> Just to add to everyone elses suggestions.  Take a look at the
> MAX472. Should be able to design it for whatever range you want.

Thanks, it looks useful for many current-related applications. The
only fly in the ointment at nA ranges might be this, (but I haven't
gone through all the ANs in detail yet) -

http://pdfserv.maxim-ic.com/en/ds/MAX471-MAX472.pdf

Output Offset Error at Low Load Currents.

Large RG values reduce IOUT for a given load current. As IOUT
gets smaller, the 2.5=B5A max output offset-error current becomes
a larger part of the overall output current. Keeping the gain
high by choosing a low value for RG minimizes this offset error


_______________________________________________
http://www.piclist.com
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist