At 03:07 PM 9/4/2005 -0400, you wrote:
>Hi everyone,
>
>In my thread last week Peter posted a schematic for a virtual shunt current
>measurement circuit. I wanted to know more about this type of circuit, but a
>search on "virtual shunt" combined with some other terms turned up no useful
>links. Could this type of circuit have a different common name?

It's just an op-amp with a "virtual ground". The op-amp forces the input
to ground (give or take Vos of the op-amp) through a feedback resistor.
The output voltage is -(Iin * Rf), ignoring the effect of input bias
current of the op-amp and offset voltage. Those two details (and others)
affect the accuracy, of course.

>Also, if anyone has suggestions or tips for measuring small (10nA-100uA)
>currents feel free to share them! :)

A regular DVM will measure small currents fairly accurately. Often there is
no resistor on the input on lower ranges (eg. 199.9mV F.S.). So a simple 10M
1% shunt  resistor will allow you to measure current with a resolution of 
10pA.
Putting a small *film* capacitor eg. 0.1uF polypropylene (or polyester if you
don't have PP) across the 10M will help smooth the reading. The voltage
drop will be < 200mV, if you can read it, obviously. Go with a lower
resistor for higher currents.

My HP33401A can be set from the front panel to have no input resistor, so with
1uV resolution, I can measure current with a resolution of 100fA, which is
sufficient for many applications.

If you want to go lower still, check into some of the 'electrometer' op-amps,
and the special construction techniques required (I think Bob Pease had an
article on it), which will allow you to delve well into fA territory at
room temperature.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


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