MT-091: Digital Potentiometers
Contents
1. 7 5 5V T DIGITAL CONTROL VouT ADP3336 500mA max in LDO ViN SD LDMOS Vpp AD5227 cs AD5173 SZ DIODE DigiPOT Figure 8 Two Circuit Applications for Digital Pots Similarly electronic equipment makers use digital potentiometers in power supplies by adjusting the supplies to the tolerances that cover all supply voltage conditions during reliability testing This voltage margining approach accelerates the burn in process and therefore reduces the system time to market Page 8 of 10 MT 091 Because of the optimized cost performance benefits digital pots have been gaining popularity in replacing traditional DACs in many applications For example in wireless basestations the optimum threshold voltages of the RF power amplifiers vary widely in production Such variation affects the transmitted signal linearity and power efficiency Too much power delivered from a poorly regulated amplifier can also interfere with neighboring cells within the wireless network Although DACs are widely used in biasing RF power amplifiers many users find digital pots to be more suitable in such applications because of the availability of non volatile memory which simplifies the designs As shown in Figure 8B the one time programmable digital pot is used to calibrate the dc bias point of the RF power amplifier and the calibration is programmed by factory software without the need for any external controlle2. allowed before programming the resistance value into the 20 Time Programmable memory These parts do not require any external voltage supply to facilitate fuse blow and there are 20 opportunities for permanent programming During 20 TP activation a permanent blow fuse command freezes the wiper position analogous to placing epoxy on a mechanical trimmer DIGITAL POTENTIOMETER AC CONSIDERATIONS Digital potentiometers can be used in ac applications provided the bandwidth limitations created by the internal capacitance are considered Figure 7 shows an ac model of a digital potentiometer where the capacitances are modeled as Ca Cg and Cw Rap A B T O Ce L SOURCE FOR AD5245 5kQ 1 0MHz 10kQ 500kHz Cy p 90pF 50kQ 100kHz Cy 95pF 100kQ 50kHz BW MEASURED FROM A TO W WITH B GROUNDED MIDSCALE CODE DRIVEN FROM A LOW IMPEDANCE SOURCE Figure 7 Digital Potentiometer Bandwidth Model The bandwidth of the digital pot is configuration dependent It is also dynamic because of the variable resistance For example if A terminal is the input B terminal is grounded and W terminal is the output then the bandwidth can be approximated by BW 1 27 Rwal Rwa Cwl The lowest bandwidth occurs at midscale where the equivalent resistance is at its maximum in this configuration The typical values for the AD5245 are shown as well as the corresponding bandwidths for the various resistance options measured at midscale This si
3. is believed to be accurate and reliable however no responsibility is assumed by Analog Devices regarding technical accuracy and topicality of the content provided in Analog Devices Tutorials Page 10 of 10
4. three terminals of the potentiometer are available for use The operating supply voltage can range from 2 7 V to 5 5 V The power supply current is 8 uA maximum and an internal command bit is available to shut down the device into a state of zero power consumption The voltage noise is approximately the thermal noise of Rag Recall that the thermal noise of a 1 kO resistor at room temperature is approximately 4 nV VHz The AD5247 shown in Figure 2B is similar to the AD5245 except it has 128 positions 7 bits the B terminal is grounded and the part comes in an SC70 6 lead package The AD5247 does not have the AO function Finally the AD5246 shown in Figure 2C is similar to the AD5245 but is connected as a rheostat with the W and B terminals available externally In addition to single potentiometers such as the AD5245 AD5246 and AD5247 digital potentiometers are available as duals triples quads and hex versions Multiple devices per package offer 1 matching in ganged potentiometer applications as well as reducing PC board real estate requirements Figure 3 summarizes some of the characteristics and features of modern digital potentiometers Resolution wiper steps 32 5 Bits to 1024 10 Bits Nominal End to End Resistance 1kQ to 1MQ End to End Resistance Temperature Coefficient 35ppm C Thin Film Resistor String 500ppm C Polysilicon Resistor String Number of Channels 1 2 3 4 6 Interface Data Cont
