How does isolation prevent input signals to float/saturate for differential ended inputs?

Question

A differential ended input data-acquisition board has the following architecture:

Here more information.

Through screw terminals it takes the voltage inputs and each input first goes to a signal-conditioning module which also forms a channel to channel isolation.

Normally for floating sources until now I've encountered differential ended data-acquisition boards which needed around 10k bias resistor between the signal return and the AI GND of the board. It was something like at this link. And I remember without this resistor saturation was happening.

But the manufacturer says for the board above, one does not need bias resistor because channels are "truly isolated".

What does truly isolated mean in this context? How can this Take care of the need for bias resistor for return currents? Can it be explained with a simple opamp or circuit model?

Answer 1

Those input channel devices are not op-amps, but isolated analog signal conditioning modules. Internally I suspect they contain op-amps, but that is transparent to the user. All you see are connections for power, input, and output, and a set of specs that characterize performance. Bias resistors are internal, manifesting themselves externally as part of the input resistance spec.

I believe the one who told you it is "Truly Isolated" meant it is galvanically isolated. The inputs are floating with respect to ground and the power supply common. A block diagram of the 8B voltage input module is shown below, obtained from here.