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Contents
1. o HERE AAA AAA AAA AAA EA u Q i w Q s w a SECTION 1 GENERAL INFORMATION 1 1 INTRODUCTION The Model 261 Picoampere Source is a secondary standard for use in calibrating picoammeters and electrometers The output of the Model 261 can be adjusted between 104A and 1 1x 104A in eight decade ranges The source has 3 digit resolution for current outputs between 10 12 and 104A The current output of the Model 261 is derived from a variable voltage source and a selectable range resistance The voltage supply is highly regulated for stability and uses high quality components throughout for reliability The low side of the output can be floated to avoid possible ground loop pro blems 1 2 MODEL 261 FEATURES important Model 261 features include 1 11V Internal Source The internal voltage source can be set to a value between 0 and 11V in 0 01V increments This permits accurate calibration of picoammeters and elec trometers with input voltage drops up to 10mV without calibration correction 2 Stable High Meg Resistance Standards The high megohm resistance standards are selected for maximum stability Individual temperature coefficients and resistance values are supplied with each instrument 3 Line Operation The Model 261 is line operated from either 105 125V or 210 250V power sources Voltage selection is easily accomplished with the rear panel line voltage selec tion switch 4 Eigh2. REFERENCE DESIGNATIONS NOT USED eaf T Ee ess RES E E we uiv Bab Vo SO 80 Z3ev Ac u 4 540810 A Figure 7 8 Model 261 Schematic Diagram Dwg No 18323E 7 7 7 8 KEITHLEY Service Form Model No Serial No Date Name and Telephone No Company List all control settings describe problem and check boxes that apply to problem LJ Intermittent LJ Analog output follows display CJ Particular range or function bad specify Q IEEE failure L Obvious problem on power up CJ Batteries and fuses are OK LJ Front panel operational Q All ranges or functions are bad l Checked all cables Display or output check one LI Drifts J Unable to zero CI Unstable LJ Will not read applied input LI Overload Q Calibration only C Certificate of calibration required LJ Data required attach any additional sheets as necessary Show a block diagram of your measurement system including all instruments connected whether power is turned on or not Also describe signal source Where is the measurement being performed factory controlled laboratory out of doors etc What power line voltage is used Ambient temperature F Relative humidity Other Any additional information If special modifications have been made by the user please describe Be sure to include your name and phone number on this service form
3. Range AMPERES Switch The range switch selects one of eight decade current ranges between 105 and 10 12A The actual output is affected by the positions of the three multi plier switches When the multiplier switches are in the 1 00 positions the range switch indicates the actual output cur rent Multiplier Switches The multiplier switches control the in ternal voltage source of the instrument These switches pro vide multiplying values that determine the actual output of the source To determine the current output multiply the current range by the multiplier setting The multiplier switch es control the output through the use of a 3 decade Kelvin Varley divider The X1 and X 01 switches each have 11 posi tions between 0 and 10 the X 1 switch has 10 positions be tween 0 and 9 The illuminated clecimal point between X1 and X 1 switches also serves as a pilot lamp OUTPUT Receptacle The OUTPUT receptacle is a Teflon insulated UHF type connector The low threaded side floats from ground unless the COM and GND terminals on the rear panel are connected together A chassis ground terminal G is located next to the OUTPUT connector 2 3 REAR PANEL CONTROLS AND TERMINALS Figure 2 2 shows the rear panel of the source The purpose of the various controls and terminals is described below Voltage Switch The voltage switch sets the source for 105 125V or 210 250V operation A screwdriver can be used to set the switch to the des
4. 1 9 OPTIONAL ACCESSORIES by 19 inches deep Two different covers are provided for use with either 10 inch deep or 13 inch deep instruments Contact The Model 4003A Rack Mounting Kit allows the Model 261 your nearest Keithley representative or the factory for infor to be conveniently mounted in a standard 19 inch rack The mation on ordering the Model 4003A overall dimensions of the Model 4003A kit is 5 inches high SECTION 2 OPERATION 2 1 INTRODUCTION This section contains information necessary to set up the Model 261 and operate the unit Also included are compen sating equations necessary when using the source with measuring instruments with high input voltage drops 2 2 FRONT PANEL CONTROLS AND TERMINALS Figure 2 1 shows the front panel of the Model 261 The func tions of the various controls and terminals are described below POLARITY Switch The POLARITY switch has four posi tions which are used to control power turn the instrument output off and on and set the polarity of the source output in the AC OFF position the Model 261 power is discon nected in the OFF position power is applied to the instru ment but the source is disconnected from the output jack When the POLARITY switch is in this position the output resistance of the source is equal to the reciprocal of the AMPERES switch setting regardless of the multiplier settings in use The and positions select positive or negative cur rent output values
5. D205 Q203 C gt D206 O i Lu R207 Figure 7 1 Voltage Supply Board Component Location Drawing 2 8p0 ON Dan BuimeiQ 1018207 3ueuoduio pieog uoneiqe z e15D14 s a 2 a SC ECH FT ZZ Oar bo BF F ae eee pepa rd Cee EE Leg Cl Ce E E e a a 2 a 304830 7 2 Table 7 1 Model 261 Parts List Circuit Keithley Desig Description Location Part No Capacitor 1uF 50V Mylar Chassis C 41 0 1 Capacitor 5004F 25V Aluminum Electrolytic Voltage Supply C 94 500 Capacitor 5004F 25V Aluminum Electrolytic Voltage Supply C 94 500 Capacitor 224F 500V Ceramic Disc Voltage Supply C 22 01 Capacitor 5004F 25V Aluminum Electrolytic Voltage Supply C 94 500 Rectifier Silicon 1N645 Voltage Supply RF 14 Rectifier Silicon 1N645 Voltage Supply RF 14 Rectifier Silicon 1N645 Voltage Supply RF 14 Diode Zener 1N706 Voltage Supply DZ 1 Rectifier Silicon 1N645 Voltage Supply RF 14 Rectifier Silicon 1N645 Voltage Supply RF 14 Diode Zener 1N823 Voltage Supply DZ 36 Rectifier Silicon 1N645 Voltage Supply RF 14 LED Hed Front Panel PL 67 Fuse 1 8A 250V 105 125V Operation Rear Panel FU 20 Fuse 1 16A 250V 210 250V Operation Rear Panel FU 21 Binding Post G Front Panel BP 15 Binding Post GND Rear Panel BP 11G Binding Post COM Rear Panel BP 11B Receptacle UHF Front Panel CS 64 Test Jack Voltage Supply TJ 4 Test Jack Voltage Supply TJ 5 Cord Set 6 foot Rear Panel CO
6. ILR The output voltage may be used to drive either a recorder or the meter M The input voltage drop V is Vi V k Equation 2 Typical values for the preceding parameters for a picoam meter are Input current to picoammeter 10 124 Amplifier gain k 104 Picoammeter feedback resistor R 10120 Amplifier output voltage V 1V Input voltage drop V 1004V Figure 2 5 Simplified Circuit of Feed Back Type Picoammeter 2 8 2 Shunt Type Picoammeter This type consists of a voltmeter V calibrated in amperes which responds to the IR drop across a shunt resistor R Refer to Figure 2 6 The voltmeter can be one of two kinds A high impedance voltmeter such as an electrometer which has an extremely high input impedance Z or a low im pedance voltmeter High Impedance Voltmeter The input voltage drop IR of a picoammeter using a high impedance voltmeter is usually 1mV or more due to voltmeter sensitivity limitations Rise time is usually slow possibly several minutes for very low cur rent levels Typical values are Input current to picoammeter 1012A Picoammeter shunt resistor R 10100 Voltmeter input impedance Z 10 40 Input voltage drop V 10mV Low Impedance Voltmeter The circuit of a shunt type pico ammeter using a low impedance voltmeter is essentially the same as Figure 2 6 except a more sensitive but lower input impedance voltmeter is used A lower input voltage drop results
