The circuit is a follow on from this topic "Monitoring it's own current draw"http://arduino.cc/forum/index.php/topic,64870.0.html
I'm measuring the voltage drop across a 0.2 Ohm resistor in series with the positive wire before it goes into the Arduino.
I'm using two 10:1 matched resistive dividers, one on either side of the sense resistor to drop the voltage to about 0.8v.
The differential voltage is about 3mv.
Currently I'm using 3 LM324 op-amps in an instrumentation config to amplify the signal which works very well except for the monster offset which is of the same magnitude as the measured signal. This is of course can be subtracted in software, but I'm using this as a learning experience so I started looking at low offset op-amps then realized that an real instrumentation op-amp works out cheaper, but the cheapest of these require a split rail.
What I'm doing is probably over the top for this particular application, but as I said it's a learning experience.