'Capacitance Meter Principal'
Anyone have an idea on how I can measure capacitance?
I wanted to charge a cap with a constant current and measure the time it
takes to reach Vref. I have some _serious_ doubts about this though.
How do commercial meters work then?
This'll give you something to talk about - better than ASCII vs
eric van es
cape town, south-africa
|On Tue, 1 Jul 1997 19:17:02 +0200 EJA van Es <ilink.nis.za> writes: vanes
>Anyone have an idea on how I can measure capacitance?
>I wanted to charge a cap with a constant current and measure the time
>takes to reach Vref. I have some _serious_ doubts about this though.
>How do commercial meters work then?
Constant current timing could work. A commercial one (inside a
DVM) that I saw used a wein bridge sine wave oscillator to drive the
"unknown" capacitor. The other end of the capacitor was connected to an
inverting current to voltage converter (an op-amp with a negative
feedback resistor). Since capacity is proportional to current with a
constant voltage and frequency, they just measured the voltage out of the
op amp and displayed it.
A fancier method would be to drive the unknown impedance with a
sine wave, then measure the resultant magnitude and phase of the current.
this would measure the impedance of any circuit at that particular
frequency. You could then, in software, convert R+jX at some frequency
to L and Q or C and D.
Finally, there was a thread on this subject back in April or so.
There are a number of ways to do this. The difference between them
relates to the highest and lowest impedances that they can handle.
The neetwork analyzer method can only handle about 0.5 ohms to 500
ohms. The I/V method is good for about another order of magnitude
on both ends of the range. The auto balancing bridge is good for
milli-ohms to about 1 Gig ohm. I don't have data for the DC method,
but it is used in some semiconductor ""Quasi-static" C-V meters such
as the HP4140B.
You might get the "Impedance Measurement Handbook" from your HP
Field Sales Engineer and get a data sheet on the 4140 C-V meter
for information on that method.
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