I need help understanding a SMU voltage measurement circuitry

[DanielDlouhy]

Hello everyone!
I am working on designing my own Source Measure Unit and I was looking how to make the voltage measurement circuitry. The output is floating, so it has to be a differential measurement. Because my biggest inspiration is the mighty Keithley 236, I looked there for inspiration. And I don't seem to understand the circuit fully... Because in my head it can't work up to 110 V, as it should. Even the simulator (falstad - website) didn't help.

I also found a simplified schematic made by Jaromír Sukuba, that he used in his diy SMU. (see attachment)
https://github.com/jaromir-sukuba/J-SMU

Just to clarify, the 15V0+ and 15V0- are floating. But still, the voltage difference between SEN- and SEN+ is up to 110V and I don't know how this circuit is able to measure that. When the input op amps can't handle more that about +-15V and it's even clamped using diodes D4 and D5 . I know it has to be something with the floating power supply, but I can't quite understand it, can someone help me understand it, please?

Thank you in advance!

[jaromir]

Hi there.

Potential on SEN+ is close to +-15V ground potential (the difference being votlage drop on current sense resistor)
Potential on SEN- is negative with respect to +-15V ground when output voltage is positive. +-15VO ground potential is floating near SEN- potential. Voltage on U11 output follows its input (that is, SEN- voltage), and is divided down using R29/R28 divider, and therefore brought to levels within +-15V range.

My name is Jaromir Sukuba.

[DanielDlouhy]

Thank you very much, or should I say Děkuji mockrát!
I didn't realize that SEN+ is so close to the ground potential, now it makes much more sense... I will take a look at this more, and I may write again But for now, a big thanks!

And I am really sorry that I misspelled your name, I will fix it right away. 

[MathWizard]

So what do people test with high voltages like that? And do SMU's use AC sweeping voltage like a Curve Tracer ? I gather a SMU can be a current or voltage, sink or source, is it basically just DC tho, and people still use the HV for testing transistors/diodes or even leakage testing caps?

I copied a Heathkit Curve Tracer, but still never finished the op-amp section, and now I want to use a micro-controller with it. I'd love to try to copy an old, low voltage version of these, or just leave out most the HV section.