Design of a low current source

[anushaas]

Sir,

I need to design a low current source to test an electrometer circuit.The expected currents inputs are in the range from 1pA - 1 nA.This will to converted voltage of a measurable range say 0-10V by the electrometer circuit

Can anyone tell me how to design a current source generating current of say a few tens of pA?

 

[dick_freebird]

I would use the smallest MOSFET you can get, with a high
value source resistor that you sense to control current
(needing a very high impedance input op amp). You will
need to think carefully about wiring, using approaches like
triaxial (shield, equipotential guard, signal) cabling to cut
leakage in the harness and so on.

10pA on a 10Mohm resistor is still sub-mV. That's going
to be tricky.

You might consider very-low-duty-cycle PWM techniques,
delivering a quantized charge to a filter. But you have the
problem of leakage there as well. I hope you do not need
a very high and repeatable accuracy.

 

[Easyrider83]

Sir,
1pA - 1 nA.

I think, it is impossible.

 

[FvM]

I think, it is impossible.

You can buy commercial instruments that generate respective currents. They have been mentioned in previous threads of the O.P.

 

[Easyrider83]

You can buy commercial instruments that generate respective currents. They have been mentioned in previous threads of the O.P.

I mean impossible in home conditions.

 

[mtwieg]

It's not hard to generate currents in that range, but regulating it accurately is very difficult. In order to do so you would need a device that has a higher impedance than the electrometer you're trying to test, so you end up in a sort of catch 22 situation.

If I were you I would try to obtain a picoammeter (you can find ones from keithley used for cheap, I bet) and just measure the input current manually.

 

[FvM]

I assume, that it's easier to verify zero and scale factor of your current amplifier than to design a calibration or reference current source. The problems starts of course, if source impedance and similar additionnal parameters have to be considered. If the current source connected to the amplifier in operation is known to have suffcient high impedance, it's less critical.