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DIY-SMU Project

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first of all: awesome project! Using LTspice and looking a bit into Dave's curcuit, I saw that the output voltage can overshoot when the instrument comes out of compliance, depending on the set voltage and compliance. Here's an example: Force voltage is set to 1V, and the current clamp is set to 10mA. The load is just a 1Meg resistor. At 5ms within the simulation I am shorting the 1Meg RLoad resistor with a 1 Milliohms resistor, and keep it shorted until 7.5ms. During the short, the circuit goes into compliance as expected, limiting the output current to the programmed 10mA.

However, there are two transients on the output when applying and, respectively, removing the short:

* When applying the short, there is a short and large current spike (above 100mA), which is ok because I am basically shorting the 135pF output capacitor of the SMU. There is also some oscillation visible which even drives the output voltage below 0V(!), but this does not concern me so much at the moment.
* What concerns me more is that when removing the short, the output volage overshoots quite a lot, it goes up to 7V, after which it takes 1.5ms to recover to the programmed 1V. From my understanding of the DIY-SMU circuit, a voltage overshoot after removing a short on the output is hard or even impossible to avoid due to the fact that the circuit has to deal with the voltage drop on the current sense resistor. As I had set the current sense resistor to 500 Ohms, the voltage drop on that resistor is 5V when the instrument is in compliance, and these 5V plus some additional voltage will end up on the output when the load current suddenly drops from 10mA to a few microamperes due to removing the short.

Looking at the Keithley 236 schematics, I was looking for some circuitry to deal with this situation and reduce the overshoot, but so far I have not found anything that looks like doing so. So, what I am curious of is whether such overshoot is also encountered on other SMUs, such as the Keithley 236?

Thanks and kind regards,


Thanks for the feedback. I tried the same test on 2 units.   For the same conditions: 10mA, 15V range, 1V force, switching from short to 1Mohm load, I get a similar amplitude, but the response is faster, 10uS wide, not the ~1mS you see in simulation.
Send me your .asc file and I'll try to figure out what's up.


Hi Dave,

apologies first, as I saw when looking through my last simulation again a part of the problem was self-made, I was running the circuit in 150V mode (R4=300K!), which is not what I wanted for a 1V output voltage. The attached .asc file has this corrected, I have set R4=20K (that roughly matches R64||R65 on the schematic you published on your web site) and V3=0.333V (which gives Vout=1V as we now have a voltage gain of 3). I also stopped playing with C5, it does not have an effect on what I see.

Please correct me if I am wrong, but from my understanding these settings for R4 and V3 should be what I need for 1V output in +/- 15V mode.

With the fixed LTspice schematic file I see the same behaviour as previously described, but now the transient when removing the short is considerably shorter, in the same ballpark as what you measured (thanks for that, very interesting!). However, the spike amplitude has grown to 16V in the simulation...

Thanks and kind regards



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