159 19 Extemal c XE 3010019 50 10 u 100 0 1 s 249K 01 NZ SGND FIGURE 3 1 Bridge with PWM Interference The RTD s resistance is a quadratic polynomial in temperature see Appendix C Conversion Polynomials It can be approximated as a linear polyno mial with a temperature error less than 1 3 but that is not accurate enough for applications requiring an RTD The point
45. ed voltages into RTD resistance then into RTD temperature This approach adds overhead to the PIC MCU so it was not done on this design the firmware converts directly from INA voltage to temperature The firmware gives two estimates of the same RTD temperature based on the outputs of INA 1 and INA 2 EQUATION 1 RTD temperature from INA 1 based on V4p4 Trtp2 RTD temperature from INA 2 based 4 4 1 Polynomial Estimate of Temperature Section C 3 INA Voltage to Temperature discusses the conversion of Vina to TRTD This section summarizes those results To accommodate fixed point arithmetic Trtp and Vina are scaled as follows EQUATION V Vina Vpp 2 w Tgrp 1 Where Vina for INA 1 for INA 2 Vpp 2 2 5V Tg 256 The firmware uses a cubic polynomial for the conversion Its accuracy supports the 12 bit ADC we are using 1 LSB is between 0 055 C and 0 088 C nominally EQUATION w Ko v K v K3 v K3 W Where Ko 0 00002 K4 0 50417 K 0 07463 0 01370 w r EwTg ler 0 021 1 Tgrpo INA 2 The GUI collects 10 temperature estimates per second for each INA 2012 Microchip Technology Inc DS52031A page 33 MCP6N11 and MCP6V2x Wheatstone Brid
46. es all RoHS compliant compo nents 2012 Microchip Technology Inc DS52031A page 61 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design NOTES DS52031A page 62 2012 Microchip Technology Inc MCP6N11 AND MCP6V2X WHEATSTONE BRIDGE REFERENCE DESIGN USER S GUIDE MICROCHIP Appendix C Conversion Polynomials C 1 CIRCUIT RESPONSE Table C 1 shows the nominal performance of critical circuit parameters used in this appendix Veripce is the differential output voltage of the Wheatstone bridge Vina out is the output voltage of the INAs VREF shifts the INAs output above ground about 1 80V Vina IS Vina our Vrep Which corrects the measured voltages for Vggr errors about 0 03V TABLE C 1 NOMINAL VALUES VBRIDGE ViNA OUT ViNA CC V V V 55 78 32 0 00565 0 665 1 136 50 80 31 0 00510 0 774 1 026 45 82 29 0 00456 0 883 0 918 40 84 27 0 00403 0 989 0 811 35 86 25 0 00351 1 095 0 706 30 88 22 0 00299 1 199 0 601 25 90 19 0 00248 1 303 0 498 20 92 16 0 00197 1 404 0 396 15 94 12 0 00147 1 505 0 295 10 96 04 0 00097 1 605 0 196 5 98 04 0 00048 1 703 0 097 0 100 00 0 00000 1 801 0 000 5 101 95 0 00048 1 897 0 096 10 103 90 0 00095 1 992 0 191 15 105 85 0 00142 2 086 0 285 20 107 79 0 00188 2 179 0 378 25 109 74 0 00234 2
47. esistance in R2 s place For example use the 0 value of 1000 0 01 4 8 PWM OUTPUT The firmware sets the Vpwm output pin 13 to produce a square wave with 5 Vp_p 25 kHz and 50 duty cycle It runs continuously The manually set options on JP1 allow different coupling methods to the Wheatstone bridge including no connection 4 9 USB COMMUNICATIONS The USB PIC MCU device s clock runs on a 20 MHz ceramic resonator which supports the microcontroller s firmware overhead and USB communications see Section 3 5 PIC Microcontroller The GUI provides a means for sending the following configuration information to the board via the USB Temperature calibration coefficient Number of averages Strobe signal to trigger a Vos calibration event mCa1 in INA 1 The firmware will send the following data to the GUI in this order VAp4 VAp2 VAD3 Vap4 1 TRTD2 RRTD1 and 2012 Microchip Technology Inc DS52031A page 35 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design NOTES DS52031A page 36 2012 Microchip Technology Inc P MCP6N11 AND MCP6V2X WHEATSTONE BRIDGE REFERENCE DESIGN USER S GUIDE MICROCHIP Chapter 5 Software GUI 5 1 INTRODUCTION This chapter discusses the PC s GUI Items discussed in this chapter include Platform Requirements USB Communications Display Data Calibrate Temperature Calibrate the MCP6N11 Configuration Options Export Data to a File