7. but response speed and maximum current sensitivity are usually less The voltage drop is R Typical values are Input current to picoammeter 10 10A Picoammeter shunt resistance R 1060 Voltmeter input impedance Z 9 x 1070 Input voltage drop V 1004V Figure 2 6 Typical Shunt Type Picoammeter 2 9 COMPENSATION FOR INPUT VOLTAGE DROP OF FEEDBACK TYPE PICOAMMETERS Errors caused by picoammeter input voltage drops of 1mV or less are negligible for Model 261 outputs between 1 1 x 104 and 10 A Following the instructions in paragraph 2 6 will bring the output current accuracy to between 0 25 and 10 796 2 9 1 Calculating Voltage Drop Error When either the picoammeter voltage drop or input resis tance is known use either Equation 3 or 4 to determine the error When the voltage drop is known E V x 100 Equation 3 V s where E is the percent of error due to the picoammeter in put voltage drop V is the picoammeter input voltage drop in volts V is the Model 261 source voltage read directly in volts from the multiplier switch settings When the input resistance is known E R x 100 R s Equation 4 where R is the picoammeter input resistance in ohms R is the Model 261 source resistance which is the reciprocal of the range switch setting 2 9 2 Compensating Equation If the voltage drop is known the actual Model 261 output current can be calculated Equation 5 gives the outp
8. equipment may be used as long as the accuracy specifica tions are at least four times better than Model 261 specifica tions f less accurate equipment is used additional allowances must be made for test equipment inaccuracy 4 4 INITIAL CONDITIONS Before performing the following procedures make sure the Model 261 meets the following conditions 1 If the unit has been subjected to temperatures below 18 C 65 F or above 28 C 82 F allow sufficient time for it to reach a temperature within this range Generally it takes one hour to stabilize an instrument that is 10 C 18 F out side the allotted range 2 Turn on the power to the Model 261 and allow it to warm up for at least one hour before beginning verification or cal ibration procedures 4 5 VERIFICATION PROCEDURE Model 261 operation is based on connecting an accurate known resistor in series with a stable accurate voltage source The resulting current is easily predicted with Ohm s law This same principle can be used to verify Model 261 per formance if both the resistance and voltage are within speci fications it can be assumed that the output current is accu rate as well 4 5 1 Top Cover Removal The procedure in this section require that the top cover be removed to allow access to test points and the CAL OPERATE switch To remove the top cover proceed as follows WARNING Disconnect the Model 261 from the power line and other instrumentation be
9. 12 8 Resistor 12 80 1 WW Wirewound 103 R 67 12 8 Resistor 12 80 1 AW Wirewound 103 R 67 12 8 Resistor 12 80 1 WW Wirewound 103 R 67 12 8 Resistor 12 80 1 AW Wirewound 103 R 67 12 8 Resistor 12 82 196 W Wirewound 103 R 67 12 8 Resistor 12 80 1 4W Wirewound 104 R 67 12 8 Resistor 12 80 1 W Wirewound 104 R 67 12 8 Resistor 12 80 196 W Wirewound 104 R 67 12 8 Resistor 12 82 1 W Wirewound 104 R 67 12 8 Resistor 12 80 1 4W Wirewound 104 R 67 12 8 Resistor 12 89 1 WW Wirewound 104 R 67 12 8 R108 is not field installable Table 7 1 Model 261 Parts List Cont Circuit Keithley Desig Description Location Part No Resistor 12 802 1 1 4W Wirewound 104 R 67 12 8 Resistor 12 80 1 1 4W Wirewound 104 R 67 12 8 Resistor 12 8Q 1 1 4W Wirewound 104 R 67 12 8 Resistor 12 80 1 1 4W Wirewound 104 R 67 12 8 Resistor 64Q 1 1 4W Wirewound 105 Resistor 640 1 1 4W Wirewound 105 Resistor 64Q 1 1 4W Wirewound 105 Resistor 64Q 1 1 4W Wirewound 105 Resistor 6422 1 1 4W Wirewound 105 Resistor 64Q 1 1 4W Wirewound 105 Resistor 642 1 1 4W Wirewound 105 Resistor 642 1 1 4W Wirewound 8105 Resistor 6422 1 1 4W Wirewound 105 Resistor 64Q 1 1 4W Wirewound 105 Resistor 64Q 1 1 4W Wirewound 105 Resistor 64Q 1 1 4W Wirewound 105 Resistor 32Q 1 1 2W Wirewound 10
10. CIENT When the Model 261 is used for range switch settings above 1038 or when the unit is operated at the temperature indicated on the calibration certificate no significant errors will occur in the output However for range switch settings between 103A and 102A the change in range resistance because of temperature coefficient can affect the output considerably The Model 261 is calibrated at a specified temperature 0 5 C which is stated on the calibration certificate for the various ranges The certificate also lists the temperature coef ficients by range Table 2 2 Causes of Discrepancy in Model 261 Output Current and Correction Index Cause of Discrepancy in Output Current Picoammeter input voltage drop more than 1 1000 of source voltage Feedback Shunt Type Shunt Type Picoammeter Type High Impedance Low Impedance Paragraph Equation to Compensate Reference for Discrepancy Affected all Model 261 ambient temperature differs from all that stated in Calibration Certificate 108 to 10 14 Compensating Equation The effect of temperature dif ferences on the Model 261 current can be calculated using equation 9 This gives the output current value if the temperature effect is the only significant source of error ME 1 jeter Tool Equation 9 T TIte 100 where is the actual or compensated Model 261 current out put L is the indicated
11. Connect the HI lead to the grey white wire on the X 01 switch and connect the LO lead to the black blue wire connection on the X1 switch See Figure 4 1 Connect the Model 261 to the power line Set the unit to the 105 range set the polarity switch to and set the muitiplier switches to 10 00 The CAL OPERATE switch should be in the OPERATE position Adjust R210 Figure 4 1 for a reading of 10 000 0 005V on the meter Switch each multiplier switch through the settings listed in Table 4 4 and verify that the required reading is observed on the meter Return the multiplier switches to the 10 00 setting and ad just R210 for a reading of 10 012 0 005V on the meter NOTE This last step must be performed to return the unit to the stated specifications 6 CALIBRATION alibrate the ranges after performing the previous procedures in this section paragraph 4 5 Range calibration is based on al other adjustments and verifications being complete and accurate Perform this calibration any time the voltage supply is adjusted or if any Kelvin Varley resistor or range resistor is replaced The basis of this calibration is V R The range potential is adjusted to correspond to the exact range resistor value so that the current produced is equal to one times the range Multiplier Switch 1 Set the Model 261 CAL OPERATE switch to OPERATE Settings Connect the Model 261 to the proper line source Connect the
12. E ee dn Compensating Equations Zo ce e ER REM be EEN dE epe edente deve ed Compensation for Input Voltage Drop of Shunt Type Low Impedance Picoammeters Range Resistor Voltage Coefficient Compensation for Temperature Coefficient 0 0 heme SECTION 3 APPLICATIONS Introduction ocio o ERR CERES dade Re XR eed Ege ee dure UR pr a ane ne les ab Current Suppressigh u u uyu sede y CHER AY RP Ideen es an aded Venir Cal EET LS X A Gaivanometric Measurements Ohmm ter Calilbration 2225 25 eisdemque px d a RAN NATO RP CER na cua EO ees SECTION 4 PERFORMANCE VERIFICATION AND CALIBRATION introduction L u EN sure sew cred reg EEN mee a GET A ERG EORR UE de Prid ed edo rune MES Environmental Conditions csse eee m mde re a eee Recommended Test Equipment s ssu rauana srra ehe hee rae Initial Conditions dee sage o n Ra uha est eode Lima Verification Procadure 4 4 44444 o Top Cover Removal 444444 4444 dieser r9 e E ye ry ere rh rere Range Resistor Verification Multiplier Switch Setting Verification ciis Hh Calibration aida ar a Ed a Ad AE A A lin A asa sa SECTION 5 THEORY OF OPERATION Introduction vn a Eae Geb A A E eai ees pi Block Diagrami vus ee mex rx so Ree EE Eege a Se dd e Sos giaa a Circuit Descrip EE Voltage Supply 428 ia x gx A A ES ER s Range Resistors s a diis ori ta anb queer ee
13. Had leans ER AG ERG EG EUR ES nues dre dE Ue dra dS RN dd je opa Safety Terms eos usi she dad hosed pe a ree en Ra deed E ae e hr ck Ru SEENEN ER Unpacking and Inspection Using the Model 261 Instruction Manual 2 0 00 cee hee Specifications EE Optional Accessories 4 4 40 a ei aere sh dh e rhe rna SECTION 2 OPERATION Introduction oec CO o dd ER tne ER p ip E ti aid pa ap venues gua rines Front Panel Controls and Terminals Rear Panel Controls and Terminals Preparation For Use csse mere CERA ERR CR UE ENS Wed EELER DR eb eod eer ea die p hn Basic Operating Procedure io we kd seek kara rd o ar der he eee NEEN dA ca te Accuracy Considerations cocer SERRURIER ens EBORE I RARO Or EAT Ree ERR EN eg input Voltage Drop Voltage Burden Proper Multiplier Settings sees I eel Output CONNECTIONS ee iad eed a er eg bus w naiss LSU EN be ROLE LAE Parece s aaa Input Voltage Drop for Feedback and Shunt Type Picoammeters Feedback Type Picoammeter e hee Shunt Type Picoammeter aee ah ehe ee eee Compensation for Input Voltage Drop of Feedback Type Picoammeter Pur ids Calculating Voltage Drop Error Compensating EQuation cic u esed eed er ee Ra RN ic CC PH D deet CAR UR ae a rade dee do ha Compensation Drop for input Voltage Drop of Shunt Type High Impedance Picoammeters Calculating the Error e a o ette Red RA Oe EE EE REENEN Eo N
