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Hacking the Siglent SDM3055 Bench DMM
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alexvg:
I've found some interresting things :
- Only low-23 of 24 bits are used in the software (the 24th bit is used as a polarity bit - shame)
- I've done some precision simulations using datasheet ADC and I presume the ADC used have an internal precision of 24bits (instead of probably 32bits ADC used in the SDM3065X). Using software filtering and fast ADC sampling rate, it's seems to be possible to have a some better results.
- I've found some better Op Amp at the same price (ADA4528-2 instead of AD8629 and LTC1151 instead of AD706)
- I've received a LM399AH-based voltage reference calibrated at 10.00000V (my SDM3055X displays up to 10.001V)
- Resistance measurement (2 or 4 wires) is more stable than current or voltage measurement.
- The internal +5V, +15V and -15V have some 100Hz (from 50Hz AC).

Kleinstein:
It is kind of normal that 1 bit is used for the sign. So 23 bits + sign is kind of normal. Chances are the noise of the ADC is high enough, that there is very limited value to more resolution. With long time averaging a slightly high resolution can be possible, but this does not need the numbers from ADC chip itself.

The ADA4528-2 OP is not per se better than the AD8629 - they are different:  the ADA4528 is very low voltage noise, but higher bias and much higher current noise. The ADA4528 is good for signal sources with less than some 10 K impedance. However the AD8628 is much better for high impedance sources, like the input divider for the higher voltages.

In case one really want's to get sub 100 nV resolution, it would be more like an extra external preamplifier based on an OP like ADA4528. This could also add more gain and have a higher precision protection part (though possibly lower maximum voltage, more bias). Chances are in the lower cost meters the protection is just a resistor, maybe PTC. This could add enough resistance and noise so that it does not make sense to use the ADA4528.

Similar the LTC1151 and AD706 are good for different applications. So it depends on the use. AZ OPs can add quite some spikes, that might upset other circuit parts.

If resistance measurement is more stable than voltage and current measurements, this indicates that the reference noise is a mayor contribution. This might also be the external reference. A lower noise reference may be one of the few actually possible hardware mods, especially if the reference noise is really that bad. Usually the voltage readings near 0 V should be not effected by reference noise, but a reading near to the full scale (e.g. 2 V range, maybe 20 V range) should.

The chip SD ADCs are usually more like good for the lower end 6 digit meters, as the INL can be relatively high. The other problem is the limited range, so that there can be additional gain errors from resistors. So one may get more resolution (low enough noise), but the accuracy is still limited. These SD ADC chips are good for 5.5 digits, and they can even have a good point: the bandwidth for the input noise can be low, as they may not loose time to AZ phases.
alexvg:
Kleinstein> Thanks for your very interresting post !!!

I've intercepted data from the FPGA to get the used protocol.
Now I could convert the raw value into voltage value using the DMM calibration XML.

I've detected the real range of the analog input (VDC mode) :
- 200mV range : -312.33mV to +312.55mV (without ADC overflow)
- 2V range : -2.4984V to +2.5110V (without ADC overflow)

In VAC mode (100Hz sine)
- 2V range : 0 to +5.0356V (without ADC overflow)

Measurements done using Rigol DG4102 (as a DC voltage generator) and UNI-T181A (as a voltage measurement)

The VDC mode uses 23bits and 1bit for the polarity (sign)
The VAC mode uses 24bits but within a range from 2^23 to 2^24-1 (like 23bits)
The resistance mode (2W and 4W) uses the same limitation as VAC mode (23bits)

I'm also working on 2 mods in parallel : power-supply low-noise preregulation (plug and play) and multiple LTC6655BHLS8-2.5 chip for the voltage reference (need soldering).
alexvg:
It was hard to do but now I could access to ADC raw data.

Done :
- Read keyboard state
- Access to display
- Read ADC raw data

To do :
- Control capture mode
- Speaker
- External trigger
- VMC Output



alexvg:
I've done the investigation of the 20V VDC range... It's very funny :
-31.25V to +31.25V (without calibration)

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