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The aging drift is insignificant (perhaps fractions of a %). If your design is sensitive to C, you will have more issues with normal sample-sample variations (perhaps 20 % or more from one manufacture …

5 mV AC signal will only cause a minuscule change in the capacitance of the varactor and therefor the resonant frequency. This will be quite hard to detect. …

That way the voltage difference between the guard and the sensing electrode will be (ideally) zero; thus that capacitance won't draw any current and so the guard won't affect the sensed signal. …

The pin capacitance may be about 2 pF. This gives an L.C resonance of about 1.6 GHz, or a rise time of 100 ps. … If you are operating at edge rates much slower than that, the system basically just looks like the capacitance. …

If you measure reactance at a single frequency, you can't distinguish between negative capacitance and positive inductance. … If Im(Z) is positive (which occurs with a inductance), the meter will display a negative capacitance. …

You don't need R24, and would have better performance by adding an R (1k) between base and emitter of VT6. This circuit is basically an emitter follower from the (filtered) signal on C30.

The previous calculations of 76 mF are reasonable -- if the capacitor is charged to only ~ 6 V, but basically you need to store a certain amount of energy -- 1.2 W for 500 ms == 0.6 mJ. You could stor …

Capacitance is \$C=\large\epsilon\frac{A}{d}\$, where \$\large\epsilon\$ = the permittivity of the medium (usually vacuum or air, \$\epsilon = 8.85*10^{-12}\$). … Imagine that \$d\$ increases by separating the plates -- the capacitance decreases. …

To get an intuitive feeling look at it like this: When a V is applied at Vb (NODE3), some current will flow through R2 into the capacitor. The Opamp's '+' input will be at some voltage. The opamp will …

The FET's drain-source (and drain-bulk) capacitance also had to discharge -- this current is also conducted by the FET. … This increased VGS also takes some of the gate driver available current to incrementally charge the VGS capacitance. …