Hello,

I'm looking into capacitive sensors and would like to make my own so I've been looking at different topologies and found this one in particular. I understand how the regulators work, the oscillator and the filter and gain section but what seems to confuses me is how can he say he is measuring say a 125pF capacitor if you have R11 and CA1C in parallel with the 125pF cap. When caps are in parallel there capacitance are added there for 10.125nF???

Here's the circuit and description...

http://www.edn.com/design/test-and-measurement/4423158/Precision-capacitance-meter

http://m.eet.com/media/1200981/DI5345sch.png

Thanks

 

If you assume that U4 non-inverting input is basically a DC level, which is close since it's highly filtered, then U4 will maintain the inverting input at the same level, so really there is no current flowing in CA1C once it achieves the DC bias voltage.

 

Ok so let me get this straight once CA1C is charged its capacitance value is ignored with the capacitor being measured?

Thanks again

 

Yes, that's how I'd analyze it without going into more detail with the frequency, etc. You'll get current across R11 and R10 (and R13) as the op-amp maintains the balance at its inputs, but CA1C will not be changing or affecting the measurement.