I am trying to make a detector to pick up small changes in capacitance. Basically both electrodes have a common ground and are closely spaced. One is driven by oscillator at 0.1-1MHz and the other one is suppose to pick up the signal (current?) that changes. Both electrodes are attached to a glass capillary that is filled with ionic solution. This is the circuit (the right part) that is a typical design for converting I to V, amplifying it, rectifing and digitizing (?) it. I am wondering how can I simplify it without loosing the sensitivity and maybe use newer ICs (that have better performance). Also I was thinking to just detect the DC voltage with my voltmeter that can be coupled to PC. If I just lose the last part (ADC) in circuit, would it be unstable? Many thanks people.
There's a method of measuring small capacitances and their variations that I find quite interesting and it's much simpler than the circuit you're working with. This method is used in capacitive touch sensors.
The method requires only a digital I/O pin from a microcontroller connected in your sensor device (which is the capacitor). It works by sending micro pulses of constant duration and reading the value back. Now, as we are talking about digital inputs there exist a voltage threshold where the microcontroller starts reading as logic "1". The amount of pulses that are necessary to charge the capacitor so it reaches this threshold and input value changes from "0" to "1" is directly related to its capacitance.