How can I buffer a high voltage?

Question

I'm trying to build a high voltage capacitor leakage tester with current limiting feature, below circuit is the simplified and raw idea:

^{simulate this circuit – Schematic created using CircuitLab}

Problem with that circuit is the current passing through the OA1 resistors, that's causing error, discharging the capacitor and essentially makes this circuit useless.

I can't buffer such high voltage and can't increase the resistor value too high because then I have to deal with noise...

• How can I buffer the input voltages of OA1 differential amplifier?

Additional info:

The current is being measured in differential mode but I didn't include that in the schematic.

I want to test capacitors leakage and diodes (LED, Zener), that's why I need to measure the voltage too.

Maximum current is 100 mA.

Input voltage is selectable from 10 V to 400 V in 12 steps.

Answer 1

I suppose the point of OA1 is to measure the voltage?

You can use an instrumentation amplifier with very big resistors (100M perhaps) as a voltage divider.

But you will always have some sort of leakage if you measure something.

Note that OA1 can be at 400V in regards to the ground if M1 does not conduct, so you better be careful with your supplies. The instrument op-amp won't have this issue.

Added: you can reinject the leakage current into the cap to compensate for it, quickly drawn - probably incorrect - schematic to show the point.

^{simulate this circuit – Schematic created using CircuitLab}