Capacitance measurement needed

[eam]

Hi,<p>Sorry to bother you guys again. What I really needed is to find a equipment to measure the capacitance change on my device and output a low voltage signal (DC). And then I can make a feedback control circiut.<p>Does anybody konw this kind of capacitance measurement?<p>
Thanks a lot

[haklesup]

If you the capacitor in an oscillator circuit, a change in value would equate to a change in frequency. This signal (frequency) can be used in a FB control system maybe using a VCO of some type. Alternatively the frequency can be converted to voltage using an integrator (op-amp wired just so).<p>For higher precision, the RC oscillator output can be compared to a reference frequency signal (perhaps a divided crystal oscillator) and a difference frequency is produced. the FB circuit endevers to maintain this signal at 0.<p>If you use the cap in an RC timing circuit and measure (with a clock or counter) the time to takes to reach a referecne voltage after being discharged by a reset signal you would also have another metric to feedback (better suited to a digital control loop).<p>The nominal value, range of change and voltage rateing of the capacitive sensor would limit some possibilities. Do you know these values?<p>You indicated you want to use DC voltage as feedback but frequency feedback is less succeptable to being disturbed by local electrical noise. If your FB signal is not too small this may not be a probale even with V FB.

[philba]

what's the sensor and what are you sending it to? I agree that keeping the signal in the frequency domain is far more reliable than a voltage level but a lot depends on the answer to those two questions.<p>Phil

[Robert Reed]

Not enough info!! What is your DEVICE And What are you trying to CONTROL.

[Enzo]

I agree, we need more info, and directly measuring capacitance is not likely the way to do it. If you have imagined capacitance as a measure of proximity for example, then you are trying to detect proximity, not capacitance per se.