5. ANALOG MT 091 DEVICES TUTORIAL Digital Potentiometers INTRODUCTION Mechanical potentiometers have been used since the earliest days of electronics and provide a convenient method for the adjustment of the output of various sensors power supplies or virtually any device that requires some type of calibration Timing frequency contrast brightness gain and offset adjustments are just a few of the possibilities However mechanical pots have always suffered from numerous problems including physical size mechanical wearout wiper contamination resistance drift sensitivity to vibration temperature humidity the need for screwdriver access layout inflexibility etc Digital potentiometers avoid all the inherent problems associated with mechanical potentiometers and are ideal replacements in new designs where there is either a microcontroller or another digital device to provide the necessary control signals Manually controlled digital potentiometers are also available for those who do not have any on board microcontrollers Unlike mechanical pots digital pots can be controlled dynamically in active control applications The digital potentiometer is based on the CMOS String DAC architecture previously described in Tutorial MT 014 and the basic diagram is shown in Figure 1 Note that in the normal string DAC configuration the A and B terminals are connected between the reference voltage and the W wiper terminal is the DAC output Ther
6. D is the decimal equivalent of the step value N is the number of bits Rag is the nominal resistance and Rw is the wiper resistance The switches are CMOS transmission gates that minimize the on resistance variations between any given step and the output The voltages on the A and B terminals can be any value as long as they lie between the power supply voltages Vpp and Vss MODERN DIGITAL POTENTIOMETERS IN TINY PACKAGES Figure 2 shows three examples of digital potentiometers that are all offered in small packages The I C serial interface is a very popular one but digital potentiometers are also available with the SPI Up Down Counter and Manual Increment Decrement interfaces A POTENTIOMETER DIVIDER B POTENTIOMETER DIVIDER ALL TERMINALS AVAILABLE ONE TERMINAL GROUNDED Voo 2 7V 5 5V Voo 2 7V 5 5V O O AD5245 AD5247 SCL SDA AO C INTERFACE SOT 23 8 PACKAGE ALL NOMINAL RESISTANCE 5kQ 10kQ 50kQ or 100kQ C RHEOSTAT SPAO my 35ppm C SC70 6 B PACKAGE GND Figure 2 Typical Examples of Digital Potentiometers in Tiny Packages Page 2 of 10 MT 091 The AD5245 shown in Figure 2A is available in an 8 lead SOT 23 package and has 256 positions 8 bits The AO pin allows the device to be uniquely identified so that two devices can be placed on the same bus The thin film resistor string Rag is available in 5 KQ 10 kQ 50 KQ or 100 kQ and the Rag temperature coefficient is 35 ppm C All
7. e converted for factory programming applications The AD5172 AD5173 are available in a MSOP 10 package All parts are guaranteed to operate over the automotive temperature range of 40 C to 125 C Besides their unique OTP features the AD5172 AD5173 lend themselves well to other general purpose digital potentiometer applications due to their programmable preset superior temperature stability and small form factor The AD5170 Reference 6 is a two time programmable 8 bit digital potentiometer and a functional diagram is shown in Figure 6 Note that a second fuse array is provided to allow second chance programmability Like the AD5172 AD5173 there is unlimited programmability before making the permanent setting The electrical characteristics of the ADS5170 are similar to the AD5172 AD5173 RDAC REGISTER Gees Figure 6 AD5170 256 Position Two Time Programmable PC Digital Potentiometer Page 6 of 10 MT 091 DIGITAL POTENTIOMETERS WITH 1 ACCURACY The AD5291 AD5292 are single channel 256 1024 position digital potentiometers with less than 1 end to end Resistor Tolerance error nominal temperature coefficient of 35 ppm C and 20 Time Programmable Memory These devices are capable of operating at high voltages supporting both dual supply 10 5 V to 15 V and single supply operation 21 V to 30 V The AD5291 AD5292 device wiper settings are controllable through the SPI digital interface Unlimited adjustments are
