Re: ESR meter project, again.

On Dec 14, 11:49 am, whit3rd <whit...@xxxxxxxxx> wrote:
On Dec 11, 2:15 am, Jan Panteltje <pNaonStpealm...@xxxxxxxxx> wrote:

Or you could use a small FPGA, put sine and cosine tables in block ram, and use 2 DACs.
It could also drive a LCD  display at the same time, and do the rest of the logic.
I do not know the frequency of the sine/cosine output that is needed for this application,
but if below say 5 MHz you could use a R2R network as DAC.

Alas, that isn't the complete treatment: DAC output will simply
have distortion components at/above the Nyquist limit, which
can contaminate later measurements unless they're filtered.

If you have an ADC sampling at least at the same rate as the DAC is
updated, the processed values from the ADC won't be badly effected by
the DACs imperfections. It is better to do most of the work digitally
than just a little of it, if you don't go with an all analog circuit.

The filter introduces its own phase shift.

Analog sinewaves from a VCO make a 1:100 frequency range
available, and the DAC trickery (or the other solution, with
switched capacitor filters) can't get far from the Nyquist post-filter
target frequency.   It's also a tad irritating that the DAC has
a settling time issue.  Characterizing a big capacitor requires
low frequency (to get C), medium frequency (where R dominates)
and high frequency (to get L), and either a large frequency
range, or very precise sinewaves, is needed.  With analog,
you get both, relatively cheap.

It seems to me that a square wave or other non sine wave of current
may be a better way to go. The harmonics will give you multiple
frequencies all at once. The multiple measurements can be done all at
the same time.

A fairly simple demodulator can pull out the in phase part of the
signal. Since the capacitance and the ESL are both at 90 degrees,
this measurement will be fairly easy to do.

The ESL would show up as a change in the amplitude of the 90 degree
part with frequency. I think that you could get the ESL with two
demodulators and simple math on the outputs of them.

.

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