14. the voltage at the anode of D204 it should be 17 1V 1096 If this bootstrap voltage is not sufficient Q203 will not operate and Q201 and Q202 will be cut off Check the reference amplifier and error amplifier stages using the schematic diagram voltage levels as a guide 6 6 FUSE REPLACEMENT A line fuse protects the Model 261 from possible damage in case of excessive line current H the unit is totally inoperative the fuse may be open To replace the fuse proceed as follows 6 2 Probable Cause Defective voltage supply Defective Kelvin Varley divider Check S103 through S106 check R120 through R152 No current output on one Defective range resistor Check corresponding range range resistor for open circuit Output too high on all ranges Defective voltage supply Check Q202 for short Output too high on one range Defective range resistor Check Q202 and 0208 for open Check corresponding range resistor WARNING Disconnect the instrument from the power line and other instrumentation before replacing the fuse 1 Locate the fuse holder on the rear panel rotate the fuse carrier counter clockwise until the carrier is free of the holder Remove the fuse carrier from the holder then remove and discard the defective fuse Replace the fuse with the type recommended in Table 6 2 CAUTION Use only the recommended fuse type replacing the fuse with a unit with a higher ratin
15. 3 Resistor 1 8kQ 10 1 2W Composition R 1 1 8k Resistor 760Q 1 1 2W Wirewound Voltage Supply R 58 760 Resistor 2 2kQ 10 1 2W Composition Voltage Supply R 1 2 2k Resistor 10kQ 10 1 2W Composition Voltage Supply R 1 10k Resistor 100 1 1 2W Carbon Voltage Supply R 12 10 Resistor 2 7kQ 10 1 2W Composition Voltage Supply R 1 2 7k Resistor 10kQ 10 1 2W Composition Voltage Supply R 1 10k Resistor 6340 1 1 8W Metal Film Voitage Supply R 88 634 Resistor 4 7kQ 10 1 2W Composition Voltage Supply R 1 4 7k Resistor 270kQ 10 1 2W Composition Voltage Supply R 1 2 7k Potentiometer 200 10 1 2W Calibration Board RP 104 200 Resistor 470Q Nominal 1 1 2W Wirewound Voltage Supply R 58 470 Slide Switch Cal Operate Chassis SW 45 Rotary Switch less Components AMPERES Front Panel SW 180 Rotary Switch with Components AMPERES Front Panel 18524B Knob Amperes Switch Front Panel KN 55 Rotary Switch less Components X 01 Multiplier Front Panel SW 180 Rotary Switch with Components X 01 Multiplier Front Panel 18463B Knob 0 10 Readout Front Panel KN 57 Rotary Switch less Components X 1 Multiplier Front Panel SW 159 Rotary Switch with Components X 1 Mutliplier Front Panel 18456B Knob 0 9 Readout Front Panel KN 56 Rotary Switch less Components X1 Multiplier Front Panel SW 182 Rotary Switch with Components X1 Multiplier Front Panel 18465B Knob 0 10 Readout Front Panel KN 57 Rotary Switch POLARITY Fron
16. 5 Transistor PNP Germanium 2N1372 Voltage Supply TG 8 Transistor PNP Silicon 40319 Voltage Supply TG 50 Transistor PNP Germanium 2N1372 Voltage Supply TG 8 Transistor PNP Germanium 2N1372 Voltage Supply TG 8 Transistor PNP Germarium 2N1372 Voltage Supply TG 8 Resistor 100kQ 02 W Wirewound 102 R 47 100k Resistor 1MQ 1 W Carbon 102 R 12 1M Resistor 10MQ 1 W Carbon 102 R 12 10M Resistor 100MQ 1 102 R 289 100M Resistor 1GQ 1 102 R 289 1G Resistor 10GQ 2 102 R 289 10G Resistor 100GQ 2 102 R 289 100G Resistor Assembly 1T2 2 102 32042B Resistor 10MQ 10 W Composition 102 R 1 10M Resistor 1 5MQ 10 ZW Composition 102 R 1 1 5M Resistor 150kQ 10 Y W Composition 102 R 1 150k Potentiometer 2000 Y W 10 Calibration Board RP 104 200 Potentiometer 2000 1 4 W 1096 Calibration Board RP 104 200 Potentiometer 2000 4 W 1096 Calibration Board RP 104 200 Potentiometer 2000 V W 10 Calibration Board RP 104 200 Potentiometer 2002 12 W 10 Calibration Board RP 104 200 Potentiometer 2000 2 W 10 Calibration Board RP 104 200 Potentiometer 2000 Ya W 10 Calibration Board RP 104 200 Resistor 12 80 1 W Wirewound 103 R 67 12 8 Resistor 12 80 1 4W Wirewound 103 R 67 12 8 Resistor 12 80 1 W Wirewound 103 R 67 12 8 Resistor 12 80 1 WW Wirewound 103 R 67 12 8 Resistor 12 80 1 W Wirewound 103 R 67 12 8 Resistor 12 80 1 4W Wirewound 103 R 67
17. 6 Transistor Q203 operates at a high gain by connecting its col lector to a negative supply voltage This arrangement permits linear operation of Q202 under wide variations in supply voltage To supply Q203 one side of the T201 secondary voltage is rectified by D201 and filtered by C201 Resistor R202 and zener diode D204 regulate this voltage to a stable 6V referred to the collector of Q202 Resistors R204 and R205 and diode D205 provide current overload protection Excessive current drawn from the supply causes an increased voltage drop across R204 which for ward biases D205 preventing the collector of Q203 from go ing more negative Since this prevents Q203 s error signal from increasing any further so Q202 cannot increase con duction and further current increase is prevented 5 3 2 Range Resistors The range resistors R101 through R108 are mounted on a specially designed rotary switch range switch S102 which has silver plated contacts The 105 range resistor R101 is a 0 0296 wirewound resistor Resistors R102 through R104 are 196 carbon film resistors High megohm resistors R105 through R107 are special glass sealed resistors that are in dividually selected after a 3 month stability test R108 is made up of ten 1010 resistors mounted on a separate assembly which is then mounted on 102 Capacitor C101 and resistors R109 R110 and R111 form a damping network to compensate for the capacitance across high megohm resisto
18. AMPERES Switch is in the 109 position The exact resistance for any position of the AMPERES switch can be found listed on the supplied calibration certificate The basic procedure for ohmmeter calibration is as follows 1 Set the POLARITY switch to the OFF not AC OFF posi tion 2 Set the AMPERES switch to the reciprocal of the desired resistance 3 Connect the ohmmeter to the Model 261 OUTPUT con nector using an appropriate coaxial cable See Section 2 for precautions to be taken when making connections 4 Carefully shield connections when making measurements greater than 1080 Use guarded connections for measurements of 1090 and more as shown in Figure 3 3 Guarded connections are necessary in such cases to minimize the shunting effects of cable and connector leakage resistance For example using the 10190 range a 10720 leakage resistance will add an error of 196 To make the proper connections within the unit place the CAL OPERATE switch in the CAL position see Section 4 3 2 WARNING Disconnect the instrument from the power line before removing the top cover Failure to observe this precaution may result in personal injury or death due to electric shock Remove the link between the GND and COM terminais on the rear panel of the source The outer shell of the OUT PUT jack is the guard terminal while the G binding post next to it is connected to the low side of the ohmmeter 5 Observe the ohmmeter reading and comp
19. ANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE THE REMEDIES PROVIDED HEREIN ARE BUYER S SOLE AND EXCLUSIVE REMEDIES NEITHER KEITHLEY INSTRUMENTS INC NOR ANY OF ITS EMPLOYEES SHALL BE LIABLE FOR ANY DIRECT INDI RECT SPECIAL INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OF ITS INSTRUMENTS AND SOFTWARE EVEN IF KEITHLEY INSTRUMENTS INC HAS BEEN ADVISED IN ADVANCE OF THE POSSIBILITY OF SUCH DAMAGES SUCH EXCLUDED DAMAGES SHALL INCLUDE BUT ARE NOT LIMITED TO COSTS OF REMOVAL AND INSTALLATION LOSSES SUSTAINED AS THE RESULT OF INJURY TO ANY PERSON OR DAMAGE TO PROPERTY KEITHLEY Keithley Instruments Inc 28775 Aurora Road e Cleveland OH 44139 216 248 0400 Fax 216 248 6168 e http www kcithley com CHINA Keithley Instruments China e Yuan Chen Xin Building Room 705 No 12 Yumin Road Dewai Madian Beijing China 100029 e 8610 2022886 Fax 8610 2022892 FRANCE Keithley Instruments SARL BP 60 3 all e des Garays 91122 Palaiseau C dex 31 6 0115155 Fax 31 6 0117726 GERMANY Keithley Instruments GmbH Landsberger Strade 65 82110 Germering 49 89 849307 0 Fax 49 89 84930750 GREAT BRITAIN Keithley Instruments Ltd The Minster 58 Portman Road Reading Berkshire RG30 1EA 44 01734 575666 Fax 44 01734 596469 ITALY Keithley Instruments SRL Viale S Gimignano 38 20146 Milano 39 2 48303008 Fax 39 2 48302274 JAPAN Keithley KK Aibido Bldg 7 20 2 Nishishinjuku e Shinjuku k
