I have an unknown value of R in parrallell with an unknown value of C - I need to measure R through an AC coupled circuit (there are High DC potentials I must isolate). C is likely to be in the order of a few nF, but MIGHT be a lot larger (100nF?).. And the circuit must produce 2 alarm levels, one when R is less than 200k, and one when R is less than 40k.

My plan was to inject (via known resistance, and large capacitance of say 10uF, so that the Z of this capacitance did not affect result) sine signals switching between (say) 100Hz and 1kHz, and to measure the resulting attenuation for each frequency

(F) --[== 10k ==] --(M)--| 10uF|---[ZZ Unknown R || Unknown C]---(GND)

F = Frq switching between 100Hz and 1kHz, through (say) 10k. Monitor amplitude at (M) with respect to Gnd. Decouple through 10uF, Unknown R||C connect to Gnd.

Total Z of R||C = 1/((1/R)+(1/(1/(6.28*F*C)))). (correct?)

There should be a simple way to compute R from the above, having Z for two known frequencies, and I feel sure someone must have done this before and be able to give me an answer.. But annoyingly, I cannot seem to find it !

Any help would be appreciated!