Electronics > Metrology

poor man's calibration? (calibrate 5.5 digit with 6.5digit dmm)

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cnt:
So I have a keithley 2461 SMU and a 6.5 digit DMM at work.  Both new in good condition.  I want to calibrate an old keithley 197 (5.5digit).  I'm not going for NIST traceable, perfect calibration, I just want to do the best I can for hobby stuff at home. 

Is there anything wrong with using the SMU as a voltage/current source for calibration and tweaking the output until I get the correct value on the 6.5 digit DMM.  The SMU has 6ish digits of precision but only 3-4 worth of accuracy.  The 6.5 digit DMM is much more accurate.


For ohms I'm thinking I'll make a resistance box with a high resistance pot in parallel to fine tune.  I should be able to dial it into the resistance I need (1.9eN) and assume its stable for the 1min I need it to be stable.


Any tips, pitfalls, or caveats I should be aware of, or can I just do the obvious thing?  All the equipment is Keithley so I shouldn't have to worry about any off brand electrons :P

(I know won't be able to calibrate the 200V and 1000V ranges this way, or AC, which I don't care about)

bdunham7:
Yes, that will work well enough if your 6.5 digit DMM is good.  You don't have to use the exact listed voltage on the calibration chart, and I think you can even use a much lower voltage for the higher voltage ranges, with some loss of precision.  I've done this sometimes for a 'rough' initial setting when I don't have a calibrator on and ready, and it usually turns out to be pretty close.

cnt:

--- Quote from: bdunham7 on November 25, 2021, 12:49:37 am ---Yes, that will work well enough if your 6.5 digit DMM is good.  You don't have to use the exact listed voltage on the calibration chart, and I think you can even use a much lower voltage for the higher voltage ranges, with some loss of precision.  I've done this sometimes for a 'rough' initial setting when I don't have a calibrator on and ready, and it usually turns out to be pretty close.

--- End quote ---

Ah thanks for the tip!  below is the manual for DC voltage calibration.  It doesn't explicitly say I can use other voltages but am I really just "software trimming" it? 

Could I skip a step and output "1.9v" on the SMU, and then just enter in whatever the 6.5 digit DMM says, rather than trying to fine adjust the SMU to 1.90000v?

Same goes for ohms?  maybe i don't have to manually trim just make the two displays agree on a number?

bdunham7:

--- Quote from: cnt on November 25, 2021, 12:59:37 am ---Same goes for ohms?  maybe i don't have to manually trim just make the two displays agree on a number?

--- End quote ---

Yes, most of the better closed-case-calibration instruments give you the option of calibrating to at least a slightly non-standard value.  In the case of this one, I think it is just like a digital trimpot and you can use any value, but of course your results may not be as good.  For resistance, all you need is stability and a good way of accurately measuring.

alm:
The value should be somewhere in the ballpark (so not 2.0V or 1.8V). Apart from that, it's the noise and stability of the source (over the time span of the calibration), and the accuracy of the reference DMM, that will limit the accuracy of the calibration. For example, according to the Keithley 2461 datasheet, it has 2 uV noise in the 2V range, or about 1ppm of a 1.9V signal. I don't see a spec for short-term stability, but sourcing accuracy (which will include stability and temperature coefficient over the specified temperature range of +/- 5 K) is 0.015% + 300 uV, or 300 ppm of a 1.9 V signal. So the upper limit for uncertainty would be 300 ppm. Short term stability is likely to be much better, but it would be up to you to characterize how much.

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