20. COEFFICIENT 0 1 C from 15 C to 30 C on 10 7 to 10 54 range settings Approximately 0 15 C on 10 12 to 10 8A range settings Exact values for these ranges supplied with instrument WARM UP TIME 1 hour LINE REGULATION 0 01 for 10 change in line voltage SOURCE VOLTAGE 0 to 11V in 0 01V steps RESOLUTION 3 significant figures from 10 12A to 1 1 x 10 4A RANGE RESISTORS 105 to 10120 in decade steps 5 RANGE RESISTOR ACCURACY Value with power on given on certificate 10 5 108 to 10120 30 1 95 107 to 106 Q 0 02 105Q OUTPUT ISOLATION Low to ground gt 109Q shunted by 0 001pF CERTIFICATION A Calibration Certificate is furnished including range resistor values thermal coefficients temperature and date of calibration Certification traceable to the National Institute of Standards and Technology is also available POWER 105 125V or 210 250V switch selected 50 60Hz 6W DIMENSIONS WEIGHT 155mm high x 225mm wide x 300mm deep 6 25 in x 9 in x 12 in Net weight 4 1kg 9 Ibs ACCESSORY SUPPLIED Model 2611 Test Cable TABLE OF CONTENTS Paragraph Title Page See oon sch ch oa LD OO JO n G N NON DS S S IS ECT I NO o m p O N N d N EI 2 8 1 SECTION 1 GENERAL INFORMATION troduction o RE ee u Gs lege AA ab ab enun aber ure pq Fal a OT Modeli261 Features ocio sor ada EE Warranty Information DQ oe eee tas oh VR rece VAR E RUE GET ARTE Fn ri ie e Ea ey co Manual Addenda x ocio ou E
21. Model 179A Digital Multimeter across the Kelvin Varley divider Connect the HI lead to the grey white wire on the X 01 switch S103 and the LO lead to the black blue wire on the X1 switch S105 Figure 4 1 2 Set the Model 261 controls to Polarity Switch Range Switch 105 Multiplier Switch 10 00 3 Adjust the 105 CAL potentiometer R210 Figure 4 1 for 10 012V 5mV when read on the Model 179 Setting the voltage 12mV high compensates for any loading errors on the 10 SA range 4 Set the multiplier dials to 10 00 switch the Model 261 through all ranges Use the internal potentiometer for each range to set the range potential to 10R volts the toler ance listed in Table 4 5 R is the exact resistor value found in paragraph 4 5 2 excluding the exponent value Exampte The 10 1 range resistor is measured paragraph 4 5 and its value is 1 019 x 1009 The range potential is now adjusted when the range switch is set to 10 11 and the multiplier switches to 10 00 Using Table 4 5 potentiometer R118 is set so the voltmeter reads 10 190V 5mV Table 4 4 Voltage for Muitiplier Switch Settings Voltage Reading Tolerance Table 4 5 Range Calibration Multiplier Range Switch Switch 10 x R Volts Setting Setting Adjustment R Range Resistor 10 00 R 10 Range Resistor 10 00 R 107 Range Resistor 10 00 R 108 Range Resistor 10 00 R 109 Range Resistor 10 00 R 101 Range Resistor 10 00 R 2 10
22. Model 261 Picoampere Source Instruction Manual Contains Operating and Servicing Information KEITHLEY WARRANTY Keithley Instruments Inc warrants this product to be free from defects in material and workmanship for a period of 1 year from date of shipment Keithley Instruments Inc warrants the following items for 90 days from the date of shipment probes cables rechargeable batteries diskettes and documentation During the warranty period we will at our option either repair or replace any product that proves to be defective To exercise this warranty write or call your local Keithley representative or contact Keithley headquarters in Cleveland Ohio You will be given prompt assistance and return instructions Send the product transportation prepaid to the indicated service facility Repairs will be made and the product returned transportation prepaid Repaired or replaced products are warranted for the balance of the origi nal warranty period or at least 90 days LIMITATION OF WARRANTY This warranty does not apply to defects resulting from product modification without Keithley s express written consent or misuse of any product or part This warranty also does not apply to fuses software non rechargeable batteries damage from battery leakage or problems arising from normal wear or failure to follow instructions THIS WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES EXPRESSED OR IMPLIED INCLUDING ANY IMPLIED WARR
23. Range Resistor 10 00 R 10 Range Resistor Tolerance 4 5mV 9mV 8mV 7mV 6mV 5mV 4mV 3mV 2mV 1mV 1 84mV 1 64mV 1 44mV 1 24mV 1 04mV 0 84mV 0 64mV 0 44mV 0 24mV 310uV 280V 250uV 220uV 190uV 1604V 1304 V 1004 V 100uV 1004 V 4 3 4 4 SECTION 5 THEORY OF OPERATION 5 1 INTRODUCTION This section contains a block diagram and a detailed descrip tion of Model 261 circuit operation 5 2 BLOCK DIAGRAM The basic block diagram for the Model 261 is shown in Figure 5 1 The source is made up of three basic sections a voltage source a variable voltage divider and a selectable range resistor The voltage source uses a stable zener regulated supply to provide a constant 11V A 3 dial Kelvin Varley voltage divider which has 0 1 accuracy is used to set the voltage to a value between OV and 11V in 0 01V increments The three front panel multiplier switches contain the divider resistors The range switch controls the output current by selecting an appropriate series range resistor For any combination of front panel controls the output current can be found simply by dividing the voltage divider output value by the selected range resistance value RANGE OF RESISTOR ONE OF EIGHT VARIABLE VOLTAGE SOURCE TN DC CURRENT OUTPUT VOLTAGE SUPPLY CALIBRATE Figure 5 1 Block Diagram 5 3 CIRCUIT DESCRIPTION For component designations refer
24. are it to the ap propriate value listed on the calibration certificate Range resistor values will be within the stated tolerance for up to three months after factory calibration MODEL 261 TO OHMMETER Figure 3 3 Guarded Circuit For Ohmmeter Calibration SECTION 4 PERFORMANCE VERIFICATION AND CALIBRATION 4 1 INTRODUCTION This section contains information necessary to verify that Model 261 performance is within specified accuracy and to calibrate the instrument Moclel 261 specifications may be found at the front of this manual ideally performance verifi cation should be performed when the unit is first received to make sure that no damage or change in calibration has occurred during shipment The verification procedure may also be performed if accuracy is suspect or following calibra tion If instrument performance is substandard calibration may be performed as described at the end of this section NOTE tf the instrument does not meet specifications and less than three months have passed since the date of shipment contact your Keithley representative or the factory to determine the action to be taken 4 2 ENVIRONMENTAL CONDITIONS All measurements should be made at an ambient temperature between 18 C and 28 C 65 to 82 F with a relative humidi ty less than 80 4 3 RECOMMENDED TEST EQUIPMENT Recommended test equipment for Model 261 performance verification and calibration is listed in Table 4 1 Different
25. aru de s Pp edle een Bd Kelvin Varley Voltage Divider 066 e heme SECTION 6 MAINTENANCE Introduction oc rrr ane ER SEENEN wa ORY Hee eee daa Servicing Schedule Top Cover Removal Parts Replacement ces ds posed center A e A eine Bebe eee dd Figure 22 TABLE OF CONTENTS CONT Troubleshooting u us Ee ER EXE GG EI DEENEN AER NES RENE AE 6 1 Troubleshooting Equipment 2 ccc ccc seas Rh heh a 6 1 Common Faults degen A vd pA AST Y Ae ds 6 1 Troubleshooting Guidelines 6 2 Fuse Replacement 4 44 4444444 suu eere era 6 2 SECTION 7 REPLACEABLE PARTS Intt ductiOn cocida ee em rec ra A at ade Parts iii a nda oa a cas Ordering Parts uuu ori A A ac Factory Service Schematic Diagram and Component Layouts mee tete he d rg nn nn nas s i sans mere ns LIST OF FIGURES Title Page Front Panel Controls and Terminals Rear Panel Controls and Terminals Connecting the Model 261 Source High Resistance Paths in Output Connections Simplified Circuit of Feed Back Type Picoammeter Typical Shunt Type Picoammeter Current Suppression Circuit Connections Galvanometric Measurements metre nes hah e d hod nn es er hh ep a i rra o cere hes nu nm rd Bras hh or noa md ox eism h ho sn mm mms hr e mb e meh mao s e e v t ose des smhe hh amod
