There was a neat idea I saw for isolated voltage measurements where the measuring circuit is totally powered by a pulse from the micro to initiate a measurement. After a bit of searching, I found a very similar circuit in this app note: (See figure 9)
http://www.ti.com/lit/an/snoa604a/snoa604a.pdfI was thinking of adapting this idea to use an ethernet pulse transformer powering a remote micropower 6 pin micro with a 10 bit A/D. Hadn't looked into the details to see if it is feasible.
I just liked the idea of a micro being able to power a remote circuit just from a single enable pulse and then possibly use the same micro pin to read in the data.
Another idea that works really well for measuring a high remote DC voltage is a flyback transformer. The primary connects from ground to a PNP transistor or mosfet to the +5V. You put some current into the primary then turn it off. This causes the voltage to shoot up on the secondary until it can turn on a diode on the secondary to the 200VDC. You capture the peak negative voltage on the primary with a 1n4148 diode and capacitor with a simple RC filter before the diode to eliminate the switching spike.
It is not hard to get better then 1% and the reading is very stable. It all comes down to transformer winding ratio that is exact. The only parts on the secondary are the transformer secondary and a diode. I have used this method for monitoring the voltage on isolated 48V batteries, and it would work even better at 200V for low impedance, slow changing voltages.
Richard