Methods to accuractly measure capacitance

[Topher925]

I need to measure the capacitance of something as accurately and as quickly as possible. I also need the method to be very noise tolerant and not depend upon signal attenuation. I've been looking at several different methods so far on how to do this and so far I have found methods like the UTI technique, and using frequency locked or phase-locked-loops. Does anyone know how those high accuracy capacitance meters work or know of any commonly used methods to measure capacitance?

 

[waht]

Some of those Agilent Impedance analyzers are crazy. They measure capacitance with femto-farad accuracy. Check out some of their app notes on their website.

One method that was used in older HP impedance analyzers is by using an Auto-Balance Bridge.

Basically there is a variable power AC source, voltage difference amplifier between the unknown impedance, and a trans-impedance amplifier that converts current into voltage.

By using feedback loop, the CPU can set up a constant voltage source over an unknown impedance and measure current flowing into it, or a constant current source and measure voltage across the unknown impedance.

Now there are two AC voltages. One is proportional to voltage across impedance, and the other is proportional to current.

The ratio between these voltages it taken using another complicated feedback, IQ demodulator to separate the phases, and feed everything to ADCs and the CPU.

The advantage of this method is that is gets rid of any effects of ESR, and stray inductances

 

[f95toli]

Yes. you need a capactance bridge.
The ones we use for routine measurement where I work are easy to use and very reliable, while still being -as far as I know- the most accurate capacitance meters you can buy.
(they are made by Andeen-Hagerling); for more accurate measurements we have other systems but they we are talking about VERY specialized equipment (which are still based on bridges).

Also, do you need to know the relative change (fairly easy) or absolute (not so easy) capacitance?
At what frequency? kHz is easy, hundreds of MHz is not so easy...

 

[zagaraa]

hi, you can use capacitance bridge, resolution limit is 0.5 attofarad.