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[cleaningOut]

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[dannyf]

I'm not having much luck with an LM324-based transimpedance amplifier circuit.

The input bias current would be too much for that opamp.

I would get a cmos or at least jfet opamp for that.

Short of it, use a jfet or mosfet pair in front of your lm324 and you are ready to go.

National (?) has a few excellent appnote on pa-level current measurement.

[dannyf]

The datasheet shows typical input bias current of 40na and max of 100na, depending on which ones you see.

That's almost 10% of 1ua.

On the other hand, most (genuine) opamps will come within the spec so maybe you will get luck so give it a try - if it doesn't pass, you can try other alternatives:

1) alternative #1: use a jfet. Go to your local radioshack and get a TL0xx: they have a typical input bias current of less than 1na (10-20pa typically).

2) alternative #2: put a mosfet pair in front of your opamp - this is often used to improve input stage performance or to add output offset adjustment capabilities.

Here is a basic nanoammeter, with a nmos input buffer, with a 1v/1ua gain, set by R4.

For it to be practical, you will need to implement offset adjustment (easily done on the input's legs), etc. Resistor values, aside from R4, are not critical.

[dannyf]

Size R4 to suit your project.

Yes, higher current is easier to measure.

[dannyf]

You will need to make minor changes for single rail operation.

[dannyf]

Glad that it worked for you.

Those ammeters are fairly easy to make, especially if your read-out is a coil meter or a battery powered dmm - single rail would be sufficient in those cases.