8. e is also one more R resistor in the string DAC configuration which connects the A terminal to the reference VDD TERMINAL A 3 T0 8 3 BIT O DECODER lt O DIGITAL R gt INPUT R R POLYSILICON OR THIN FILM O WIPER W RESISTOR R STRING R CMOS FOR N BITS SWITCHES 2 SWITCHES R AND RESISTORS R QO TERMINAL B Figure 1 3 Bit CMOS Digital Potentiometer Based on String DAC Architecture Rev 0 01 09 WK Page 1 of 10 MT 091 The digital potentiometer configuration essentially makes use of the fact that the CMOS switches common mode voltages can be anywhere between the power supplies the switch selected by the digital input simply connects the wiper to the corresponding tap on the resistor string The relative polarity of A to B can be either positive or negative The resistor string represents the end to end potentiometer resistance and the traditional DAC output becomes the wiper of the digital potentiometer The resistors can be either polysilicon TC 500 ppm C or thin film TC 35 ppm C depending upon the desired accuracy The number of resistors in the string determines the resolution or step size of the potentiometer and ranges from 32 5 bits to 1024 10 bits at present The value of the programmable resistors are simply Rws D D 2 Rag Rw and Rwa D 2 D 2 Rag Rw where Rwe is the resistance between W and B terminals Rwa is the resistance between W and A terminals
9. ht adjustment electronic volume controls e RF Communications RF power amplifier biasing DDS PLL amplitude adjustment VCXO frequency tuning varactor diode biasing log amp slope and intercept adjustment quadrature demodulator gain and phase adjustment RFID reader calibration e Automotive set points in the engine control unit sensor calibrations actuator controls instrumentation control navigation entertainment display adjustments e Industrial and Instrumentation system calibration floating reference DACs programmable 4 to 20 mA current transmitters Page 9 of 10 MT 091 e Optical Communications laser bias current adjustments laser modulation current adjustments optical receiver signal conditioning optical attenuators wavelength controllers REFERENCES 1 Walt Heinzer Design Circuits with Digitally Controllable Variable Resistors Analog Dialogue Vol 29 No 1 1995 http www analog com 2 Hank Zumbahlen Tack a Log Taper onto a Digital Potentiometer EDN January 20 2000 3 Mary McCarthy Digital Potentiometers Vary Amplitude In DDS Devices Electronic Design Ideas for Design May 29 2000 4 Alan Li Versatile Programmable Amplifiers Use Digital Potentiometers with Nonvolatile Memory Analog Dialogue Vol 35 No 3 June July 2001 5 Reza Moghimi Difference Amplifier Uses Digital Potentiometers EDN May 30 2002 6 Mark Malaeb Single Chip Digitally Controlled Data Acquisitio
10. mic changes in wiper setting a left right bit shift command adjusts the level in 6 dB steps The AD5235 is available in a thin TSSOP 16 package All parts are guaranteed to operate over the extended industrial temperature range of 40 C to 85 C ONE TIME PROGRAMMABLE OTP DIGITAL POTENTIOMETERS The AD5172 AD5173 are dual channel 256 position one time programmable OTP digital potentiometers which employ fuse link technology to achieve the memory retention of resistance setting function Reference 5 A functional block diagram is shown in Figure 5 Note that the AD5172 is configured as a three terminal potentiometer while the AD5173 is pinned out as a rheostat The AD5172 AD5173 is available in 2 5 kQ 10 kQ 50 kQ and 100 kQ versions The temperature coefficient of the resistor string is 35 ppm C The power supply voltage can range from 2 7 V to 5 5 V A1 W1 B1 A2 W2 B2 Wi B1 W2 B2 V FUSE V na LINKS gt DD RDAC RDAC RDAC RDAC ene ee cenlineaeessll ene Presi Beco ADO AD5172 AD1 SDAQ SERIAL INPUT SDA sc Reester Scu Figure 5 AD5172 AD5173 256 Position One Time Programmable Dual Channel FC Digital Potentiometer OTP is a cost effective alternative over the E MEM approach for users who do not need to program the digital potentiometer setting in memory more than once i e set and forget These devices perform the same electronic adjustment functions most mechanical trimmers and variable resistors do bu
