Kai,

Perhaps I'm guilty of only considering AC excitation as a cure all for zero drift - mainly due to thermal emfs, which are considerable. I've managed to control these to a point where they are less than our desired approvals so, commercially, that is far as I will go. DC excitation gives us:
1. Simpler, cheaper design.
2. Better ultimate speed (due to the inherent delay in the EMC input/output circuitry).
3. Removes yet more error inducing components from the signal/reference input chain.
4. Easier for a service technician to fault find on the bridge circuitry.

That said, I'm always open to discussion!

I'm eager for your views on how AC excitation can remove gain noise that I assume is being introduced by the ADC (which is back in the DC domain). Unless you mean I should include the ADC within the AC domain - I think not - it takes around 40ms at my sampling rate to clear the ADC digital filters - not to mention the anti-aliasing input filters. Please explain.

Dave