# Example of a variable capacitor input op-amp to voltage...

1. May 25, 2015

### thankz

example of a varible capacitor input op-amp to voltage level circuit.

I'm not sure how to turn reactance to a voltage level using an op-amp?

2. May 25, 2015

### meBigGuy

maybe you could try to explain what you are doing in the initial post. What capacitance range, what accuracy, how fast to respond, what voltage range, etc.

First read up on "how to build a capacitance meter" or "measure capacitance" to get some ideas. Measuring capacitance requires a time or frequency based solution.

3. May 25, 2015

### thankz

a capacitive probe being measured by a resistive ladder adc. 0-5 volt range, response time not important, 10 bit accuracy but the increments will be determined by the probes range which is unknown.

4. May 25, 2015

### thankz

did I say resistive ladder I was think of a dac, prob successive approximation but I'm not looking at the data sheet.

5. May 26, 2015

### davenn

that doesn't make sense

that did

Dave

6. May 26, 2015

### jim hardy

One could apply direct current and measure rate of voltage rise, or measure voltage after known charge has accumulated,
or apply voltage with known dv/dt and measure current,

this circuit's period of oscillation will be in proportion to capacitance... and slope inversely so.
One could perhaps follow it with a differentiator and precision rectifier to arrive at inverse of C.......

you've given no hint what you're trying to do.

I could interpret this as relating to varactor diode chopper opamps from 1950's, but i doubt it's what you meant.

.As Davenn observed, a question well stated is half answered and yours is neither.

Last edited by a moderator: May 3, 2017