Well, I do not exactly know, what you are doing there, but if R and C of standard components is to be determined I would proceed with the following circuit:



As you can see a 50kHz sinus oscillator is buffered by a high speed operational amplifier providing low distortion at 50kHz. This buffered sinus signal Uin(t) is fed via resistor R0 to the parallel circuit consisting of R and C. The signal voltage at this parallel circuit Uout(t) is also buffered by a high speed operational amplifier providing low distortion at 50kHz.
R and C in combination with constant R0 is now introducing a certain phase shift 'alpha' between Uin(t) and Uout(t). Also, there's a certain dampening 'D' of Uin(t). If now this phase shift and this dampening is measured, then the value of R and C can be determined.

Kirchhoff's laws yield:



What about value of R0?
R0 is necessary, because R and impedance of C can be down to zero Ohm. Without R0 first buffer would be short circuited! Also, some resistance at output is necessary, when a capacitive load is connected to the output of an operational amplifier: To prevent instability and oscillation!
R0 should be in the range of some hundreds of Ohm. Many operational amplifiers can work with this load. But only rather small amplitudes are achievable. I think much more than 1V is bad anyway, if organic material is measured. I'm not quite sure about maximum voltage that can be applied without any risk to organic material, so, you should contact a doctor or scientist in any case!!!!!!!!!!! I would only apply some hundreds of millivolts!
There are some operational amplifiers arround which can drive such low ohmic loads very well, and with very low distortion! E.g. NE5534. And even if these buffers are not 'strong' enough, it's very easy to strengthen them by a set of added transistors.

How can the phase shift and amplitudes be measured?
There are many ways. One way is to use analog peak detectors at the output of each buffer. An ADC is then converting the analog voltages into digital data.
Phase shift could be measured by the help of two fast comparators, connected as 'zero-crossing' detectors. The micro could then measure the time delay between the two zero-crossings.

Another methode is to sample Uin(t) and Uout(t) many times with very precise clock timing (low aperure jitter of data aquisition) and to put all samples in RAM. Then, micro can calculate amplitudes and phase shift from this data.

If R or impedance of C becomes very small, at output of second buffer signal becomes also very small. If second buffer is provided by a programable gain (e.g. x1, x10 and even x100), micro can automatically adjust this gain relative to detected output signal. By this, range of detectable impedances can be drastically extended.

Micro can also be used to provide some power-on-calibration: Output voltages of analog peak detectors will show some offset voltage, which can be exactly detected, when the oscillator is muted.
Also, exact frequency of oscillator can be measured, so that error of phase shift calculation can be highly minimized.


Kai