48. ew Select GUIs la RTD Resistance RTD Temperature Data View Additional Configuration Sup Chat INA 1 w Temperature mm RTD Temperature V V ace Reference Pts V 2066 1 654 Simplified Circuit Diagram Font Size Q all Firmware Version V1 0 0 Data Acquisition RTD 1 10927 Q 2379 Viewing Style Border Style Temperature Show Legend Numeric Precision www VAD Data Shadows Grid Options Graph and or Table Point Label Orientation Mark Data Points Undo Zoom Maximize Voltage V Customization Dialog Export Dialog Chart Update Speed Sampling Interval FIGURE 5 14 Chart Menu With More Chart Options Right Click inside the Strip Chart Area 2012 Microchip Technology Inc 0 52031 49 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design 5 9 4 11 ZOOMING IN AND OUT Figure 5 15 shows the result of clicking and dragging inside the Chart Area this will cause the Strip Chart to display the selected area only To zoom out to full view click the Zoom out button second from right button under arrows 1 in Figure 5 2 Thermal View Select GUIs Help Firmware Version V1 0 0 Data View Additional Configuration Strip Chart
49. g scheme for speed or noise performance e Convert Vina to then to TRTD if Rrro is a required output 2 4 3 Component Substitutions The following component substitutions may be of interest to the user Replace the MCP6V26 with either the MCP6VO6 or the MCP6V01 lower power Replace the MCP6V27 with either the MCP6VO07 or the MCP6VO2 lower power Replace the MCP6N11 100 with another MCP6N11 GMIN option need lower gain Replace the RTD requires a change in firmware to evaluate another RTD 2012 Microchip Technology Inc DS52031A page 19 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design 2 5 CONFIGURING THE PCB 2 5 1 Test Points Table 2 1 lists the test points and describes their functionality TABLE 2 1 TEST POINTS Test Point Comments Ref Des Label Connector I O A D TP1 5 0V 5 0V A Power supply also labeled Vpp Note 1 TP2 GND for TP1 TP3 Rtd1 Vrtd l Connected to external RTD R2 TP4 Rtd2 VL 5 1 Vo1 Un filtered output of U2 INA 1 TP6 Vad1 Vad1 A Filtered signal output of INA 1 TP7 Vad2 Vad2 Filtered reference output of INA 1 TP8 GND for TP5 TP6 TP7 TP9 Vo2 Vo2 O Un filtered output of U3 Side A TP10 Vo3 Vo3 O Un filtered output of Side B TP11 Vo4 Vo4 Un filtered output of U4 TP12 GND A for TP
50. ge Reference Design 4 5 RESISTANCE CALCULATIONS For convenience of the user the firmware gives two estimates of the RTD resistance We do not need these estimates since we convert directly from voltage to temperature They are useful however to a user that wants to change the firmware or hardware The firmware gives two estimates of the same RTD resistance based on the previously calculated of TRTD1 and TRTD2 see Appendix C Conversion Polynomials EQUATION Rrtp1 RTD resistance from INA 1 based on Rrtp2 RTD resistance from INA 2 based on Tprp2 To accommodate fixed point arithmetic TRrp and RRTD are scaled as follows EQUATION Ts q Rgrp Rso Rs Where 256 Rso 120 Rs1 64 Q The RTD s resistance is estimated with this polynomial approximation with scaling EQUATION q Ag t 0 4 Rs teg Where 0 31251 A 1 56353 0 05946 g Errorinq ER g Ri leg lt 0 0053 INA 1 INA 2 ER dRgrp dTgrD The GUI collects 10 resistance estimates per second for each INA 4 6 CALIBRATING THE MCP6N11 When is set to position 1 the PIC MCU can send an mCa1 signal to the MCP6N11 device This happens when the GUI sends a signal to the PIC MCU via USB to start an mCal event The effect of this signa
51. icrochip Technology Inc DS52031A page 52 Schematics and Layouts BOARD TOP SILK SCREEN AND PADS Ha o JP1 Couple TA e Rtd2 i WM R37 cio AE ICSP RTD 25 Rice Vol m b Os fap pi n20 iz I OON m C13 mCalSu 58508 Vadi MANN e MICROCHIP C23 e TP R21 C14 6 11 rpeuex Wheatstone Bridge Re erence Design Brso R40 RDOO354 TP10 TPS TP11 TP12 TP14 TP15 2012 Microchip Technology Inc DS52031A page 53 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design A4 BOARD TOP METAL LAYER Schematics and Layouts A 5 BOARD GROUND PLANE SECOND LAYER wt C 79 M 5 r MH a 2012 Microchip Technology Inc DS52031A page 55 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design A 6 BOARD POWER PLANE THIRD LAYER _ oae A w wssst E qn s Q Msc s DS52031A page 56 2012 Microchip Technology Inc Schematics and Layouts BOARD BOTTOM METAL LAYER TOP VIEW 8 cCE00 01 e 2012 Microchip Technology Inc DS52031A page 57 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design A 8 BOARD BOTTOM SILK
