Hi Bill (et al - this is a global reply):
The main reason I was looking at the 358 is that I bought twelve reels
of them a while ago, and still have six and a half left. It's the only
dual op-amp in an 8-pin package that I have in my home lab, in fact. I
have a regulated +12V rail in my application, and for that matter a
rather wiggly +/-10V available at very low current. I had some crazy
plan to power the 358 off +12, and bias the input up off the 0V rail,
but I kind of stopped there.
Well, you didn't say anything about having 12V available (and we
assumed you didn't). In that case, you probably can use the LM358, but
you'd have to clamp the output so it doesn't cause problems with the
ADC. The LM358 also has a rather large maximum Vos (that means a DC
offset error! perhaps several bits), but aside from that it works
almost down to the - rail. You could get a -0.6V with a diode and
resistor and a +12V from your regulated supply if it was necessary to
get down to the rail. If your ADC can tolerate a couple of K you could
use the second amplifier as a clamp using a diode and the 5V supply as
a reference. If you only need to go to 50mV or so then you wouldn't
need the - supply, ground would do.
I've downloaded the datasheets for the parts suggested by everybody,
thanks for the input. I happen to have a friendly Microchip rep keen to
get her parts into my work, so it will PROBABLY wind up being the uChip
part....
The CMOS parts from Microchip are nice, and not insanely expensive,
but they cost a lot more than an LM358 bought in bulk (probably under
a dime). You do get 500uV Vos, almost no bias current, and few
external parts for the money. OTOH, there are some applications where
the bias current is actually an advantage (detect an open connection).
Best regards,
Spehro Pefhany