Hi,
You can increase the sensitivity by
reducing the ADC Reference voltage (which is the full supply voltage by default) by using the
ADCCONFIG 2 command and the internal "Fixed Voltage Reference". Then setting the FVR to 2 volts (with
FVRSETUP FVR2048 will increase the READADC10 sensitivity to 2 mV per digit. It's not "recommended" by Microchip, but in practice you
can use the 1 volt FVR1024 which will give 1mV (or one Amp) per digit input, which might be "good enough".
Beyond that, it depends if you want to measure smaller
changes in current or more accurate values near to "zero". A "rail to rail" Op Amp with a gain set between +10 and +50 will certainly give a better resolution of changes in current, but getting the display to read zero when there is no current flowing is much more difficult. You need to look at the specification of the (maximum) "Input Offset Voltage" of the Op-Amp, because that gets multiplied by the "gain" of the amplifier and upsets the values measured around zero output.
Of course you can "fake" the zero reading by the software displaying any measurement of under perhaps half an Amp (or mV) as zero. This seems to be a trick used by the manufacturers of some of the cheap digital panel meters, but of course means that you still won't get any useful measurement of low currents.
Cheers, Alan.