52. idge Reference Design Appendix B Bill Of Materials BOM Lists the parts used to populate the MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design Also lists alter nate components Appendix C Conversion Polynomials Gives background information on the polynomials used in firmware Appendix D Board Validation Summary Summarizes analog performance of two boards e DS52031A page 6 2012 Microchip Technology Inc CONVENTIONS USED IN THIS GUIDE Preface This manual uses the following documentation conventions DOCUMENTATION CONVENTIONS Description Represents Examples Arial font Italic characters Referenced books MPLAB IDE User s Guide Emphasized text is the only compiler Initial caps A window the Output window A dialog the Settings dialog A menu selection select Enable Programmer Quotes A field name a window or Save project before build dialog Underlined italic text with A menu path File gt Save right angle bracket Bold characters A dialog button Click OK A tab Click the Power tab N Rnnnn A number in verilog format where N is the total number of digits R is the radix and n is a digit 4 b0010 2 hF1 Text in angle brackets gt A key on the keyboard Press Enter F1 Courier New font Plain Courier New Sample source code define START Filena
53. l is to internally re calibrate the MCP6N11 devices input offset voltage its output is high impedance while this re calibration is in progress DS52031A page 34 2012 Microchip Technology Inc Firmware 4 7 TEMPERATURE CALIBRATION 4 7 1 Single Point Calibration One of the user options available in our Thermal Management GUI see Chapter 5 Software GUI is a single point offset calibration usually at 50 C it helps to correct for both circuit and RTD errors The GUI calculates the errors and sends calibration coefficients to the USB PIC MCU which correct all subsequent temperature values These corrections are simple subtractions Note 1 The demo board initially has the calibration coefficient set to zero 2 Only a single point calibration will be provided for our demo board 4 7 2 Higher Order Calibrations Higher order calibrations are potentially more accurate With two temperature measurements e g at 0 C and 100 C it is possible to do a linear correction offset and gain With three temperature measurements e g at 0 C 50 C and 100 C it is possible to do a quadratic correction offset gain and bow These corrections require firmware modifications by the user 4 7 3 Circuit Calibration Separating the circuit calibration from the temperature calibration gives more insight into error sources but not more accuracy It is possible to do a separate circuit calibration by inserting a r
54. mes autoexec bat File paths e mecl8 h Keywords asm _endasm static Command line options Opa Bit values 05 1 Constants OxFF Italic Courier New A variable argument file o where file can be any valid filename Square brackets Optional arguments mcc18 options file options Curly brackets and pipe character Choice of mutually exclusive arguments an OR selection errorlevel 0 1 Ellipses Replaces repeated text var_name var_name Represents code supplied by user void main void 2012 Microchip Technology Inc DS52031A page 7 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design RECOMMENDED READING This user s guide describes how to use MCP6N1X Other useful documents are listed below The following Microchip documents are available and recommended as supple mental reference resources MCP6001 Data Sheet DS21733 Provides detailed information on the op amp that is used for VREF MCP6N11 Data Sheet 500 kHz 800 uA Instrumentation Amplifiers DS25073 Provides detailed information on the INA that is used for INA 1 MCP6V26 7 8 Data Sheet 620 uA 2 MHz Auto Zeroed Op Amps DS25007 Provides detailed information on the auto zeroed op amps used for INA 2 PIC18F2455 2550 4455 4550 Data Sheet 28 40 44 Pin High Performance Enhanced Flash USB Microcontrollers with nanoWatt
