Hacking the Siglent SDM3055 Bench DMM

[alexvg]

Hi,

I've done some little improvements since last post, here is the list :

2023-03-25
- final correction of the clamping measurement
- bug fix of function/range selection

2023-03-26
- better precision (NPLC) of measurement interval (microsecond instead of millisecond)

2023-03-30
- bugfix of random bad measument, the ADC (AD7190) lost the serial interface synchronisation. Apply a fast reset.
- add a better calibration processing from the ADC (AD7190)
- add a better CPU usage (not less, but a better distribution to all threads) 56% (NPLC=1 @ 50Hz)

2023-03-31
- reduce the global CPU usage to 42% (NPLC=1 @ 50Hz)

2023-04-01
- Add measurement of ADC input configurations : Vd (AIN3/AIN4), Vz (AIN2/AIN2), Vr (AIN3), Va (AIN4), Tc (Temperature Sensor)


The first public release will arrive in the some weeks, only if enough features are considered safe and fully functional.

https://twitter.com/AlexiaGossa/status/1643362760892227587

Alexia from France.

[alexvg]

I've done more investigations...

Before the ADC "AD7190", there's a Zero-drift chopper operationnal amplifier, the AD8629.
This op-amp runs at 15KHz and generate some artefacts.

The AD8628/AD8629/AD8630
family uses both auto-zeroing and chopping in a patented pingpong arrangement to obtain lower low frequency noise together
with lower energy at the chopping and auto-zeroing frequencies,
maximizing the signal-to-noise ratio for the majority of
applications without the need for additional filtering. The
relatively high clock frequency of 15 kHz simplifies filter
requirements for a wide, useful noise-free bandwidth.

Source : https://www.analog.com/media/en/technical-documentation/data-sheets/ad8628_8629_8630.pdf

Note :
The Siglent SDM3065 uses a better ADC (the AD7175-2) but with the same op-amp AD8629 (with the same artefacts...).




All magenta marks are power line frequency/harmonics.
All red mars are artefacts :
233Hz at -124dB
461Hz at -141dB
539Hz at -142dB
561Hz at -136dB

Frequencies of 233Hz and 561Hz are artefacts between 15KHz output of the AD8629 and internal sampling frequency of the AD7190.
Not sure for 2 others... (461 and 539Hz, a link with the 500Hz power line harmonic ?)

I need to experiment a filter to remove the 233Hz frequency.

An other solution should be to replace the AD8629 with a ADA4528-2 (better features and an internal frequency of 200KHz)... For the moment, I'll not to make any hardware modification.


Alexia from France.

[Kleinstein]

The AD8629 is a reasonable good compromise between input bias current, current noise and voltge noise.  The ADA4528 has much higher input bias current and current noise - so it would not be a good  choice. Chances are there could be similar interference with other amplifiers too. Overall the artifacts looks small and at an acceptable level.

The normal reading would be averaging over some 100 ms and this alone would attenuate the 233 Hz quite a bit, even if the frequency is not a notch of the response curve.

[alexvg]

The AD8629 is a reasonable good compromise between input bias current, current noise and voltge noise.  The ADA4528 has much higher input bias current and current noise - so it would not be a good  choice. Chances are there could be similar interference with other amplifiers too. Overall the artifacts looks small and at an acceptable level.

The normal reading would be averaging over some 100 ms and this alone would attenuate the 233 Hz quite a bit, even if the frequency is not a notch of the response curve.

With this kind of artifacts, my value search algorithm is disrupted. Perhaps I probably will add an auto-calibration to detect all artifacts on all calibrations and remove them before processing my algorithm. My algorithm uses noise to try to improve accuracy.
Actually, I've reached 7.4 digits / 24.6bits at NPLC=100 over 24 hours (with shorted ADC inputs) without my algorithm.

[alexvg]

Hello,

I've divided my software into 3 main parts: acquisition (hepha), processing (swani), and display (zunia).
Now I've finished the acquisition part and then I've discovered a problem with the Siglent SDM3055.

When there is a power outage for 20 to 40ms, at best, the measurement is distorted (compared to a Keysight 34470A) and at worst, the device reboots !
I discovered this after experiencing small power outages because I had left my UPS in line-interactive mode (my UPS has several modes: 100% online or 100% line-interactive, changing the output frequency to 50 or 60Hz...).
This means that the SDM3055's power supply is under-dimensioned. Not very good for a measurement device.

Since this week, I have been working on rewriting the processing part (swani).


Globally the rewrite is going well, and I am satisfied with the functionality, which is much simpler and more stable but it takes time.

Thank you for your support.


Alexia from France.

[Bill3745]

Hello Alexia,
I have been following your work on reverse engineering the hardware and redesigning the firmware/software of the SDM3055 and congratulate you on how far you have come and what you have been able to accomplish without any hardware modifications to this fairly low end benchtop DMM.  Per your comment regarding powerline outage distortions, do you know if the Keysight uses a linear or switching power supply? Since it costs around 10x the Siglent, I guess we shouldn't expect the same level of performance 

Thank you for your continued work on this and I would be willing to sign up as a beta tester when you are ready to enter that phase of development.

--Bill

[Kleinstein]

Most of the bench meters use a classical transformer - ideally a custom one with extra shields and isolation.  A few than use a switched mode regulator for the supply of the computer part - though less so with the more modern ones as the power needs have come down.
The point to get better tolerance to missing mains periods would be larger filter capacitors or more voltage reserve (and thus more heat).  A larger/additional filter capacitor may be an easy change if really needed. For a reboot chances are this would only be for the logic / µC parts not for the analog part.

