Under conditions of very high source impedance or conditions that create a ground loop (other conditions apply as well), you may want to try removing R41 from the ADMM-CP (VOM) board (or testing with a board that doesn’t have it installed); if, after firmware updates and confirmation of measurement protocol, measurement values still seem inaccurate, outside of calibration error allowances, or highly variable.
Bench testing under simulated anomalous or difficult source conditions demonstrated that in some conditions, removing R41 may potentially improve measurement results. Or it could replaced with an 0402 TVS protection diode, as a test.
Examples of potential high source impedance scenarios, depending on other factors, could include: AC voltage coupled onto the structure from nearby powerlines, or measuring native or naturally occurring electrochemical phenomena with high soil resistivity. Another is measuring soil electrical properties where local energy is quickly depleted while recharge from surrounding areas is limited by higher bulk soil resistivity. Measuring voltage/current imposed on a structure by a rectifier is probably least likely to be a high source impedance situation.
Conditions really do depend on several factors. However, the “R41 thing” is nonetheless a useful adjustment option to understand and/or test.
With R41 installed, the hardware still meets specifications and passes tests for a range of conditions, including higher source impedances. However, this component is a very real difference between a pure handheld DMM and this ADMM/DLITE/online system.
R41, if installed, is used to limit the common-mode voltage imposed on the measurement circuitry – that is, it is there to keep the measurement circuitry on the board from floating too far out of bounds, relative to the main board or the main system. The measurement circuity on the ADMM is isolated and can generally float higher or lower than the main (DLITE0100A1) board’s ground level. R41 is intended to keep it from floating too far. It may rarely be needed and is a sort of protection component.
R41, if populated, also typically has a very high value, depending on the component selection (maybe 10 to 100 Meg). Which is why generally, operation is “transparent” to its presence. Especially in bench/lab conditions.
However, in some situations, the current movement (we could almost call it leakage) through that resistor could potentially shift or disrupt measurements.
On the bench reference ADMM board here, the component currently remains unpopulated.
If you experiment with this adjustment, what you find may be very useful, so please do communicate any findings.