26. cale is possible with most picoammeters The basic procedure for current suppression is as follows 1 Connect the Models 261 and 610C to the unknown current as shown in Figure 3 1 use the Model 2611 test cable or other similar coaxial cable 2 Set the Model 261 to the desired current as described in Section 2 Set the output polarity of the source opposite to that of the unknown current Also set the source output value as close as possible to the unknown current 3 Set the Mode 610C FEEDBACK switch to FAST In this mode the Mode 610C acts as a feedback type picoam meter 4 Set the Model 610C to the least sensitive current range Vary the Model 261 source output until a null is reached 5 Gradually increase the Model 610C sensitivity nulling the current with the Model 261 controis in each case Keep the Model 610C multiplier switch on 1 6 Set both the Models 261 and 610C range switches to the same setting If the electrometer is set to a lower range the Model 261 source resistance wili divide the Model 610C feedback current impairing its zero stability 7 Switch the electrometer multiplier setting gradually from 1 to 0 001 In the lowest setting the Mode 610C current sensitivity is 1000 times the unknown current The basic procedure for using this method with other instru ments is the same Check the picoammeter s specifications to determine any circuit limitations Always set the picoam meter for the best possible
27. cessary to ensure rated accuracy as described in the following paragraphs Table 2 1 Current Output Examples Desired Current Multiplier 6 00 2 17 8 95 10 0 1 35 2 50 7 50 4 55 3 66 ES 261 PICOAMPERE SOURCE MODEL 261 MODEL 2611 CABLE INPUT MEASURING INSTRUMENT Figure 2 3 Connecting the Model 261 Source 2 6 ACCURACY CONSIDERATIONS The range switch setting times the multiplier switch setting gives the Model 261 current output The accuracy of the out put however depends upon several factors Output connections from the source to the picoammeter are very important especially for the lower ranges The accuracy of the current at the Model 261 OUTPUT receptacle will mean little if poor or improper hookups are made from the source to the picoammeter 2 6 1 Input Voltage Drop Voltage Burden For specified output accuracy the voltage drop of the measuring instrument must be less than 1 1000 of the inter nal source voltage of the Model 261 The internal source voltage in volts is read directly from the multiplier switch set tings regardless of the range switch setting The voltage drop of the picoammeter is a characteristic of that instru ment it can be found in its instruction manual or it must be calculated For picoammeter with a 1mV or less voltage drop set the Model 261 multiplier switches to 1 00 For a voltage drop be tween 10 and 1mV set the multiplier switches to 10 00 and set the ran
28. current output range switch setting times the multiplier switch settings T is the room temperature in C T is the calibration temperature in C tc is the temperature coefficient of the range resistor in 96 9C given on the calibration certificate 2 7 2 8 SECTION 3 APPLICATIONS 3 1 INTRODUCTION The Model 261 was designed to serve as a secondary source in calibrating picoammeters and electrometers While this is the primary purpose of the unit it can also be used for various other applications including current suppression galvanometric measurements and ohmmeter calibration This section briefly covers the methods for each of these ap plications these examples by no means exhausts the possibi lities of using the Model 261 3 2 CURRENT SUPPRESSION The Model 261 can be used to suppress steady background currents so that a picoammeter or electrometer can measure small variations in signal For example the Model 261 can be used to cancel phototube dark currents while the picoam meter measures small variations in the normal output of the phototube The basic circuit for current suppression is shown in Figure 3 1 Because of its popularity the Keithley Model 610C is used as the current measuring device Other instruments with similar characteristics will serve equally well in this applica tion Using the Models 261 and 610C suppression up to 1000 times full scale is possible Suppression up to at least 100 times full s
29. e returned to Keithley Instruments inc for repair services Contact your nearest Keithley representative or the factory for information 6 5 1 Troubleshooting Equipment The following equipment is recommended for use when troubleshooting the Model 261 Keithley Model 179A or similar DC voltmeter with 0 0496 basic DC accuracy and a minimum input impedance of 10MQ 6 5 2 Common Faults Table 6 1 lists the most common troubles that might affect the Model 261 If the steps listed in the table do not rectify the problem a step by step circuit analysis may be required Use the operating theory covered in Section 5 for this purpose Table 6 1 Troubleshooting Guide Difficulty No current output on all Ranges 6 5 3 Troubleshooting Guidelines If the instrument does not operate at all check the fuse line cord and power source If these are all found to be in good working order use the following procedure to troubleshoot the voltage supply Refer to the schematic at the end of Sec tion 7 for component designations 1 Set the front panel POLARITY switch to and connect the meter between J202 and the emitter of Q202 The voltage should be 11 6V If the voltage is about 17V Q202 might be shorted If the voltage at J202 is much less than 11 6V check the volt age at the collector of Q202 it should be 17V 20 Absence of this voltage indicates the rectifier circuit is not working properly Measure
30. e Model 261 was carefully inspected before shipment Upon receiving the unit carefully unpack all items from the shipping carton and check for any damage that might have occurred during transit Report any damage to the shipping agent at once Save the original packing material in case reshipment becomes necessary Contact the nearest Keithley representative or the factory if the unit fails to function pro perly The following items are included in every Model 261 ship ment 1 Model 261 Picoampere Source 2 Model 261 Instruction Manual 3 Model 2611 Test Lead Cable 4 Additional accessories as ordered 1 7 USING THE MODEL 261 INSTRUCTION MANUAL This manual is intended to familiarize the operator with the operating controls and features of the Model 261 Picoampere Source Also included is information on calibration maintenance and spare parts The manual is divided into the following sections 1 Sections 2 and 3 contains pertinent operating information including applications and possible pitfalls to avoid when using the source 2 Section 4 covers performance verification and calibration procedures 3 A complete description of operating theory is contained in Section 5 4 Troubleshooting and maintenance procedures are covered in Section 6 5 Parts ordering information can be found in Section 7 1 8 SPECIFICATIONS A complete list of Model 261 specifications can be found im mediately preceding this section 1 1
31. eadout device indicates the voltage drop on these type picoammeters 2 10 2 Compensating Equations Since the picoammeter voltage drop can be found the Model 261 output current can be corrected Equation 6 gives the output current vaiue if the input voltage drop is the only significant source of error and if the voltage drop is less than 1 10 the source voltage 1 1 V V Equation 6 where I is the actual output current from the Model 261 is the range switch setting reciprocal of range resistor value V is the Model 261 source voltage read directly in volts from the multiplier switch setting V is the picoammeter input voltage drop in volts Using this correction accuracy is nominally 0 5 Model 261 accuracy This error results from the factory calibration method used for the Model 261 that adjusts the source volt age from that indicated to match the range resistor used For greater accuracy or if the input drop is more than 0 1 times the source voltage use equation 8 to compensate for the error The following equation is based on the calibration procedures Va Ve R 4 NA Ral Equation 7 1 1 where V is the actual source voltage V is the Model 261 source voltage read directly in volts from the multiplier switch setting R is the actual source resistance from the calibration cer tificate supplied with the Model 261 L is the Model 261 range switch setting reciprocal of range resistor value Usi
