- Exterior, controls and initial condition
- Internal construction, design features
- Diagnostics and secret menu
- Performance test
- Application example, measuring resistance
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.
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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 17-18: High impedance, low leakage amplifier front-end
Image 19-20: High impedance low current circut design
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
Diagnostics and secret menu
Image 31: Vacuum display burn-in mode for segment brightness correction
Image 32: Test with 10pA current with Keithley 6485
No repair was required.
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.
|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
Modified: July 2, 2017, 11:49 a.m.