Fix SR830 lock-in amplifier unit error

[liuyichao82]

Fixed the issue that the units of the parameters "R", "X", "Y" of the SR830 lock-in amplifier were incorrect.
(should be current "A" but were written as voltage "V")

[liuyichao82]

@microsoft-github-policy-service agree

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.92%. Comparing base (dee6288) to head (723b1ce).
Report is 10 commits behind head on main.

Additional details and impacted files

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##             main    #7016   +/-   ##
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  Coverage   59.92%   59.92%           
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  Files         342      342           
  Lines       31426    31426           
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  Hits        18832    18832           
  Misses      12594    12594           

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[jenshnielsen]

@liuyichao82 Could you provide some documentation that the output unit is A (current)
In the manual I see

Which explicitly says that the output unit of this command is either V or degree.

Page 5-15 in this pdf https://www.thinksrs.com/downloads/pdfs/manuals/SR830m.pdf

[liuyichao82]

Sorry for the late reply. The confusion comes from how the SR830 handles current input: it converts current to voltage internally using a transimpedance gain (e.g., 1 MΩ = 1 V/µA). So even in current mode, OUTP? returns volts, not amps. When I saw that the value I read using the current mode was in voltage, I thought there was something wrong with the program.Maybe we need to add ISRC? to determine the current input mode. Thanks for catching this!

[jenshnielsen]

Thanks @liuyichao82 I suspect that the qcodes driver does not capture that usecase. If you can find a way to make the driver work better in current mode (while still retaining the functionality in voltage mode) We are happy to merge that.

[liuyichao82]

I noticed that there is a _set_units function in the driver to set the units correctly. I will find an SR830 machine in a few days to continue testing why this function didn't work when I used it last time.

[liuyichao82]

First of all, I need to admit that I made a mistake earlier: when using the SR830 to read, you don't need to consider the transimpedance gain. I believe that the manual's statement that the units returned are always volts is a typo. I tested it with my SR830, and the unit error mentioned earlier will only appear after connecting the instrument and changing input configuration. When connecting the instrument, self.input_config() will be executed to ensure that the units are correct at the time. This error can actually be fixed - just ask ISRC? every time you ask OUTP?, but doing so means asking twice for each reading, which will cause additional resource overhead, and the benefit is only to correct the unit error that sometimes occurs. Is this cost-effective?

[liuyichao82]

I should add that what I just said about "changing input configuration" refers to manual adjustment through the SR830 front panel rather than using qcodes adjustment. If use the input_config parameter to change, the unit will automatically adjust correctly, so I think there is nothing wrong with the program if use qcodes completely, and no changes are needed.