32. fore removing the top cover 1 Remove the four screws securing the top cover to the chassis 2 Grasp the top cover by the handle and gently pull the cover up and back until it clears the chassis 3 To replace the cover reverse the above procedure 4 5 2 Range Resistor Verification The following procedure outlines a method which can be used in the field to measure the actual value of the high megohm range resistors The inherent drift of these resistors sets the 3 month accuracy limit of the Model 261 Note that resistor verification is also necessary to complete calibration Table 4 1 Recommended Equipment for Verification and Calibration Manufacturer and Model Use Electro Scientific Industries Measurement of 105 Range Resistor Precision Resistance Measuring System Model 242A Digital Teraohmmeter Guildline Model 9520 Digital Multimeter Keithley Model 179A Measurement of 109 through 10 12 range resistors Measurement of Power Supply Kelvin Varley Voltage Divider Table 4 2 Range Resistor Verification Model 261 Amperes Switch Setting Range 106 4 196 107 4 196 108 4 196 10 196 1010 296 10 42 1012 4 2 Resistance Value Ohms 105 0 0296 Measuring Measurement Potent Volts Accuracy 0 0296 4 0 0396 4 0 02596 0 03596 0 05 0 07 0 1 0 2 Accuracy of equipment listed in Table 4 1 Disconnect the Model 261 line cord from the powe
33. g may cause instrument damage Replace the fuse carrier in the holder and rotate the carrier clockwise until it seats in the holder Table 6 2 Fuse Types Keithley Operating Limits Fuse Types Part Number 105 125V 50 60Hz 3AG 1 8A FU 20 250V Glow Blow 210 250V 50 60Hz 3AG 1 16A FU 21 250V Slow Blow SECTION 7 REPLACEABLE PARTS 7 1 INTRODUCTION This section contains replacement parts information and a schematic diagram and component drawings for the Model 261 7 2 PARTS LIST Table 7 1 lists the replaceabie parts for the Model 261 Parts are listed alphabetically according to circuit designation 7 3 ORDERING PARTS To place an order for Model 261 parts or obtain parts informa tion contact your Keithley representative or the factory See the inside front cover of this manual for addresses When ordering parts include the following information R202 Oum D201 D202 D203 Q201 Instrument Model Number Instrument Serial Number Part Description Circuit Designation where applicable Keithley Part Number m P N 7 4 FACTORY SERVICE If the instrument is to be returned to the factory for service complete the service form which follows this section and return it with the instrument 7 5 SCHEMATIC DIAGRAM AND COMPONENT LAYOUTS Parts list schematic diagram and component location draw ings are shown on the following pages O D204
34. ge switch one decade lower This setting does not affect the Model 261 accuracy for outputs greater than 108A Note that even for a voltage drop 1 100 of the source voltage an error of only 196 is added to the specified accur acy 2 6 2 Proper Multiplier Settings Setting the multiplier switches to 10 00 ensures the most ac curate output current given the proper voltage drop source voltage ratio Setting the multiplier switches to other than 10 00 does not affect the accuracy for outputs greater that 103A in ali cases however the Model 261 output accuracy is at least that of the worst case accuracy listed in the accur acy specifications When the Model 261 is used on the 10 to 10 12 ranges at other than the calibrated temperature stated on the calibra tion certificate furnished with the source a discrepancy in the output current will occur On other ranges the effect is not significant 2 7 OUTPUT CONNECTIONS Make all connections carefully between the Model 261 and meters having fast response speeds Tie down cables to avoid vibrations Cable movements cause meter jitter on the 109A and lower ranges due to capacitance changes For currents 103A and less carefully shield the output con nections and the input connections of the picoammeter Unless the shielding is thorough any changes in the elec trostatic field near the input circuitry will cause definite distur bances on the measuring instrument Use high resistance l
35. he fuse is good 2 5 BASIC OPERATING PROCEDURE Operation of the Model 261 is fairly straight forward Once the unit is connected to the measuring instrument the operator need only select the appropriate range and multiplier switch settings and select the output current polarity The basic operating procedure is as follows 1 Plug in the instrument to the appropriate power source and rotate the POLARITY switch to the OFF position The pilot light should indicate that power is applied to the source Allow a one hour warm up period for rated accuracy ES 261 PICOAMPERE SOURCE 2 O 6 o A 4 gt lt A 9 wj A Y Aug M 86 928 POLARITY OFF KEITHLEY INSTRUMENTS INC 3AG SLOW BLOW FUSE VOLTS AMPS W 14 234 1 6 50 1000 CPS GROUND AND VOLTAGE SWITCH FUSE LINE CORD COMMON TERMINALS Figure 2 2 Rear Panel Controls and Terminals 2 Connect the source to the measuring instrument as shown in Figure 2 3 Use the supplied Model 2611 cable to make the necessary connections 3 Select the desired range and multiplier settings according to the required output current Some examples are shown in Table 2 1 Note that accuracy figures are given with the multiplier settings at 10 00 4 Place the POLARITY switch in the or position depending on the desired output polarity This procedure covers the basic steps for using the Model 261 however some precautions may be ne
36. hould need replacement except the fuse Ideally the high megohm range resistors shouid be checked every three months to ensure instrument accuracy Refer to Section 4 for procedures to be used The source may also be calibrated if accuracy is suspect Use the procedures outlined in Section 4 when calibrating the unit Alternately the Model 261 may be returned to Keithley Instruments Inc for calibra tion 6 3 TOP COVER REMOVAL Maintenance or troubleshooting of the unit will require that the top cover be removed WARNING These instructions are intended for qual ified servicing personnel only Do not remove the top cover unless qualified to do so Also disconnect the Model 261 from the power line and other instruments before removing the top cover Failure to observe these precautions may result in serious personal injury or death because of the possibility of electric shock To remove the top cover use the following procedures 1 Disconnect the Model 261 from the power line 2 Remove the screws securing the top cover to the chassis 3 Grasp the handle on the top cover and carefully separate the cover from the chassis The cover can be installed by reversing this procedure Be sure to line up the holes in the cover with those in the chassis before replacing the screws 6 4 PARTS REPLACEMENT Section 7 lists the replaceable parts available for the Model 261 When replacing parts be sure to use only reliable replacemen
37. ired position FUSE By unscrewing the fuse holder cap the fuse is ac cessable See Section 6 for fuse replacement procedures Power Cord The 3 wire cord is permanently attached to the rear panel The opposite end of the power cord has a stan dard 3 prong plug attached This plug should be used only with grounded outlets COM and GND terminals The COM terminal is connected to the low side of the source output The GND terminal is connected to chassis ground which is connected to the power line ground wire in the 3 wire power cord Normally the GND and COM terminals are connected together with a shorting link which is provided The link can be removed to float the low side from ground 2 4 PREPARATION FOR USE Before operating the Model 261 perform the following steps 1 Set the line voltage selection switch to the appropriate set ting depending on the local power line voltage Check to see that the appropriate fuse is installed CAUTION Operating the source on an incorrect line voltage may damage the unit possibly voiding the warranty 2 Set the Model 261 front panel controls as follows POLARITY Switch AC OFF Range Switch Desired current range Multiplier Switches 10 00 3 Plug in the source to AC power and rotate the POLARITY switch to the OFF position The decimal point pilot light should come on indicating the unit is receiving power If not disconnect the instrument from the power line and check to see that t
38. ng equation 7 in equation 6 and substituting V for V l l V R Vi Equation 8 s isVs s where is the actual output current from the Model 261 V is the picoammeter input voltage drop in volts read from the meter Using equation 8 completely compensates for the error due to the input voltage drop 2 11 COMPENSATION FOR INPUT VOLTAGE DROP OF SHUNT TYPE LOW IMPEDANCE PICOAM METERS For the most part the voltage drop for this type picoammeter is about the equivalent of that of the feedback type picoammeter about 1mV or less Following the directions in paragraph 2 6 will ensure that Model 261 output currents are within the specified accuracy To compensate for output current difference caused by input voltage drops use equation 8 This calculation will be precise since the voltage drop is read directly from the picoammeter 2 12 RANGE RESISTOR VOLTAGE COEFFICIENT Ordinarily the voltage coefficient of high megohm resistors can lead to significant errors However the range resistors used in the Model 261 have an extremely low voltage coeffi cient of only 5ppm V This low value results in a worst case current change due to voltage coefficient of only 0 005596 with the multiplier switches set for 11V Thus the output discrepancy because of voltage coefficient is one or two orders of magnitude smaller than other sources of error and can be ignored for all practical purposes 2 13 COMPENSATION FOR TEMPERATURE COEFFI
