Isolation Amplifier Gain Problem
at low input voltage (1mV) I'm seeing a gain of 25


The input offset voltage of the HCPL-7800 device is bigger than your 1 mV signal and this will play a major role in screwing your numbers. Look at the data sheet on page 7 - it states that it could be up to +/- 2mV. If you inverted your DC input signal you would probably find you'd get another silly value but different again.

Gain measurements should be made with an AC signal so that offsets can be ignored. However, if you need good DC performance then you might need to choose a better amplifier or add gain at the front-end using an op-amp with excellent DC characteristics.