The circuit below is a simplified and partial front end schematic of a solution that measures main voltages and takes action if over voltage, under voltage or "wrong voltage" (e.g. user plugged into 220 Vac and appliance is not automatic). Main measurement should be within 1% error.
(Still need a DC shift before R4 reaches secondary ground. Or dual supply to the op amps.)
From the rectifier bridge, a LM317 supplies power to all the ICs (not shown). Under_voltage, Over_voltage and Wrong_voltage are triggers, coming from comparators and going into latches and logic (also not shown)). A NO relay at the end of the circuit must be activated to allow appliance to be main powered (just guess, not shown lol).
There is a difference between the secondary and the bridge references, indicated on the schematic by the common ground and chassis ground (should be earth gnd, but no such symbol in LTSpice :) ). The difference is, of course, a 0.7 V diode drop, and I should account for it in measurement.
One way to go is just use an instrumentation amp, and that ground shift is solved. But that looks like an overkill, since I don't need all the CMRR, either extra high input impedance an instrumentation amp has to offer. Also, it is somewhat expensive (although I've just found a $0.54/1k unit in amp: http://www.ti.com/product/ina332).
A differential amp would be not ideal, since I can't cheaply match input resistances (at least not precisely), and I have surprisingly found that the cheapest diff. amps is more expansive than the INA332 in amp mentioned above: http://www.ti.com/product/ina154. I am not sure if other manufacturers follow this price tag as well.
I was considering maybe using a charge pump for powering the ICs, then there is no 0.7 V drop. In the charge pump case, its power capability should be well calculated - just closing a miniature 10A relay might need about 30mA, indefinitely.
What do you think, can you suggest me a strategy? Keep the bridge and deal with the ground shift, going with the charge pump, or something else?