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  • Author Author: Jan Cumps
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DIY Source Meter Unit - initial thoughts

Jan Cumps

I'm thinking about building a low power SMU.

We're playing with OpAmps and discrete semiconductors regularly over here, and there's a Keithley 2450 SMU with I-V Tracer Software  road test going on.

Learnings from those activities can be applied here.

 

image

image: over simplified concept

 

Goals:

Only two;

  • Make a Source Measure Unit.
  • Make it programmable or scriptable

 

Possible capabilities:

This would be a low power instrument, µA up to maybe a few mA. Low voltage. A utility that can be used with analog low power devices.

Source and sink voltage and current. positive and negative.

Constant current, constant voltage.

(undefined) precision and speed

 

It should be possible to make this modular:

  • the driver part
  • the measure part
  • controller and converters
  • human and digital interfaces

 

I'm going to read up on the subject now. Ideas and support are welcome.

  • in reply to jc2048
    I was thinking of separate modules for driving current and voltage, though I can see how they might be combined.

     

    I had some inspiration from the DAC8775 used on the Quad-Channel, Analog Output Module  RoadTest kit (I got 's board, traded it with him for other electronics).

    They use a separate current and voltage driver, then combine the outputs:

     

    image

    from https://www.ti.com/lit/ug/tiducv5/tiducv5.pdf?ts=1591530663544&ref_url=https://www.ti.com/tool/TIPD215 , page 2, 3 and 5

  • This is interesting. Several times when I was doing the transistor blogs I wondered about about doing some kind of modular test setup [though it was fun designing the ad-hoc circuits and wiring the prototype boards each time]. I couldn't make up my mind whether separate boxes for each function would be best or some kind of mainframe with plug-in boards.

     

    It shows how ignorant of test and measurement I am that I didn't even know that SMUs were a thing, though I had come across curve tracers and was vaguely conscious of automated testers with 'pin drivers' and all that. I obviously don't move in the right circles.

     

    I was thinking of separate modules for driving current and voltage, though I can see how they might be combined [though, if you do, it kind of then starts to move towards what Michael is doing with his impedance analyser - I'd worry then that the spec is going to run away faster than you can chase after it]. I also toyed with the idea of using an isolated serial communication bus between modules [maybe something like a fast MIDI, or differential like DMX], to save the cost of working with ethernet [I'd have had some kind of a hub, with a small SBC, to relay stuff back to a laptop].

     

    One complication for me, with the MOSFET tests, was the necessity for doing quick tests because of the effect of temperature from the dissipation. That then moves you away from DMM type techniques for the measurement to something that works sample and hold, with the timing and synchronisation issues that introduces [as far as I can see, SCPI won't be very good at synchronising and sequencing things at speed]. Although you might think that temperature effects would only be important for power devices, I was seeing such effects at much more modest levels with very small packages like SOT-23 and smaller.

     

    Anyway, it will be interesting to see what you come up with.

     

    One request: please don't put it all in one blog. That programmable load blog with 16 pages of comments is getting impossible to trawl through.

  • in reply to genebren

    Certainly for the controller - converter part. Right away.

     

    The interface part can wait as long as it's defined what core functionality is needed to enable programming and front panel control.

  • I would like to contribute.  If I can keep up with you guys.

  • Jan,

     

    Let me know if you need any help. My strengths would be in the area of controllers/converters and human/digital interfaces.

     

    Gene