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Picoammeter Design

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Does anyone know of any reference material which I could use to design a decent picoammeter? I know of the Low Level Measurement Handbook and a few other articles but I want to know as much as I can because it's such a noise prone design.

I don't know about decent but this design using the LMC662 might be a useful starting point...


Edit: Apart from that, the Keithley Low Level Measurement Handbook that you've already found is the best reference I know of.

I'll second the Keithley Low Level Handbook.

I've found digging out older service manuals for instruments, the ones that have proper schematics, to be very helpful as examples of good design. You could start with the Keithley picoammeter that Dave repaired a way back. The service manual is linked from the episode page.

I watched a documentary about madam Curie measuring the discharge rate of pF capacitor (leak caused by radioactivity of Uranium ore) using electrometer with rotating mirror and beam of light. The electrometer was working as a nullmeter where opposite side was connected to piezocrystal, generating an accurate DC by pulling end of crystal with weight.

So for picoammeter level accurate comparative measurements one needs a piezo DC source, scale, set of weights, electrometer, capacitor with shield, clocks and earth's gravity.

This design can give so many ideas for calibration of picoammeter.

In terms of practical advice....

- Put the picoammeter, or at least your sensing head containing the input amp and feedback resistor as close as possible to what you're measuring (ie, make it small).

- Don't use any cables (in line with keeping it close). Even the low noise ones are piezoelectric / triboelectric / leaky.

- Shield everything in a metal enclosure.

- Air wire everything that's high impedance - lift the IC input pin if using a PCB. Air is the best insulator and isn't piezoelectric / triboelectic.

- Use a decent input connector, BNCs are available in high voltage variants that have an extended ribbed ptfe rear insulator. Use it as one of the supports for the internal air wiring. Don't use a mating BNC plug - insert a suitably sized solder pin on wire into the center contact (you could use the guts of a BNC plug)

- Wash everything in IPA and allow to air dry, then never touch it with fingers.

The close proximity and screening ought to help keep noise under control.


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