55. n level of the document For the most up to date information on development tools see the MPLAB IDE online help Select the Help menu and then Topics to open a list of available online help files INTRODUCTION This chapter contains general information that will be useful to know before using the MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design User s Guide Items discussed in this chapter include Document Layout Conventions Used in this Guide Recommended Reading The Microchip Web Site Customer Support Document Revision History 2012 Microchip Technology Inc DS52031A page 5 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design DOCUMENT LAYOUT This document describes how to use the MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design The manual layout is as follows Chapter 1 Product Overview Important information about the MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design Chapter 2 Installation and Operation Covers the initial set up of this board required tools board setup and lab equipment connections Chapter 3 Analog Circuit Discusses the analog functionality of the circuit on this board Chapter 4 Firmware Discusses the firmware on the PIC device Chapter 5 Software GUI Discusses the GUI on the PC Appendix A Schematics and Layouts Shows the schematic and board layouts for the MCP6N11 and MCP6V2x Wheatstone Br
56. nd sends data to the PC via the USB The ICSP header allows the user if desired to re program the microcontroller In many cases the installed firmware will provide the necessary functionality for an evaluation of this design The GUI Thermal Management Utility runs on the PC It handles communications through the USB displays results on a strip chart stores single point calibration coef ficients on the PIC MCU tells the PIC MCU how to configure the averaging filter initi ates offset calibration events for the MCP6N11 and exports data to CSV files DS52031A page 12 2012 Microchip Technology Inc Product Overview 1 3 2 Sensor The RTD converts board temperature to resistance with a nearly linear response It is placed in a Wheatstone bridge to convert its resistance to a small DM voltage The bridge s characteristics are Ratiometric current reducing hardware and firmware complexity Sets the CM voltage at mid supply 2 5V for the greatest headroom 1 3 3 Interference The PIC device outputs a PWM waveform at 25 kHz when it is enabled by the GUI and by JP2 which can be coupled onto the top of the bridge using a jumper JP1 this is a CM interference signal about 0 2 Vp p at the bridge The coupling mechanism can be either a capacitor or a resistor with a DC blocking capacitor the bridge s DC bias point is not affected This interference demonstrates how bridges are susceptible to CM noise The follo
57. nology Inc Analog Circuit J1 1 USB PIC A Ce U5 SGND e e AME s 2 E 8 Mad gt H ANO 2 5 aH m ET Vai pe MH ans NC 6 ras NE vum USB NIC RAS RB 2 SGND GND 8 Si C28 se vss 21 100nF 50V 10 RESONATOR CSTCE E Osci ya e L 1 vss 9 SGND z ed xi 5 i Nn s n L iai nci psp oo vp 16 S vus 55 ue a TDH wus 1 0uF 25V 10 PIC18F2553 SOIC2S CONN USB MINI SGND FIGURE 3 7 PIC Microcontroller Section Chapter 4 Firmware and Section Chapter 5 Software GUI discuss in detail how the digitized data is processed displayed and stored 2012 Microchip Technology Inc DS52031A page 29 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design NOTES DS52031A page 30 2012 Microchip Technology Inc E MCP6N11 AND MCP6V2X WHEATSTONE BRIDGE REFERENCE DESIGN ER IDE MICROCHIP ar i ac SA Chapter 4 Firmware 41 INTRODUCTION This chapter discusses the PIC microcontroller s firmware Items discussed in this chapter include Analog Signals at the ADC Inputs Data Conditioning Temperature Calculations Resistance Calculations Calibrating
