> 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.

Yes. Easily enough calculated from the datasheet.
Ioffset x Rin is error from offset current.

> 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.

As Bob noted

>> The LM358 has a PNP input stage and is fully valid to
>> the negative rail. Input voltage is only limited near the
>> positive rail for this opamp.

So, that says that the cheap op amp will handle a 0 volt input.

The output needs to drive the FET into cutoff. This is at somewhat
below Vth  (threshold). This is typically 1+ volts for most FET's
unless they are intended for very low gate voltage operation. The
LM324 is liable to be able to easily drive a MOSFET into cutoff unless
you choose an exceptionally low Vth one.

> When the accuracy requirement is not as high, we
> use Rail to Rail input OpAMP like OP213FS.


See above.


           R
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