Hi Spehro,

On Sun, Sep 04, 2005 at 04:36:43PM -0400, Spehro Pefhany wrote:
>
> 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.

Thanks for the information. This is very helpful!

Hopefully, I'll be able to take some better measurements soon.

Matthew

-- 
If at first you don't succeed, destroy all evidence that you tried. 
    -- Steven Wright
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