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| Keithley 196 AC Voltage Measurements Incorrect |
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| RPP:
Thanks KleinStein. I'll take some readings on U27 and run some tests on the lower AC ranges this weekend. Rich |
| RPP:
Some progress. The AC readings on the 300 mV, 3 V and 30 V all read fairly close to actual, so it's just the 300 v range. I measured pin 14 on U27 with the leads shorted and got 0 VAC (using pin 3 for ground). With the leads open, it also measured 0 VAC. So it would seem it just needs a calibration. Anything else I should check? Rich |
| Kleinstein:
If U27 pin 14 reads near zero and the DMM a totally off, this really looks like bad calibration constants. There could be a slight offset or background from noise, but this should not be much. Subtracting is tricky with with AC anyway, especially if there are 2 parts. If the DMM allows separate calibration of the 300 V AC range it is fine. These cal constants are known bad, so nothing to loose there. If the calibration could only be done as a whole it would be more tricky to somehow save most of the old cal constants. I don't know this meter much (it is a long time ago I used one), but some meters allow to read and save the cal constants via GPIB, just in case something goes wrong with the calibration. |
| RPP:
Looks like i can calibrate just the 300 VAC range, so I'll cobble together some crude voltage sources for the 30VAC and 300VAC reference points needed. It won't be super accurate but at least I can determine if everything works properly. If it does hold the new calibration constants, I'll probably send it in for a proper calibration. Looks like i can get it done for $80 + shipping, so it would be worth it to have an accurate DMM on the bench. Thanks for your help. Rich |
| RPP:
Calibration complete. The 196 is extremely easy to calibrate - I only did the 300 VAC range and it allows you to input the actual voltages you are using for high an low points so I cranked up about 140V at 500 Hz which is the most i could get out of a 3VAC/120VAC transformer wired in reverse to my function generator. For the low point I used 20 VAC. In both cases, I just entered the actual reading from my Tek scope which is calibrated. Works perfectly now. Rich |
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