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Hacking the Siglent SDM3055 Bench DMM
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Kleinstein:
It is not a surprise to find some hum for the high resistance values. Normally the high Ohms ranges should use relatively high voltage ranges. Here this seem to be the 2 V range. It may take some shielding to get good readings. Ideally the DMM would detect if there is too much AC background (e.g. to cause clipping in peaks).

144 mV_pp is not too bad, less than some 5% for the full range.  For a good measurement one may need to use a shield to keep hum away and maybe avoid using long unshielded cables.

For the 10 M range 4 wire Ohms measurement does not make much sense.
alexvg:

--- Quote from: Kleinstein on September 14, 2019, 08:15:00 pm ---It is not a surprise to find some hum for the high resistance values. Normally the high Ohms ranges should use relatively high voltage ranges. Here this seem to be the 2 V range. It may take some shielding to get good readings.

--- End quote ---
The voltage range could go up to 2V, it's current generator of (around) 200nA


--- Quote from: Kleinstein on September 14, 2019, 08:15:00 pm ---For the 10 M range 4 wire Ohms measurement does not make much sense.

--- End quote ---
I think you're right :
- Right because it's a nonsense to mesure high resistance value.

But I need to add something I'm thinking :
- Using a 4-wire cables it's possible to use shielded cables and differential input (for sense wires) for a more precise measurement.
But I could be wrong with this, I'm not sure (never tested, never simulated...)


I've also notice a strange math simplification inside the original software :

You could write :
f1(x) = 1/(1/x)
f2(x) = x
It's okay to say f1(x) = f2(x)
But it's false for a CPU !

Original Siglent Software uses a function like f1(x) that could cause a "divide by zero". But without simplifying the function, it's possible to avoid the "divide by zero" like f2(x). I don't understand why Siglent have done this...

Kleinstein:
The capacitive coupled mains hum during the high ohms measurement is present at the DUT and the hum is thus directly added to the voltage to measure.  So a differential measurement does not help here. The main things that helps is the usually good suppression of mains hum by the ADC, because it is integrating over a multiple of mains periods.
Shielding for the high ohms measurement can be tricky, as there may be more leakage. Some high end DMMs offer a special driven shield for ohms shielding, that reduced the voltage over the isolation and also help a little with settling.

I sometimes makes sense to measure high resistors, but in the MOhms range one does not really care about cable resistance and could thus use a 2 wire measurement.  Some meters only offer 4 wire Ohms for the lower ranges.

Using only 2 wire ohms reduces the cables to 2 and thus less leakage and less surface to pick up . So there is a point at high resistors when 2 wire ohms is more accurate than 4 wire ohms. It is not just the external leakage, but also leakage at the input protection.

alexvg:
Measuring using 100M range with this device is a joke (using 4-w and 2-w).
The precision is awful (even with 50 and 60Hz filter)
Kleinstein:
The accuracy gets worse with high resistors already due to leakage and bias currents (e.g. at the protection). With only a 2 V range (compared to often 5-10 V for other 6 digit DMMs) this also makes things more tricky. 4 Wire resistance is likely worse than 2 wire. The 100 M range is about 10 times less accurate than the 10 M range as there is the 10 M in parallel and thus only a relatively small change used.  Anyway getting 1% or so accuracy is not that bad with such high resistors.

A really accurate measurement would need a special instrument, more like an electrometer. This usually is with applying a fixed (e.g. 20 V or 100 V) voltage and than measure the current. This also need shielding and special, short cables. Also touching the resistors with the hand can change things from contamination.
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