> From: Harold Hallikainen[SMTP:
RemoveMEharoldTakeThisOuTHALLIKAINEN.COM]
> Sent: Thursday, January 15, 2004 12:48 PM
> To:
spamBeGonePICLISTspamBeGoneMITVMA.MIT.EDU
> Subject: Re: [PIC:] want to make a LC meter with pic.
> Some of the inexpensive C meters I've seen have a wien bridge oscillator
> generating a sine wave. This drives one of the capacitor terminals. The
> other capacitor terminal drives an op-amp current to voltage converter (op
> amp with resistor from inverting input to output, non-inverting input
> grounded, input applied to inverting input directly). Capacitor current
> with known voltage and known frequency is proportional to capacity. So,
> they just use their AC voltage measurement capability to measure the
> voltage at the output of the current to voltage converter.
> They can't tell if it's a capacitor, a resistor, or an inductor, but it's
> a simple way of measuring a capacitor if it is indeed a capacitor.
> If you want to get fancy, you could generate a sine wave with a lookup
> table and D/A to drive the capacitor. Then, instead of just measuring the
> RMS current, do fast D/A samples of the current (as indicated by the
> current to voltage converter). Do a complex number division (using either
> polar or rectangular notation, whichever works out best), to get Z=V/I . Z
> can then be represented as R+jX . Knowing the frequency, you can convert
> from X to inducatance or capacitance and model the unknown impedance as
> L/C in series with R or L/C in parallel with a different R.
> Harold