I have not been following the thread closely, so please excues me if the
comments are off point or redundant.

Making a differential amplifier with good common mode rejection is quite
difficult due to the resistor matching required. Even if you get them to
match at 25C, carbon film resistors have a pretty high temperture
coefficient, so they won't stay matched for long. I really like just
buying a diff amp when I need one. The chip manufacturer has done all the
work for you, putting the precision and tempco matched resistors on the
chip.

>From the subject line, I see the application is current sensing. It is, of
course, much simpler to do low side sensing, since you then don't need a
differential amplifier (you'd be subtracting zero from the sense voltage).
If you need to do high side sensing, there are chips made for that
purpose. I've used one from Maxim. I think LTC also has some. Some have
internal sense resisitors and some use an external sense resistor.

Good luck!

Harold

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