C measurement tool

[Martin Czech]

Some design need low tolerance caps, so one has to measure...

I don't want to build a bridge, it is expensive and not very easy to
use. Bridges have means to adjust for cap loss, so many controlls have
to be adjusted at the same time.

Since such demanding applications need good caps polystyrene or the like)
we could assume them to be ideal.  This is a hypothesis.  This would
allow to measure the RC time constant.

I think this is the way that cheap multimeters do it.
But these meters are very limited (this starts with the
mechanical parts, clamps).

There was a cicuit in Elektor:

A regulated voltage, a known R and the C (DUT), a window comparator
(counter window), and a discharge logic.  The window comparator gives
an estimation of charge time, so a quartz counter is needed to measure
this time.  The discharge time is very long, so the whole circuit is
some kind of oscillator, but the frequency does not matter.

The charge time should be in the 1ms range, to avoid comparator swithching
time influences etc. The cicuit has some stray capacitance,
maybe it is possible to add a one shot that shortens the window
in order to trim out these stray effects.


Are there any experiences ? What possible errors could limit the accuracy?
Series R and L? Stray capacitance?

m.c.