Is it an AC or DC signal?
You'll want to pay particular attention to input offset voltage and offset current of the front end amplifier you chose. You'll want input offset voltage hopefully << your measurement range and as importantly, low drift on both parameters (worst case you can run a cal processes right before your measurement to zero out any offsets, low drift makes this possible).
You'll find that chopper stabilized amps and their variants often have these parameters.
But you should provide more info about the source. What leakage currents are acceptable?