39. o orga et b natn Fh e dg i de RENE ie r acem s occ rg e vc ohh tm o or o ase rh or ev vo vervesesseissmssss sar 446 yens Top Chassis View Block Diagram ivi a A ados Voltage Supply Board Component Location Drawing Calibration Board Component Location Drawing Dwg No 30483C AMPERES Range Switch S102 10120 Resistor Assembly R108 X 01 Multipiier Switch 103 X 01 Multiplier Switch S104 X1 Multiplier Switch ST hse raa Model 261 Schematic Diagram Dwg No 18323E HEEN DNR ENNER ENEE Cr o ar HEEN kk rh errat bres eti trot rs mese mee memes maearacecsee ve vgsig bessssessscossmsesecvevsenesveciiciciirs Bothe ehh wo don ns nm mn nn em a nee mme nm mms mms sens DEE EE EEN yao ror d rss e rtr LIST OF TABLES Current Output Examples e Causes of Discrepancy in Model 261 Output Current and Correction Index Recommended Equipment for Verification and Calibration Range Resistor Verification Range Resistor Accuracy hehe Voltage For Multipiier Switch Settings Range Calibration 4 44e eee ere Troubleshooting Guide Fuse Typos PE Model 261 Parts List sed ci lors baa car aida A Mechanical Parts Zu scc coire e A Ei ge io NS EE E eae AAA EEE AAA cr 4d n nt ran mmm rhv cg d hoe d ss samnes koe e 9 gn n mh 9 hah eh t rss hs emet e ad rosana ranas
40. ow loss materials such as Teflon recommended polyethylene or polystyrene for insulation The insulation leakage resistance of test fixtures and leads should be several orders of magnitude higher than the inter nal resistance of the source If it is not leakage losses will cause lower readings Coaxial cables used should be a low noise type which employ a graphite or other conductive coating between the dielectric and the surrounding shield braid NOTE Ground loops may occur when equipment con nected to the Model 261 has one side of the in put grounded in this case disconnect the shor ting link between the Model 261 COM and GND binding posts The low side of the output then floats output isolation from low to ground is greater than 1090 shunted by 0 0014F with the link removed High resistance paths in the output connections do not ap preciably reduce the Model 261 current output accuracy if the source is used with a low input voltage drop or feed back type picoammeter As an example the Model 261 is used with a picoammeter with a 1mV drop Figure 2 4 The source is set to 102A If the leakage resistance is 10120 the error caused by leakage R is 0 001mV 105A 10120 This is 0 1 of the source current 107A Leakage resistance of the Model 2611 Test Cable is nominally greater than 10140 Is 10 12A Rs 10120 1mV DROP MODEL 261 1 PICOAMMETER Figure 2 4 High Resistance Paths in Output Connec
41. r source Remove the line connecting the COM and GND binding posts on the rear panel Set the CAL OPERATE Figure 4 1 in the CAL position In this position the low side of the range resistor is con nected to chassis ground while the low side of the OUT PUT jack is connected as a guard between the high side of the output and ground Connect the Model 261 OUTPUT jack to the resistance measuring device Use a suitable cable Select the desired Model 261 range resistance with the AMPERES switch The nominal resistor values are equal to the reciprocal of the switch setting Set the measuring device to the voltage setting in the table Measure and record the resistance value for each range resistor The measured value for each resistor should agree with the value stated on the calibration certificate within the tolerance listed in Table 4 3 If not calibration of the affected ranges will be required e a ES 5 9 a VOLTAGE o 2 SO 2 SUPPLY CR Of BOARD c Ac SE sail CAL OPERATE SWITCH CALIBRATION BOARD X1 SWITCH X 1 SWITCH X 01 SWITCH Figure 4 1 Top Chassis View 4 Table 4 3 Range Resistor Accuracy 105 106 and 107 108 through 1012 5 3 Multiplier Switch Setting Verification This procedure checks the accuracy of the divided potential from the voltage supply Proceed as follows 1 4 C Connect the Model 179A DMM across the Kelvin Varley voltage divider
42. rs R106 through R108 This network eliminates high current transients when the Model 261 is first turned on The voltage source connects through one deck of 102 to the range calibration controls R210 and R113 through R119 These range calibration controls form part of the voltage divider that forms the feedback signal for the regulator in the voltage supply 5 1 The range switch OUTPUT jack and the range resistors are encased in a floating shield which is normally connected to the low side of the output The shield is connected as guard when the calibration switch S101 is in the CAL position 5 3 3 Kelvin Varley Voltage Divider The Kelvin Varley voltage divider uses 0 1 precision 5 2 resistors which are mounted on the multiplier switches S103 104 and S105 These resistors R120 through R152 are used to divide the 11VDC supplied by the voltage supply down to the selected voltage between 0 and 11V in 0 01 in crements The voltage is selected with the three multiplier switches 103 through S105 SECTION 6 MAINTENANCE 6 1 INTRODUCTION This section contains maintenance and troubleshooting pro cedures for the Model 261 Picoammeter Source It is recom mended that these procedures be followed as closely as possible to maintain the accuracy of the instrument 6 2 SERVICING SCHEDULE The Model 261 requires no periodic maintenance beyond the normal care required of high quality electronic equipment Normally no part s
43. sensitivity 610C NULL DETECTOR UNKNOWN CURRENT MODEL 261 CURRENT SOURCE Figure 3 1 Current Suppression Circuit Connections 3 3 GALVANOMETRIC MEASUREMENTS In practice using the Model 261 for galvanometric measure ments is very similar to the current suppression procedures discussed in the previous paragraph In this application the Model 261 bucks out an unknown current while a measuring instrument such as the Keithley Model 614 serves as a null detector The basic circuit for galvanometeric measurements is shown in Figure 3 2 is the unknown current Vs is the Model 261 source voltage and R is the source resistance I is the source current while is any residual current seen by the in strument R is the picoammeter feedback resistor k the amplifier gain Ry the multiplier resistance Follow the same basic procedure for this measurement as described in the last paragraph Once maximum nulling is achieved the unknown current value can be determined by adding the Model 261 source output value to any residual value shown on the measuring instrument Figure 3 2 Galvanometric Measurements 3 1 3 4 OHMMETER CALIBRATION The Model 261 contains calibrated resistance standards which make it useful for checking high resistance measuring instruments The front panel AMPERES switch setting indi cates the reciprocal of the internal source resistance For ex ample the source resistance will be 1MQ if the
44. t Current Ranges Eight current ranges are user select able with a single front panel control 5 Shielded Output Connector The UHF output connector on the front panel is Teflon insulated to minimize possi ble leakage problems 6 Floating Low Connection The low side of the output con nection can be floated to minimize the effects of ground loops 1 3 WARRANTY INFORMATION Warranty information may be found inside the front cover of this manual If warranty service is required contact the Keithley representative in your area or the factory to deter mine the correct course of action Keithley Instruments Inc maintains service facilities in the United States West Ger many Great Britain the Netherlands Switzerland and Austria Information concerning the application operation or service of your instrument may be directed to the applica tions engineer at any of these locations Addresses can be found inside the front cover of this manual 1 4 MANUAL ADDENDA Changes pertaining to the instrument that occur after the printing of this manual will be found in an addendum included with this manual Be sure to note any changes before attemp ting to operate or service the instrument 1 5 SAFETY TERMS The WARNING heading used in this manual explains dangers that could result in personal injury or death The CAUTION heading used in this manual explains hazards that could damage the instrument 1 6 UNPACKING AND INSPECTION Th
