>
> Note that the voltages you'll be dealing with will be on the order of 0.1V
> (probably a lot less than that, actually), so you'll need to amplify the
> signal. A single opamp can do it - you want a differential-input amplifier
> circuit. A gain of about 20 should do it.
>

On high side sensing, you need to have a pretty good CMRR on your
differential amplifier, which requires very precise resistor matching.
There are "current sense" amplifiers specifically for this purpose. I've
used a Maxim chip for this purpose (see
http://para.maxim-ic.com/cache/en/results/4884.html). Linear Technology
also makes them (see
http://www.linear.com/pc/viewCategory.do?navId=H0,C1,C1154,C1009,C1077).
I'm sure there are also others.

I used the Maxim part to regulate the current into a high power LED (about
10W). The output of the sense amplifier drives the feedback input of an
LTC boost regulator.

Harold



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