Hello, I recently bought a new “surplus” Keithley 2015-P DMM and while experimenting with it, I noticed some weird behavior when measuring the DC offset of a mixed AC/DC signal coming from a Siglent SDG-1032X generator. With the meter set to DCV and connected directly to the output of the generator, I set a sine wave, 1kHz, 4Vpp and 2V offset at the generator. Things seemed fine initially w/ the DMM reading something like 2.08V. Then I turned the generator offset down to 0V, and back up to 1V. The DMM then began reading something like 58mV. Slowly turning up the offset, a large error remained (500mV+), until suddenly at an offset of around 2V, the DMM reading jumped back up to 2.08V.
Thinking something isn’t right with the meter, I did the same test on a Fluke 87V. The reading remained stable, DCV read about 80mV high no matter what the offset or how frequently I changed it. Finally, I performed the same test on an HP 34401A, and had the same exact result as with the Keithley; the only difference being that the threshold “jump” correction voltage offset value was 1.7V instead of 2V.
Can anyone explain why this might be? Turning on and off the generator output when the offset was at 1V and Keithley was reading around 58mV (72mV on the HP) made no difference. In order for either to read the DC offset again correctly, I had to raise it up above a certain threshold, then lower it back down.
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