45. t Panel SW 189 Knob Polarity Switch Front Panel KN 46 Slide Switch Front Panel SW 151 Transformer Chassis TR 78 To 10120 Resistor Assembly R108 Figure 7 3 AMPERES Range Switch 102 Table 7 2 Mechanical Parts Description Top Cover Assembly 18553B Handle HH 18 Bottom Cover Assembly 17148C Support Assembly Bail Left 19205B Support Bail Left 14703B Support Assembly Bail Right 19206B Support Bail Right 14705B Feet FE 5 Feet Rubber Ball FE 6 Bottom Panel 17149C Front Panel Chassis Assy 18559B Cable Clamp CC 4 Shorting Link BP 6 Label MC 30 Range Switch Shield Assy 18536B Front Panei Assembly 18464C Figure 7 4 10120 Resistor Assembly R108 7 5 DECK 2 DECK 1 FRONT DECK 2 DECK 1 FRONT DECK 3 8 H DE TEA PERSCH DER REECH KT 1 12 E 1 2 3 4 M DECK 2 LB AOS lt A ET DECK 1 7 8 9 10 1 5 6 FRONT Figure 7 7 X1 Multiplier Switch S105 AND CAPACITANCE SHALL BE DESIGNATED age Z rus RESPECTIVELY UNLESS OTHERWISE NOTED gt 10 ou TPUT I I T EI D DRIVER ADJUST RIVER ADIUST Wi TE SHOWN INECW POSITION eu REFERRED TO T202 WITH iom Tv OND gt 4 ON j RANGE SWITCH ON 1078 SON LINE VOLTAGE NAL VALUE eno Ox ithe AGE Le 7 03 com CAL OPERATES HIGHEST REFERENCE DESIGNATION USED
46. tions 2 8 INPUT VOLTAGE DROP FOR FEEDBACK AND SHUNT TYPE PICOAMMETERS The output current accuracy is specified under the assump tion that the picoammeter input voltage drop is less than 1 1000 of the Model 261 source voltage Most feedback elec trometers will easily meet this condition Any error caused by the picoammeter voltage drop may be eliminated by calcula tion if the voltage drop is known However only a 196 error is added if the voltage drop is 1 100 of the source voltage 24 The general magnitude of the voltage drop is dependent upon the type of circuit the picoammeter uses Feedback types have a low input voltage drop of approximately 1mV or less Shunt types generally have a relatively large input voltage drop about 10mV or more However some low current shunt type ammeters can be calibrated as if they were the feedback type To help determine the type of picoammeter under calibration a brief description of the cir cuits foliows 2 8 1 Feedback Type Picoammeter The feedback type picoammeter is basically an operational amplifier with a feedback resistor connected between the in put and the output as shown in Figure 2 5 If the amplifier in put current is small compared to the input current an expression for the amplifier output voltage V for an input current of is LR k Yor i Equation 1 where R is the vaue in ohms of the feedback resistor k is the amplifier gain If k gt gt 0 V
47. to schematic diagram number 18323E Figure 7 8 at the end of Section 7 5 3 1 Voltage Supply The voltage supply operates from either 105 125V or 210 250V 50 60Hz depending on the position of the fine voltage selection switch The DC output of the supply is a stable 11V with 0 0196 stability for a 1096 change in line voltage The power transformer T201 is specially constructed and shielded to provide better than 10 shunted by 0 0014F isolation from the transformer secondary to ground or line The secondary of the transformer supplies 15VRMS which is the full wave rectified by diodes D202 and D203 Filter capacitor C202 filters this voltage to provide approximately 17VDC To obtain a stable accurate voltage the conduction of the series pass transistor Q202 is regulated by comparing a sample voltage with a reference voltage The sampling voltage is provided by dividing action of resistors R211 R210 R201 and one of the range calibration resistors R210 R113 R119 The reference voltage is provided by zener diode D207 Any difference between these two voltages is amplified by a differential amplifier made up of Q204 and Q205 The signal is further amplified by Q203 and applied to a Darlington pair made up of Q201 and Q202 In this manner the conduction of Q202 is controlled to oppose any tendency for the output voltage to change The 11V regulator output is applied to the Kelvin Varley voltage divider through the POLARITY switch S10
48. ts which meet original specifications Replace parts only as necessary The range resistors are specially selected and aged to ensure rate accuracy for three months after calibration Normally these resistors do not need replacement If replacements are necessary order them from Keithley Instruments Inc In an emergency these parts can be ordered from another source but their accuracy or stability cannot be guaranteed This could seriously affect the accuracy of the source on one or more ranges In any case recalibration will be necessary if one of the range resistors is replaced CAUTION Do not touch the body of the range resis tors at any time Contamination of the special insulating materiai may cause leakage affecting the accuracy of the source output The 1072 resistor assembly on S102 is not field installable the instru ment must be returned to the factory for repair and calibration if this assembly must be replaced Do not use any spray on clean ing material on the S102 assembly 6 5 TROUBLESHOOTING The following procedures include instructions for repairing troubles which may occur in the Model 261 Use the pro cedures outlined and only specified replacement parts Com ponents can be identified with the help of the schematic and component layout drawings at the end of Section 7 In addi tion Section 7 contains several drawings pertaining to resistor locations H the trouble cannot be located the instrument may b
49. u Tokyo 160 e 81 3 5389 1964 Fax 81 3 5389 2068 NETHERLANDS Keithley Instruments BV Avclingen West 49 4202 MS Gorinchem 31 0 1 83 635333 e Fax 31 0 183 630821 SWITZERLAND Keithley Instruments SA Kriesbachstrasse 4 8600 D bendorf 41 1 8219444 e Fax 41 1 8203081 TAIWAN Keithley Instruments Taiwan 1 Ming Yu First Street Hsinchu Taiwan R O C e 886 35 778462 e Fax 886 35 778455 Model 261 Picoampere Source Instruction Manual 1983 Keithley Instruments Inc Test Instrumentation Group All rights reserved Cleveland Ohio U S A Third Printing Document Number 29046 Rev C Model 261 Picoampere Source Revised 1 16 89 OUTPUT 10 14A 10 11A full range to 1 1 x 10 4A positive or negative in eight decade ranges ACCURACY Exclusive of input drop consideration ACCURACY WITH WORST CASE 10 00 SETTING WITH SETTING RANGE 10V SOURCE OTHER THAN SETTING SPAN AMPERE VOLTAGE 10 00 10 7 to10 5 107 to 1 1 x 10 4 0 25 0 25 10 8 10 8 to 10 7 i0 5 0 5 10 9 10 9 to 10 8 10 6 0 8 10 10 10 10 to 10 9 10 6 1 1 96 10 11 10 11 to 10 10 0 6 13 96 10 12 10 12 to 10 11 0 7 1 6 10 12 10 14 to 10 12 ci 2 0 All accuracies are the percentage given 10 01 x range switch setting LONG TERM STABILITY Will operate within stated specifications for three months after calibration After three months add 0 15 per month to 10 8 through 10 12A range setting accuracy specifications TEMPERATURE
50. ut current value if the input voltage drop is the only significant source of error L l V V Equation 5 where is the actual output current from the Model 261 is the range switch setting reciprocal of range resistor value V is the Model 261 source voltage read directly in volts from the multiplier switch settings V is the picoammeter input voltage drop in volts 2 10 COMPENSATION DROP FOR INPUT VOLTAGE DROP OF SHUNT TYPE HIGH IMPEDANCE PICOAMMETERS As long as the voltage drop is less than 1 1000 of the source voltage errors due to the input voltage drop of this type picoammeter will not affect the Model 261 accuracy Follow ing the instructions in paragraph 2 6 will bring the accuracy of the Model 261 output current to between 0 25 and 1 0 796 The chief point is setting the multiplier switches to higher values so that the source voltage is 1000 times the picoammeter input voltage drop If the meter sensitivity is ad justable use the most sensitive range to obtain the smallest voltage drop NOTE If the instrument can measure current on either a feedback type or shunt type circuit calibrate on the feedback circuit For example calibrate the Keithley Model 610C Electrometer in the FAST mode feedback circuit not the normal mode shunt 2 10 1 Calculating the Error Equation 3 determines the error when the picoammeter input voltage drop is known The voltage drop is easily found since the meter or other r
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