Yeh the RIO is a very nice piece of kit. I think we were talking about different things - you meant the resolution of the ADC, but what I meant was the fact that, purely in the analogue domain, if I'm stepping down a 230V signal to 200mV, noise is likely to influence the accuracy of my measurements. I don't know what typical +/- mV noise tends to have on signals, but because 200mV is so small, the noise on that signal will be scaled along with the signal to whatever the output voltage of the isolator is set to.