C measurement tool

[Martin Czech]

> The Schreiber/Curtis chip in the RadioShack DVM uses switched laser-trimmed
> current
> sources and a 16-bit timer.
> 
> EE-101 >>> I = C dv/dt
> 
> So, we switch in a known I (there are 5 current sources to cover the cap
> ranges of 10pf to 100mf),
>  measure dv/dt (that's the change in voltage over time) and calculate C with
> the on-board 6502 uP.
> 
> The ever clever reader will immediately note this is how a standard sawtooth
> VCO works!
 

Certainly are very nice design!

What I want to do differs in that 0.1% resistors and a constant VOLTAGE
are used, something that is easy to obtain for DIY. Thus the charging
waveform is not linear, but K{1-exp(t/RC)}. But the time difference of
two voltages on the charging waveform is proportional to RC.

Perhaps I can discharge fully to 0, and the use just one 3140 as comparator
to measure the difference between 0 and  x Volts.
I could use my frequency counter as stop watch.

First I should get some good styrene caps measured in the lab,
in order to calibrate...

m.c.