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How I made my SMU

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I've been intrigued by SMUs for a long time, but since I've seen project of djerickson [1], I decided to give it a go. I put together bits from his schematics, added some of mine, wrote my firmware for both microcontrollers, made two revisions of PCBs, made my own transformer (it's surprisingly difficult to obtain materials for that) and here is the result. The component crisis in 2020/2021 made me re-evaluate a lot of components sitting in my junkboxes unused, so I decided to make use of them. Therefore, there is a few questionable component choices, but I totally don't feel bad for it. I'm glad the components got second chance to do something useful.

Whole design, including manufacturing files is public on github [2] with picture dump at [3]
My design goal was to have an 4-quadrant SMU capable of delivering at least 100V and 100mA, with resolution on lowest range 1pA. That last one wasn't trivial, but it looks like the last digit on current measurement is reasonable. I can observe slight drift with temperature on the last digit, but for short term measurements it's perfectly usable.

I have no definitive specs validation done, but it looks like after proper calibration it's better than 0,01% at source accuracy and even better and measurement. I tried to run a few more tests, for example linearity test - I observed output voltage, measured by SMU itself and external DMM (HP34401A), at 120V range. I can clearly see major component of the DAC quantization error (DAC is only 16-bit), but internal measurement is in much better agreement with HP34401A, see the attached picture.
Right now I included this SMU into my instrumentation rack and I started to use it as everyday instrument, so I guess I'll do some more testing.

Attached is also documentation package, slimmed down github snapshot at the time of writing(without Kicad PCB sources, those were too large for an attachment here, but available at github).

[1] https://www.eevblog.com/forum/projects/diy-smu-project/msg3391690/
[2] https://github.com/jaromir-sukuba/J-SMU
[3] https://imgur.com/a/rR8cIFG


Tidy schematics, tidy boards, tidy build, and well documented too. Very nicely done sir.
Seriously tempted to have a go at building that.

Really impressive project.

The linearity measurement looks a bit like there is some signal effecting the ADC reading where it should not. Otherwise I would have expected better matching between the 34401 and read back ADC.  The OPA2333 to drive the ADC looks a bit odd: the OPA2333 is a very low power OP and relatively slow and thus not very good with pulse like loads from the ADC. The 1 nF cap at the output of the OP could also cause problems - though the ADC likes it.
If DAC resolution/linearity is an issue, could it help to use the 4 th ADC channel (still unused) to read the DAC ouput ?

The switching at the current reading shunts may have quite some leakage. With better parts availability I would have choosen DG21xB instead of DG444. There are some leakage paths parallel to the 2 M shunt, so some drift for the low current ranges is to be expeced.
There may be a better configuration for shunt switching, though I know that this is a rather difficult part and always needs some compromises. The more brute force way would be to use relays, at least for the higher currents.

Another great project from you... Really nice work...


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