58. nts 3 5 PIC MICROCONTROLLER Figure 3 7 shows the PIC microcontroller USB PIC MCU with analog connectors to the blocks previously shown local supply bypassing a 20 MHz ceramic resonator ICSP header and USB header The four ADC inputs Vad1 to Vad4 are multiplexed into the USB PIC MCU s internal 12 bit ADC As discussed later on the firmware takes the differences Vad1 Vad2 and Vad3 Vad4 which corrects VREF s output error present at Vad2 and Vad4 Since the analog signal processing circuitry does not totally eliminate interference from the 25 kHz PWM signal or the mains 50 Hz or 60 Hz some will appear at the ADC inputs The 12 bit ADC supports a typical temperature resolution better than 0 09 The PWM output Vpwm is a 5 Vp p digital signal at 25 kHz and 50 duty cycle It emu lates the CM interference from a motor control application The 20 MHz ceramic resonator runs the USB PIC MCU at a rate that supports the firm ware tasks overhead and the USB An internal PLL Prescaler divides by 5 to produce a 4 MHz clock The internal PLL then locks onto this clock and produces a 96 MHz output which is divided down for the USB module s operation Use a low ESR capacitor for C27 The ICSP header makes it possible to program the USB PIC MCU in circuit for further user experiments The USB header provides the data link to the PC where data is displayed on the included GUI DS52031A page 28 2012 Microchip Tech
59. quisition c Zoom Buttons resize Strip Chart Zoom Information click and drag inside Strip Chart Zoom Out d Chart Customization Button Open chart customization dialog 2 Current measured values for INA 1 RTD Resistance RTD Temperature Vap1 Reference Point voltage which changes with temperature Vap2 Reference Point voltage based VREF 2 0 52031 40 2012 Microchip Technology Inc Software GUI 2 ue c Current measured values for INA 2 a RTD Resistance b RTD Temperature C Reference Point voltage which changes with temperature d Reference Point voltage based on VREF Right edge of Information Box To resize this box Left Click this edge with the mouse and Drag Strip Chart area a Double click in this area to open the Strip Chart Dialog Box b Right click in this area to open another chart menu Strip Chart a To zoom in on a smaller area click on one corner and drag to the other corner the magnifier icon appears b To zoom out to the full Strip Chart area click on the Zoom Out button 1 Right edge of GUI window To resize this window click this edge and drag Help button Data View tab is selected Sampling Interval drop down menu interval between stored and displayed points Data display boxes for ADC input voltages to 12 13 14 15 16 17 18 Data display boxes for calculated temperatu
60. re estimates TRTD1 and Tgrp2 on a strip chart with a different y axis The RTD resistance estimates Rgrp4 and RRTD2 are not displayed on the strip chart The user will select the data to display on the strip chart The vectors in bold blue font in Figure 5 1 can be displayed when their check boxes are checked the ADC amp F W block represents the ADC internal to the PIC MCU and the firmware stored in its mem ory The GUI requests the latest data from the microcontroller after each sampling interval period has passed and uses that data to perform the plotting and data calcu lations Data between collection requests are not sent to the GUI INA ZVapi ATRTDI _ RTD 1 JI VAp2 1_ amp amp Bridge FAN INA J VAD3 JTRTD2 _ 2 A Vaa RRTD2 _ FIGURE 5 1 Selecting Vectors to Display on Strip Chart 5 5 CALIBRATE TEMPERATURE Supports a single point calibration of TRTD1 and 2 at a temperature selected by the user The GUI sends these calibration coefficients to the USB PIC microcontroller which does the actual corrections 5 6 CALIBRATE THE MCP6N11 The MCP6N11 MCU internally re calibrates its input offset voltage when any one of these three events occurs Power up The user clicks the mCal push button in the GUI which sends an mCa1 event signal to the PIC microcontroller The user
