FDC2214EVM: Setup a differential measurement with active shield and precise control over the excitation frequency

Hi,

I am looking to build a differential measurement system using two co-planar capacitance sensors for liquid dielectric constant measurement, with active in-phase shielding for both the sensors. My measurements have to be done at fixed excitation frequencies 1 MHz and 5 MHz/10 MHz(all separately, not in a single measurement). The base capacitance of my sensors (both the co-planar capacitors) is close to 0.65 pF.

I have the following questions

1. How to build an active shield for both of my sensors. I read in one of forums , one way is to use an OP-AMP with a unity gain but I am not sure which channel should be using for driving it and will the signal be in phase with the sensor.s. Meaning, I would be connecting my two sensors to say Channel0 - IN0A & IN0B with and ground the other plates. Will a signal from, say IN1A and IN1B through an op-amp, say OPA322, act as shield inputs for my sensors connected to IN0A and IN0B respectively ?

2. How do I change the excitation frequency for the device to a precise value ?

3. My sensor capacitance value is way off close to 20 pF in a single ended measurement (the LC tanke values are 18 u and 33pF) when I measure it using the EVM. Am I missing something blatant here ? The oscillator frequency I am using is 40 MHz

So sorry if any my questions are too naive, I am a biologist with limited electrical engineering knowledge!

Any help would be greatly appreciated!

Regards

Praveen