A bench meter is normally used with a more well behaved supply in a lab environment, not so much in an more problematic indistrial environment. Still the normal switch over from an UPS should ideally be tolerated.
A better tolerance to supply glitches would still be nice, when using it for automated longer time data logging or experiments. I would not complain much about a distorted measurement as a short time effect - chances are other instruments and the signal source would also no behave than well. So one would likely have a glitch in the data anyway.

[alexvg]

I felt like thinking about something else... with a bit of painting and good weather!



https://twitter.com/AlexiaGossa/status/1662904304099893249

I will answer your questions within a few hours or a few days.
Thanks for your support.

Alexia from France.

[alexvg]

Hello,

The Keysight 34470 uses a "conventional" transformer, just like the Siglent. However, the power supply of the Siglent is undersized for voltage stability variations.

View of the transformer of the Keysight 34470:
https://www.flickr.com/photos/eevblog/16605656689/

View of the Siglent SDM3055:
https://www.eevblog.com/forum/testgear/siglent-sdm3055-first-impressions/msg541312/#msg541312


And I did a bit of painting today...



Have a nice painting day ! 

Alexia from France.

[bobxxlua]

Hello, where can I download your latest updates? I can be your beta tester. Thanks for your work

[uargo]

It´s alive or dead?

[alexvg]

Hello everyone.

I have both bad news and good news, in that order.
For several months I have been absent from the forum and from my project for a new firmware for the SDM3055.
I have been facing serious health problems for several months (https://twitter.com/AlexiaGossa/status/1680524683999944705 and https://twitter.com/AlexiaGossa/status/1714078292603490674).

But the good news are here! My health is gradually improving, but it takes time. Patience is required.
Little by little I will be able to resume my work on the SDM3055.
Another good news, Mouser has informed me that my pending components are finally available.
Over the past few months, I've been jotting down ideas in a notebook for my work on the SDM3055 so as I don't forget anything.
The work on the SDM3055 has opened up another path for me, a medical one : EEG and ECG.

I have received questions about the SDM3045 and SDM3065 asking me if my firmware can work on these models. Theoretically yes for the SDM3045, but I can't test anything, I don't have this model. For the SDM3065 this is currently not possible, there are too many internal differences.
Obviously if I had permanent access to these 2 models I could adapt my firmware there, but each thing in its time.
First health then the SDM3055.


I'm doing my best, but you have to be patient.
Sincerely, I thank you for your support.

[tautech]

Alex, GWS.

Some reading for you here and in the thread and the one it links back to about SDM3045X:
https://www.eevblog.com/forum/testgear/sdm3045x-to-sdm3055x-e-improvement-thread/

[ZhuraYuk]

Wish this would be for SDM3065 as well. It is already fast enough to add one extra digit.

[christopher.robot]

Would love to know if we can support you in some way!

I certainly wouldn't mind donating just to support the work you're doing, regardless of the outcome.

Either way - wishing you a speedy recovery.

[alexvg]

Hello,

I wanted to give you a quick update. Regarding health, there has been no improvement, but the positive aspect is that it isn’t getting worse!

From time to time, when the pain is manageable, I manage to make progress on the project (in addition to doing other small things). Additionally, I’ve decided to open a Twitch channel which is currently in the testing phase (but only in French) with replays on YouTube. I will do live streams, particularly to show progress on this project where I will code and test my developments. There's no fixed schedule; I will stream at any time, "random" streams.

I'm not sure if I'm allowed to mention the names of my channels here, so I've updated my signature.

Thank you for your support and your personnal messages.
Alexia.

[rolfdegen]

I hope they get well again soon. I wish you much health and happiness 

[rolfdegen]

I have now finished my modified Siglent SDM3055. I didn't use the various low noise adapters for the Noctua fan. They had a resistance of 50 ohms and 147 ohms. Instead I soldered a 75 ohm resistor to the fan's 12V connection. He is very quiet. I also gave the processor a heat sink.

SDM3055 mod with Noctua Fan and heatsink


Used Noctua fan NF-A6x25 FLX

[Inovet]

Very good!

The original fan was too noisy. And even if it were simply noisy, it would also create resonances at unpleasant frequencies. After some time I replaced it. Nocntua is very good, but I installed Noise Blocker.

[KungFuJosh]

I installed Noise Blocker.

Do they have a CFM and dB rating like the Noctua chart?

[rolfdegen]

It seems similar to Noctua.. Should it be here!? I would take PR-1.

[KungFuJosh]

It seems similar to Noctua.. Should it be here!? I would take PR-1.

PR-1 is insufficient if you're matching the flow of the stock fan. The PR-2 would be the correct choice, but performance/noise isn't any better than the Noctua. That said, they both look great on paper.

[Inovet]

There is something in the technical characteristics. But at that time, I bought this one.
Noctua might have been better.

CFM by press... The pressure should be better. The air flow is also better. In terms of noise level, it is necessary to compare it in practice.

On my Rigol DP832 the fan was replaced, sorry - a little offtopic by Noctua NF-R8 redux-1800. It's much quieter with the same performance. But even greater silence was created by replacing the standard grille with an ordinary wire one. It turned out a little ugly - I was afraid of iron filings. Without completely disassembling the device.

[Inovet]

I measured the temperature inside the multimeter case. Without fan, with standard fan, with new NB1. With new fan - temperature were like the standard ones. Without a fan - high.

And everything would be fine if the multimeter did not freeze at the start one day. Thanks to our member tautech for the quick help. There was only a problem with calibrations, but it was relatively solved.

And, recently -2 weeks ago end of April 2024, it froze again. stopped. It’s good that the homemade calibration files were saved this time on comp and cloud.

In our Siberia, in my apartment the temperature in both winter and summer is +28 Celsius. This is almost the upper limit; it is difficult to reduce the decrease and it is not necessary. I do a little amateur electronics out of old memory.