61. res TRTD1 and 2 Y axis for ADC input voltages Vap4 to Vap4 Y axis for calculated temperatures TRTD1 and TRTD2 X axis for collected data Strip Chart curves Bottom edge of GUI window To resize this window click this edge and drag Bottom Right corner of GUI window To resize this window click and drag 2012 Microchip Technology Inc DS52031A page 41 MCP6N1 1 and MCP6V2x Wheatstone Bridge Reference Design 5 9 2 Additional Configuration and RTD Temp Tabs Figure 5 3 shows that the Additional Configuration tab 22 and RTD Temp sub tab 23 are selected and the Strip Chart is displayed Figure 5 2 has more information m Thermal Management Utility Sx View Select GUIs 9 Help 1 gt Qe Firmware Version V1 0 0 Data View Additional Configuration ip Chart xx Ciis Data Acquisition RTD Temp MCPGNTI RTD Temperature mmm em RTD2 Temperature Calibration Temperature C Calibration Offset C 0 co a INA Calibration Offset C 0 Reset 2 504 E Temperature Units E Celsius C Fahrenheit F Kelvin K 0 Number of Averages OFF eT PWM PWM Enable VADI eee VAD VAD coe VAD4 Duty 50 X Duty 50 26 Freq Adjustment 1 x SIE 40 E T Est Frequency 25 KHz 3 5 30 25 204 154 104 05 Chart Up
62. rmal settling dominates with t 4 2 fp 38 MHz Later on the PCB s thermal settling with still air laying flat on the bench dominates with 94 s fp 1 7 MHz 100 Time Constant 7 4 2s _ RTD Ze Time Constant 94s 5 6 10 o o T x Steady State 0 C t T 1 0 10 20 30 40 50 60 70 80 90 100110120 Time s FIGURE D 3 Exponential Settling of Temperature vs Time DS52031A page 68 2012 Microchip Technology Inc MICROCHIP Worldwide Sales and Service AMERICAS Corporate Office 2355 West Chandler Blvd Chandler AZ 85224 6199 Tel 480 792 7200 Fax 480 792 7277 Technical Support http www microchip com support Web Address www microchip com Atlanta Duluth GA Tel 678 957 9614 Fax 678 957 1455 Boston Westborough MA Tel 774 760 0087 Fax 774 760 0088 Chicago Itasca IL Tel 630 285 0071 Fax 630 285 0075 Cleveland Independence OH Tel 216 447 0464 Fax 216 447 0643 Dallas Addison TX Tel 972 818 7423 Fax 972 818 2924 Detroit Farmington Hills MI Tel 248 538 2250 Fax 248 538 2260 Indianapolis Noblesville IN Tel 317 773 8323 Fax 317 773 5453 Los Angeles Mission Viejo CA Tel 949 462 9523 Fax 949 462 9608 Santa Clara Santa Clara CA Tel 408 961 64
63. s were within expected ranges The difference between the two INA outputs on each board was less than 0 2 C caused by resistor tolerances The difference between the two boards was less than 1 4 C caused by RTD toler ances The first board s steady state temperature was 23 4 C 74 1 F D 2 THERMAL IMPULSE RESPONSE A quick blast from a freeze spray can onto the RTD on the first board cooled it rapidly giving the thermal impulse response of the RTD and PCB The following plots are from the second INA on that board Figure D 1 shows Vap3 which is in range 5 0 _ 4 5 4 0 S 3 5 9 3 0 amp 2 5 3 8 2 0 1 5 S 1 0 0 5 0 0 0 10 20 30 40 50 60 70 80 90 100 110 120 Time s FIGURE D 1 INA 2 s Output Voltage 2012 Microchip Technology Inc DS52031A page 67 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design Figure D 2 shows the RTD s temperature as it changes over a two minute period of time It starts out of range for this RTD sensor 55 is the specified minimum 30 20 10 e RTD Temperature 0 10 20 30 40 50 60 70 80 90 100 110 120 Time s FIGURE D 2 RTD Temperature Figure D 3 shows the difference between the steady state temperature 23 4 and the temperature at each time point Initially the RTD s the
64. terface 1 3 8 Digital The PC is a convenient interface for the user The GUI provides Communications with the demo board via the USB PIC device Data display strip chart Data storage graphics and text files Configuration inputs Temperature calibration an input sent to the USB PIC MCU Send an 1 calibration signal to the USB PIC MCU now Number of averages 1 3 9 Miscellaneous The Test Points are for evaluating analog performance and for connecting a lab power supply The 5 0V power supply is for all components on the PCB when not connected to the USB A remote RTD can be connected via wires for measuring elevated temperatures The INA outputs filtered and unfiltered are available An external PWM signal provides for alternate interfering signals The power supply provides a 5V rail from either lab equipment or from the USB DS52031A page 14 2012 Microchip Technology Inc Product Overview 1 4 WHAT DOES THIS KIT CONTAIN i i 00354 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design 102 0 USB cable Important Information Sheet IMPORTANT INFORMATION MicrocHip Read First Thank you for purchasing an Analog and Interface Evaluation Demonstration or Reference Design Board The latest Software documentation and support materials can be downloaded from http www microchip com analogtools The using the search box in the upper right
