Intermittent voltage offsets

I noticed recently that sometimes my JS220 displays a significant voltage offset when compared to another reference like a DMM. Most recently this difference was about -23mV, regardless of the voltage scale (auto or 5V). See attached pictures. I tried a second Joulescope with the same set up and observed the same issue. Any ideas what might be going on?

Hi @rdietz01 and welcome to the Joulescope forum!

I think that you are seeing the expected behavior of your Joulescope JS220. The JS220 is intentionally designed for extremely high input impedance so that you can accurately measure down to nanoamps.
One of the tradeoffs is that it intentionally omits the resistors usually found in multimeters and oscilloscopes that keep the input from floating. Without these resistors, the JS220’s inputs float and usually drift towards the isolated rails due to the very small (but not zero) opamp input bias current.

With the JS220 you must connect up Voltage + and Voltage - in such a way to keep the voltages within ±15 V of Current +. The easiest way is to short Voltage + to Current +. For more detail, see Section 9.2 of the Joulescope JS220 User’s Guide. It is the Measure only voltage section of How to Connect Your Joulescope.

Does this fix the issue for you?

Ah yes, that worked! I missed this when reviewing the user guide. Thanks very much for the prompt response!