Calibration of Keithley 6221 AC/DC low noise current source

Contents

Intro

This current source, build on the platform of 11W four quadrant SMU is common tool in scientific labs and often used for materials study and evaluation. Common use is delta-measurements for resistance using another Keithley instrument – Model 2182/2182A nanovoltmeter. Sensitivity and offset cancellation functions of such tandem allow easy and accurate measurements of very low resistances and shunts. 6221 can source very wide range of currents, from sub-nA (lowest range 1nA) to quite powerful 100mA, while maintaining up to ±105V of compliance voltage.

However this versatility comes at price, around 5520$ USD new for bare Keithley Model 6221. I’ve got unit for this article for bit over $1300 USD.

Today market niche of this instrument is overlapping with other precision SMU, such as own Keithley 2600 series and competitor Keysight B2900A series, however these cost bit more.

Disclaimer

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Exterior, controls and initial condition


Image 1: Keithley 6221 front panel


Image 2: Keithley 6221 rear connectors

Internal construction, design features


Image 3: Cover removed, analog board overview


Image 4: Analog board without shields


Image 5-6: Analog domain FPGA, rear section and interlock circuitry


Image 7: ADC/DAC and main reference section


Image 8-9: Analog board power supply


Image 10-11: Common mode inductor?


Image 12: PCB cutouts to avoid leakage


Image 13: High-current range relays and resistors


Image 14: Medium current range relays and resistors


Image 15: Low current range high value resistors


Image 16: Output power stage


Image 17-18: High impedance, low leakage amplifier front-end


Image 19-20: High impedance low current circut design


Image 21: Auxilary bias


Image 22-23: Fully shielded low-noise power supplies


Image 24-25: µu-metal shielding and board label


Image 26-27: Triaxial 3-lug connector and input circutry


Image 28-29: Voltage reference section and auxilary circuts


Image 30: Relay drivers

Diagnostics and secret menu


Image 31: Vacuum display burn-in mode for segment brightness correction


Image 32: Test with 10pA current with Keithley 6485

Repair

No repair was required.

Calibration

Calibration procedure in manual calls for HP 3458A 8½-digit multimeter and Keithley 6487 picoammeter. These ones I have, with little deviation by using Keithley 6485 instead of 6487. DMM is used to verify current ranges from 100mA to 100nA and picoammeter take care of lowest 10 nA and 1 nA ranges.

Performance test

Test condition 3458A reading Error from DMM 6221 Specification (1 year) Result
Test DCI 100 mA 99.9816 mA Datalog -184.0 ppm ±1500 ppm PASS
Test DCI 10 mA 10.00076 mA Datalog +76 ppm ±1500 ppm PASS
Test DCI 1 mA 1.000038 mA Datalog +38 ppm ±600 ppm PASS
Test DCI 100 µA 100.0056 µA Datalog +56 ppm ±600 ppm PASS
Test DCI 10 µA 9.99984 µA Datalog -16 ppm ±600 ppm PASS
Test DCI 1 µA 0.999848 µA Datalog -152 ppm ±1100 ppm PASS
Test DCI 100 nA 99.9884 nA Datalog -116 ppm ±3100 ppm PASS

Application example, measuring resistance

TBD

Author: Illya Tsemenko
Published: March 19, 2017, 6:35 a.m.
Modified: July 2, 2017, 7:49 p.m.

References

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