65. terval pull down menu shown at 100 ms per sample 2012 Microchip Technology Inc DS52031A page 43 MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design 5 9 4 Strip Chart Customization 5 9 4 1 DATA ACQUISITION CUSTOMIZATION GENERAL TAB Figure 5 5 shows the dialog box with the General tab selected To open this box dou ble click inside the Strip Chart area 5 The overall look and feel of the Strip Chart area can be changed here m General Plot Subsets Points Font Color Style Main Title Show Annotations Sub Title Border Style Numeric Precision No Border Line 50 Q1 52 9 Shadow 53D Inset Viewing Style Grid Lines Q Color Both Monochrome IV Grid in front of data C Monochrome Symbols Display FontSize Q Graph 7 Table C Large Q Medium Small FIGURE 5 5 Data Acquisition Customization General Tab 5 9 4 2 DATA ACQUISITION CUSTOMIZATION PLOT TAB Figure 5 6 shows the Data Acquisition dialog box with the Plot tab selected To open this box double click inside the Strip Chart area 5 The Strip Chart curve s axes and display types can be changed here General Subsets Points Font Color Style Plot Style Comparison Plot Style Line ji d Points Voltage V i Points BestFitCurve Axis 3 Points BestFitCurve Il Points BestFit
66. ubtraction of Vref from INA output voltages mo c VREF vddl 100nF 50V 10 R8 121K 196 R9 68 1K 196 MCP6001 SOT23 5 100nF 50V 1096 SGND FIGURE 3 2 Voltage Reference NENNEN 0552031 24 2012 Microchip Technology Inc Analog Circuit 3 3 2 First INA Figure 3 3 shows INA 1 which includes the Input Filter 1 mcal 1 and Output Filter 1 blocks Input Filter 1 is a Differential Mode DM and Common Mode CM low pass filter The CM pole is at 800 Hz so that INA 1 will reject mains interference at 50 Hz or 60 Hz its CMRR is very good at low frequencies The DM pole is at 38 Hz for low CM to DM conversion error at a reasonable price R11 and R12 are small enough to have good EMI performance C5 C7 are sized for the desired poles INA 1 U2 is setto a DM gain of 201 V V and has its output shifted up by 1 80V Vref producing an output that uses most of the ADC s input range INA 1 s CMRR at 25 kHz is excellent e g 92 dB INA 1 s PSRR rejects mains interference e g harmon ics of 50 Hz or 60 Hz R17 and R18 are 0 1 resistors to minimize gain error they can be lower precision if the user modifies the firmware to calibrate offset and gain errors To minimize thermo junction voltage effects at INA 1 s input R11 and R12 are close together and in parallel See AN1258 Op Amp Precision Design PCB Layout Techniques for more information on this topic Output Filter
67. uipment DS52031A page 18 2012 Microchip Technology Inc Installation and Operation 2 4 CONFIGURATIONS WITH MODIFICATIONS 2 4 1 External RTD Setup R2 can be external to the PCB if desired for sensing temperatures over a wider range De solder R2 from the PCB Connect wires from TP3 and TP4 to the RTD preferably the RTD that was de soldered from the board This modification has the advantage of allowing a wider temperature range you may need to change the INAs gains The main disadvantages come from the wiring resistance and connectors The wiring resistance adds directly to the RTD resistance The apparent RTD temperature increases The increase has a significant random component to it Accommodating different wire lengths and gauges in firmware is cumbersome and error prone Wires can change resistance with aging Connectors can be less than ideal due to Junctions between different metals creating thermocouple voltages Vibration wires become brittle and connections loosen over time Corrosion resistance increases plus a DC potential appears over time Other circuits take advantage of 3 wire and 4 wire RTDs to solve some of these problems The MCP6V26 data sheet s Typical Applications section shows one example 2 4 2 Modified Firmware The PIC microcontroller s firmware can be re programmed via the ICSP connector Modifications that a user might decide to code include Different averagin
