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0,2V Offset on DC Measurements with Picotest M3511A DMM
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MaxH:
The divider network is a CADOCK 1776-C44
The offset error can bee seen until the 100V range, 1000V range has .14V offset (maybe the resistance here is too high). 100mV range has an overload. (I guess it would be also 200mV)

I followed the current path through Q203 and Q202 (high and low current) to R217 (bigger chip under U404) to K201 and K203, after that it also goes to U404 (analog multiplexer)
I didnt manage to find the point where the AC part joins the DC.
Especially since the ground path is indeed a bit confusing.

I will try to draw down some of the paths to better understand where the .2V might occur the first time. (still not 100% sure against what I have to measure them to be sure)

Btw thanks a lot for this amount of help so far. And also for even contacting them.

Attached are photos of the AC part and the "middle" part with the ADC
MaxH:
Small update:

The 0,2V are only seen on the minus input side or "ground path", not on the + input.
If i measure a voltage with the DMM from + input to the symetric ground (between the +8V and -8V rails) I get correct readings.

Are .2V maybe something that is needed for measuring resistance etc and are now constantly on?

Greetings,
Max
Kleinstein:
I see 4 possible resons to have the negative side not directly connected to ground. One is the ohms mode. There are different ways to realize excitation for Ohms, especially 4 wire ohms: some keep the common ground and some make the negative side move or have the current source (some Fluke meters). 

Another reason could be getting a larger input range without using high voltage switched. The Sigilent SDM3065 likely uses this to get a high Z 20 V input range with only a +-15 V supply and also switches for more like +-15 V. In this case it would be normal to have a difference there. Not sure if the 3510 has a high Z mode at all, and likely not 10 V, as the input divider seems to use 3 divider ratios. So the 10 V range likely still goes throung the input divider and thus +-1 V (plus some overrange) for the primary range for the ADC.

A 3rd reason may be the ADC, that has a differential input. Many of the SD ADCs get the best INL when the differential input is driven symmetric, without much common mode signal. So the neg side input is no longer at the power supply ground, but movonmg opposite to the positive input. One way to get this is to generate the inverted signal and also use is as the low side input and reference level for much of the input circuit.  It is not a common solution and the divers for the ADC inputs look like 2 differential amplifiers, which is an alternative realization.

Finally it could be simply the supply to the ADC with 0 and 3.3 or 5 V. So an virtual analog ground at some 1.5 V could be a simple idea on how to have one ADC input at this fixed level and use this also for the neg side input. However 0.2 V would be rather low for this. The differential amplifier at the ADC input also doesn't fit this way.

The result from the RMS converter likely joins back at U403 (MUX)

Trouble at the ground side could also explain, why the offset is 0.2 V also for the ranges with gain.

Finding a good ground point is indeed a problem. The middle of the +8 V and -8 V is for the supply, but his may not be the signal ground.
The divider chip R503 (below the LM399 reference) may be an interesting point. This should set the possible gain for amplification (likely x 10) for U501 as kind of the main amplifier for the input. So the pin facing towards the LM399 should be the refrence level for the analog signal. This point should have the same potential as the low side input / common terminal.
David Hess:

--- Quote from: Kleinstein on May 26, 2021, 06:19:39 am ---For the high impedance (e.g. > 1 Gohms) ranges one could measure the input bias current by whatching a capacitor (1-10 nF low loss like  PP, PS or C0G) charge / discharge at the input. So there is no need for an electrometer.
--- End quote ---

If you could watch the capacitor charge or discharge, then the capacitor would not be needed and the current could be measured directly.
beenai2018:
Edit- Fixed My meter.
Notes here https://www.eevblog.com/forum/testgear/keithley-2110-documenting-fixes/
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