11. mple model can be used in SPICE simulations to predict circuit performance such as when the digital potentiometer Page 7 of 10 MT 091 is used as a part of the feedback network of an op amp The other issue to consider when placing digital potentiometers directly in the signal path is their slightly nonlinear resistance as a function of applied voltage This effect leads to a small amount of distortion For example the AD5245 has a THD of 0 05 when a 1 V rms 1 kHz signal is applied to the configuration described above at midscale Examples of the application of digital potentiometers in ac applications can be found at the end of this tutorial APPLICATION EXAMPLES Like op amps digital pots are the building blocks of many electronic circuits Because they are digitally controlled digital pots can be used in active control applications in addition to basic trimming or calibration applications For example digital pots can be used in programmable power supplies as shown in Figure 8A Typical adjustable low dropout voltage regulators such as the anyCAP series have a FB pin where applying a resistor divider yields a variable output voltage As shown R1 and R2 are the feedback and input resistors respectively The FB circuit has an internal non inverting amplifier which gains up a 1 2 V bandgap reference to the desired output voltage A PROGRAMMABLE POWER SUPPLY B RF POWER AMP DC BIASING vq Your Vre 1 R1 R2 2 6V 12V VDD 2
12. n as Core of Reliable DWDM Communication Systems Analog Dialogue Vol 36 No 5 September October 2002 7 Peter Khairolomour Rotary Encoder Mates with Digital Potentiometer EDN Design Idea March 6 2003 8 Alan Li Versatile Programmable Amplifiers Using Digital Potentiometers with Nonvolatile Memory Application Note AN 579 Analog Devices 9 Alan Li Programmable Oscillator Uses Digital Potentiometers Application Note AN 580 Analog Devices 10 Alan Li Resolution Enhancements of Digital Potentiometers with Multiple Devices Application Note AN 582 Analog Devices 11 Alan Li AD5232 Programmable Oscillator Using Digital Potentiometers Application Note AN 585 Analog Devices 12 Alan Li ADN2850 Evaluation Kit User Manual Application Note AN 628 Analog Devices 13 Walt Kester Analog Digital Conversion Analog Devices 2004 ISBN 0 916550 27 3 Chapter 8 Also available as The Data Conversion Handbook Elsevier Newnes 2005 ISBN 0 7506 7841 0 Chapter 8 Copyright 2009 Analog Devices Inc All rights reserved Analog Devices assumes no responsibility for customer product design or the use or application of customers products or for any infringements of patents or rights of others which may result from Analog Devices assistance All trademarks and logos are property of their respective holders Information furnished by Analog Devices applications and development tools engineers
13. rol SPI 12C Up Down Counter Input Increment Decrement Input Terminal Voltage Range 15V 15V 30V 3V 3V 5V 5V Memory Options Volatile No Memory e Nonvolatile E7MEM One Time Programmable OTP One Fuse Array Two Time Programmable Two Fuse Arrays Figure 3 Characteristics of CMOS Digital Potentiometers DIGITAL POTENTIOMETERS WITH NONVOLATILE MEMORY Digital potentiometers such as the AD5245 AD5246 and AD5247 are used mainly in active control applications since they do not have non volatile memory Therefore the setting is lost if power is removed However most volatile digital potentiometers have a power on preset feature that forces the devices to the midscale code when power is applied Page 3 of 10 MT 091 Obviously there is a demand for digital potentiometers with the ability to retain their setting after power is removed and reapplied This requires the use of nonvolatile on chip memory to store the desired setting The AD5235 is an example of a dual 10 bit digital potentiometer which contains on chip E MEM to store the desired settings Reference 4 A functional block diagram is shown in Figure 4 gt f RDAC1 ry REGISTER INTERFACE 5 EEMEM1 TA gt RDAC2 REGISTER f Figure 4 AD5235 Nonvolatile Memory Dual 1024 Position Digital Potentiometers These devices perform the same electronic adjustment function as a mechanical potentiometer with enhanced resolu
