by Maciej Kokot @ edn.com:
In most cases, you measure current by converting it into a proportional voltage and then measuring the voltage. Figure 1 shows two typical methods of making the conversion. In one method, you insert a probing resistor, RP, in series with the current path and use differential amplifier IC1 to measure the resulting voltage drop (Figure 1a). A second method is a widely known operational amplifier current-to-voltage converter in which inverted IC1’s output sinks the incoming current through the feedback resistor (Figure 1b).
Measure small currents without adding resistive insertion loss – [Link]