68. use ICSP connector not used This is the most convenient setup for the user Most of the work is handled by the GUI 2 3 2 Other PWM Setups 2 3 21 RESISTIVE COUPLING Changing JP1 to position 2 Couple RC gives a basically resistive coupling into the bridge For convenience C2 blocks any change in DC bias point 2 3 2 2 COUPLING Changing JP1 to position 3 Couple Open produces no coupling no PWM signal at the bridge it serves as a baseline that other options are compared to 2 8 3 3 EXTERNAL PWM Changing JP2 to position 2 PWM EXT allows the user to input an arbitrary PWM interference signal onto the board using TP16 Note Changing JP2 to a vertical connection between pins 1 and 3 allows the PIC MCU s PWM signal to be sent off board via TP16 Vpwmx This is useful for connecting to other PCBs 2 3 3 Initiating VOS Calibration Events At power up the MCP6N11 internally self calibrates The GUI allows the user to send an mCal event signal to the PIC microcontroller which uses an open drain connection to toggle the MCP6N11 s U2 EN CAL pin The mCa1sw switch SW1 also toggles this pin 2 3 4 Other Power Supply Setup The USB can be left open don t use the PC and a lab power supply connected to TP1 and TP2 This makes analog measurements easier and cleaner but does not have the convenience of the GUI The effects of crosstalk INA CMRR and analog filters can be examined in detail using bench eq
69. wing are options selected with a jumper or in the GUI GUI option Enable or Disable the PIC MCU s PWM output JP1 options Couple C uses a capacitor to put the PWM waveform on the bridge Couple RC uses resistive coupling with a DC blocking capacitor to put the PWM waveform on the bridge Couple Open disconnects the PWM signal from the bridge JP2 options PWM PIC connects the PIC MCU s PWM signal onto the bridge PWM EXT connects an external PWM signal onto the bridge 1 3 4 VREF VREF not a fixed voltage reference provides a buffered ratiometric level proportional to Vpp that keeps the amplifiers in their normal operating range When at the design point of Vpp 7 5 0V its output is 1 8V It has a low pass filter at the output whose pole is the same as the DM pole for the INAs input and output filters This minimizes mismatches in the outputs due to unequal settling after a Vpp disturbance 1 3 5 INA 1 The input filter provides a low pass function for both CM and DM signals They are fast enough to follow supply variations and to let the INA reject CM mains noise e g harmonics of 50 or 60 Hz This INA is the MCP6N11 100 It is set at a high DM gain 200 V V and has good CMRR at 25 kHz 92 dB at the PWM frequency Its Vos is calibrated using the internal mCa1 function which has the following selectable control options The DUT is calibrated at power up The GUI sends a signal to the MCP6N11 via the
70. yer 56 A 7 Board Bottom Metal Layer top view 57 A 8 Board Bottom Silk and Pads top view 58 Bill Of Materials BOM Appendix C Conversion Polynomials C 1 Circuit Response itecto d D ERROR P CIR RAS XE GE EET GRE BART ERE EYE X 63 C 2 RTD Temperature and Resistance 4 65 INA Voltage to Temperature pp 66 Appendix D Board Validation Summary 0 1 Thermal Steady State RESPONSE pp 67 0 2 Thermal Impulse Response sss 67 Worldwide Sales and Service pp 70 DS52031A page 4 2012 Microchip Technology Inc MCP6N11 AND MCP6V2X WHEATSTONE BRIDGE REFERENCE DESIGN USER S GUIDE MICROCHIP Preface NOTICE TO CUSTOMERS All documentation becomes dated and this manual is no exception Microchip tools and documentation are constantly evolving to meet customer needs so some actual dialogs and or tool descriptions may differ from those in this document Please refer to our web site www microchip com to obtain the latest documentation available Documents are identified with a DS number This number is located on the bottom of each page in front of the page number The numbering convention for the DS number is DSXXXXXA where XXXXX is the document number and A is the revisio

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