On Mon, 15 Mar 2010 13:19:09 +0800, "Xiaofan Chen" said:

> 
> Ok, just how low this current source can go down to? Say if I
> drive the Vin+ with a 16bit DAC (0...1.25V), how good the
> resolution of the current source can be? The input offset
> and leakage current may play a part though.
> 
> We do not use LM324/LM358 but rather ADI OP2177
> and sometimes we do need the negative rail to achieve
> good near 0mA current depending on the accuracy requirement.
> 
> When the accuracy requirement is not as high, we
> use Rail to Rail input OpAMP like OP213FS.

The LM358 is not a precision opamp, it has a 2mV offset voltage spec, so
untrimmed it wouldn't be better than 12 bits. 

If you trimmed it, the drift is pretty low so you'd be OK. Put a
resistor in series with the inverting input that matches your DAC and
that nulls out the offset current. 

But to hit 16 bits the offset voltage would still need to be trimmed,
and it doesn't have pins set aside for that, I'd trim it in software. 

Cheerful regards,

Bob

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