I used Hi-Voltage probes from Tektronix with a 1 Gigaohm input, as a differential pair to measure the 30,000 volt charge on the capacitor bank-based surge generator at a company I worked for. The probes were rated to 40KV, and when I opened them up I found two 500 megohm 1% ceramic resistors in series made by Dale mfg, each one rated for 20KV max. Because this was a custom ATE setup and not for a Fluke DVM, I had to disable the internal trim resistor in each of the 2 probes. I fed the resistors to a small board with 2 CA3140T op-amps (each op-amp has a 1.5 Tera-ohm input impedence). Both input's had a 100K 1% resistor to gnd (which divided the input by 10,000 - so +30KV showed up as +3 VDC at the pc), which fed a AD524CN diff op-amp, which fed a SSM2142 600 ohm balanced line driver with about a 50 foot run to my pc with an analog input card.
I used a duplicate set of probes and circuit's to measure the charge current for the capacitor's, to check for anomalies during charge-up. 30% of my code was just to check for all the thing's that might go wrong!
Giga-ohm resistors are mandatory when you have a hi-voltage source, common grounded or not, whether you are measuring volts (or a voltage "field", such as a lightning detector) or amps. I found this differential probe technique to be very accurate and stable as well. Added software can trim the accuracy to about .1%!
