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Hacking the Siglent SDM3055 Bench DMM

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Kleinstein:
The rest of the DMM (like gain stage and input divider - already used for the 20 V range) are still more suitable for 5.5 digits. So there is limited use in replacing just a a few points and hope to get much better performance.
It starts with having high impedance up to some 2.5 V only.
At some point is would be more like building your own DMM from scratch.

So I would consider anything more like a single LT6655 (with maybe some simple thermal regulation) overkill. 
The main advantage of an LM399 would be better long term stability - still needs a very stable divider stage. It would also mainly help the 2 V range. The divider (+ buffer)  from a 7 V level down to a 2.5 V (maybe 3.5 V ?) level is probably more tricky than the LTC6655.
One also has to keep in mind that the power supply and thermal design may not allow much more power.

A change in the HW would mainly make sense if there is a simple, easy upgrade possible because of something like a design fault (could be a cost driven decision)  causing an isolated weak spot. Not sure if there is one such weak point -  chances are multiple source of similar size add up.

So a first point would be an in depth analysis of the performance of the original hardware. So how much of the noise is due to the input amplifier, possibly the input buffer, the ADC and the reference ?
For the drift this is more tricky as the result is valid for the one unit only.

alexvg:

--- Quote from: Kleinstein on September 04, 2019, 08:40:41 pm ---The rest of the DMM (like gain stage and input divider - already used for the 20 V range) are still more suitable for 5.5 digits. So there is limited use in replacing just a a few points and hope to get much better performance.
It starts with having high impedance up to some 2.5 V only.
At some point is would be more like building your own DMM from scratch.

So I would consider anything more like a single LT6655 (with maybe some simple thermal regulation) overkill. 
The main advantage of an LM399 would be better long term stability - still needs a very stable divider stage. It would also mainly help the 2 V range. The divider (+ buffer)  from a 7 V level down to a 2.5 V (maybe 3.5 V ?) level is probably more tricky than the LTC6655.
One also has to keep in mind that the power supply and thermal design may not allow much more power.

A change in the HW would mainly make sense if there is a simple, easy upgrade possible because of something like a design fault (could be a cost driven decision)  causing an isolated weak spot. Not sure if there is one such weak point -  chances are multiple source of similar size add up.

So a first point would be an in depth analysis of the performance of the original hardware. So how much of the noise is due to the input amplifier, possibly the input buffer, the ADC and the reference ?
For the drift this is more tricky as the result is valid for the one unit only.

--- End quote ---
You're right, it's possible to use a 5V voltage reference but I need to check if this voltage is compatible with the analog input (AD7190 is compatible).
It's also possible to change the ADC... But I don't want to do this.
I'll need to check the noise from the input to the ADC (I'll need to build a LF amplifier probe).

I've also successfully set to 4800Hz the ADC and done a sinc filter to enhance the final resolution, the result is better than the original "Speed - Slow" (or 5 SPS)

Kleinstein:
I don't think a 5 V reference would be practica, as it's too far of the original 2.5 Vl. However a 3 V or 3.5 V (7 V / 2) ref. level could be working.

For checking the noise source, much can be done with the meter itself, reading data and look at the noise under different conditions. With s short, there should be no reference noise (at least not the normal one). With high gain in front chances are the amplifier noise would be dominant.
With the SD ADC there is however the ADC internal "gain"  (Usually actually only more frequent input sampling), that can make the analysis more complicated.

Is there an analog gain stage, or are they only using the ADC internal PGA stage for the 0.2 V range ?

alexvg:
It's possible to use a +5V voltage reference with the AD7190, but with the design of the SDM3055, I don't know...

Using VDC mode :
- 200mV - PGA = x8
- 2V - PGA = x1
- 20V - PGA = x8
- 200V - PGA = x1
- 1000V - PGA = x1

When I finish the logic analysis, I think I'll do an in-deep inspection of the analog part, and do lots of measurements.

At the moment I'm having trouble understanding the measurement of resistance of very large values (10M to 120M).

Kleinstein:
For very large resistors a common method is to add some resistance like 10 M in parallel. This allow to measure very large resistors, though with reduced accuracy and a nonlinear function to convert from the raw reading to resistance.
As the SDM3055 is more like a budget instrument, I would expect this method.

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