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How to measure 50 pA current with high "noise" current?
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David Hess:

--- Quote from: Kunibert on April 27, 2020, 04:25:32 pm ---
--- Quote from: David Hess on April 25, 2020, 05:38:02 pm ---What you can do instead though is clamp it from the output to the inverting input either with a low leakage FET or low leakage diodes. Transistor base-emitter junctions make good low leakage fast diodes.
If the high voltage edge is consistent, then charge compensation with another edge and capacitor could be used.
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But it needs to sink and source charge due to the positive and negative pulse. For this, the diodes would have to be connected in antiparallel. Wouldn't this lead to an undefined gain?

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Or anti-series so they clamp at a higher voltage.  Diode bridges can be used to clamp at a precision voltage.


--- Quote from: Marco on April 27, 2020, 04:45:29 pm ---
--- Quote from: Kunibert on April 27, 2020, 04:25:32 pm ---I've been thinking about that, too. But most photocouplers are too slow (turn-on/off time up to some ms) and the off-state current is much higher than my measuring range. Could not make it work.
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TLP3440S switches in 100 us and shows 1 pA or less leakage for a 5V signal at room temperature on its datasheet.
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2N4117 JFETs would have been used in the past and are much faster.  3N series MOSFETs would be good but are basically unavailable now.
Someone:

--- Quote from: Kunibert on April 27, 2020, 04:25:32 pm ---
--- Quote from: Someone on April 25, 2020, 10:42:06 pm ---The OP hasn't mentioned what they are trying to extract from the signals, if its just the DC then there are many other options.
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Its "just" the DC-current  :phew: What other options do you have in mind?
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As always the optimal solution depends on the specifics of your measurements, their environment, constraints etc.

You've already the simplest answer in your first post:

--- Quote from: Kunibert on April 25, 2020, 11:56:03 am ---3) The couplings are regular and predictable. It may be possible to decouple the measuring circuit for the points in time and then there would be about 50us left for settling and measuring.
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Now all you need to do is calculate some settling times and find out if there is low enough error left in the measurement.
Kunibert:
I have not found any photocouplers that switch below 1µs. The interference signal has 13 kHz and 75 ns pulse width. Anyway, I have found suitable CMOS analog multiplexers/ switches. The IC has better specifications than I could achieve with a discrete JFET solution.


--- Quote from: David Hess on April 27, 2020, 09:17:01 pm ---Or anti-series so they clamp at a higher voltage.  Diode bridges can be used to clamp at a precision voltage.

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anti-series = -----|>|---|<|----- or -----|<|---|>|----- ?
Can you explain this a bit more detailed? I do not know what that means.


I will try all suggested approaches and will report as soon as I have first results.

graybeard:
Find a way to chop (turn off-and-on) the current you want to measure.  Then run the noisy measurement into a lock-in amplifier synced with your chopping signal.
Kunibert:
I found an analog switch with a charge injection of 0.35 pC.
Do one simply connect a switched 1 pF capacitor to an adjustable power supply (~0.35 V)? Or do one use a dummy switch of the same type?
Does anyone have experience how to set up a capacitance compensation?
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