Products > Test Equipment
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.
--- Quote ---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.
--- End quote ---
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:
--- Quote from: Kleinstein on April 16, 2023, 07:53:59 am ---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.
--- End quote ---
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.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version