14. rs Note that the diode is added to the circuit to compensate for the amplifier s temperature coefficient SUMMARY Digital potentiometers offer many obvious advantages over mechanical potentiometers and trimpots and therefore they have become widely accepted in modern systems Their reliability flexibility and ease of use makes them popular replacements for the traditional potentiometer Digital pots can also be used as programmable building blocks in many active control applications There are virtually endless applications for digital potentiometers in modern electronic systems one only has to consider the many traditional applications for mechanical pots and trimpots as a starting point References 1 13 should be consulted for more ideas on how these devices can enhance a design A few applications are summarized below e General Purpose Applications sensor calibration system gain and offset adjustments programmable gain amplifiers programmable filters programmable set points traditional digital to analog converters voltage to current converters line impedance matching e Computer and Network Equipment programmable power supplies power supply margining battery charger set points temperature control set points e LCD Displays backlight contrast and brightness adjustments LCD panel common voltage adjustment programmable gamma correction LCD projector reference voltage generator e Consumer Applications PDA backlig
15. t offer enhanced resolution solid state reliability and better temperature coefficient performance The AD5172 AD5173 are programmed using a 2 wire IC compatible digital control They allow unlimited adjustments before permanently setting the resistance value During the OTP activation a permanent fuse blown command is sent after the final value is determined therefore freezing the wiper position at a given setting analogous to placing epoxy on a mechanical trimmer Unlike other OTP digital potentiometers in the same family AD5172 AD5173 have a unique temporary OTP overwriting feature that allows new adjustments if desired the OTP Page 5 of 10 MT 091 setting is restored during subsequent power up conditions This feature allows users to apply the ADS5172 AD5173 in active control applications with user defined presets To verify the success of permanent programming Analog Devices patterned the OTP validation such that the fuse status can be discerned from two validation bits in read mode For applications that program AD5172 AD5173 in the factories Analog Devices offers device programming software which operates across Windows 95 to XP platforms including Windows NT This software application effectively replaces the need for external IC controllers or host processors and therefore significantly reduces users development time An AD5172 AD5173 evaluation kit is available which include the software connector and cable that can b
16. tion solid state reliability and superior low temperature coefficient performance The AD5235 s versatile programming via a standard serial interface allows 16 modes of operation and adjustment including scratch pad programming memory storing and retrieving increment decrement log taper adjustment wiper setting readback and extra user defined E MEM Another key feature of the AD5235 is that the actual resistance tolerance is stored in the E7MEM at 0 1 accuracy The actual end to end resistance can therefore be known which is valuable for calibration and tolerance matching in precision applications The new E MEM family of digital pots AD5251 AD5252 AD5253 AD5254 also offer such a feature In the scratch pad programming mode a specific setting can be programmed directly to the RDAC register which sets the resistance between terminals W A and W B The RDAC register can also be loaded with a value previously stored in the E7MEM register The value in the E MEM can be changed or protected When changes are made to the RDAC register the value of the new setting can be saved into the E MEM Thereafter it will be transferred automatically to the RDAC register during system power on E MEM can also be retrieved through direct programming and external preset pin control The linear step increment and decrement commands cause the setting in the RDAC Page 4 of 10 MT 091 register to be moved UP or DOWN one step at a time For logarith
Download Pdf Manuals
Related Search