Hello,

First let me say that I'm very new at the electronics thing, so I apologize if there are glaring mistakes in my logic. I'm trying to figure out a good way to measure changing capacitance in a circuit. In essence what I'm doing is applying an AC current to a capacitor, which has a time dependent capacitance.
What I was going to do was set up the capacitor, putting a current meter after it in the circuit, and then I was going to compare the magnitude of the current to the magnitude of the voltage to determine the impedance of the capacitor. Unfortunately the change in capacitance will be very small, however having not run the experiment I don’t know exactly what order of magnitude the capacitance will change by. Someone suggested amplification and filtering the signal, and this is where I get completely lost. I could use a transimpedance amplifier, but I’ve been told different things about stability and signal to noise issues. As far as filters go I was thinking a simple low pass filter may work best with the set up since I don’t know what frequencies will be best yet. If anyone has any information about where I should start looking into amplifiers and filters I would appreciate it.

Thanks,
Brad

Look first toward the Internet for your needs. You can find plenty of new and used product where you are searching. Second would be a university, but a distant second at that is probable.

Hi Brad,
Firstly, a capacitor's impedance is a vector magnitude. Secondly, impedance is measured in relation to the source impedance. On a smith chart (normalized to 50 ohms, say)a capacitor will go from a complete reflection, and rotate around the smith chart passing through both short and open, depending on the frequency (and capacitance, of course)
I don't understand your question completely, but I will point out that voltage and current waves are 90 degs out of phase).
If you set up a high pass filter (pi network) the values you use will determine where the complete reflections are and where your pass falls off (moving towards the center of the smith chart.
If you have more specifics, I can help you understand what you will see.
Peace,
Ron

i have little bit knowledge in electronic field.

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bless

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