John Larkin wrote:

On 15 Jul 2007 18:26:20 -0700, Winfield Hill
wrote:


John Larkin wrote...

With some decent code, this could measure capacitance
(uF range, not pFs), true esr, resistance, millivolts,
possibly a useful range of inductances. It could find
shorted pcb traces and maybe plot diode curves.

The dac is optional, and the adc might be inside the uP.

This is a stage 0 brainstorm. Of course you can find
objections, but I'd prefer improvements.

Hi John,
Consider some co-existing values: 1 to 50us measurement
intervals, 5 to 500uF capacitors, 5 to 100nH series and
probe inductances, and 5 milliohms esr. Tell us about
proposed current-switching speeds, diff amp bandwidth,
sampling rates and resolution. Run some numbers for us.



Given that a dinky uP is running things, I'd espect much slower
measurements, up into the 10s of milliseconds at least, with a bunch
of samples along the way. Say test currents from maybe 50 uA to maybe
100 mA, a 10 bit ADC (that would be a cheapie, probably internal to
the uP) stretched by some goodly averaging, maybe 1 volt fs at the
diffamp input, 1 mV at the adc. So an lsb is 10 mohm of resistance,
again stretchable a bit by averaging.

Neither current switch speeds nor diffamp bw would be important, given
the low sample rates; we'd have microseconds to spare.

I'd think you could measure caps from maybe 1 nF up to farads, but no
usable esr until the caps got up into the microfarad range.

More gain would be nice, if we were to do stuff like super-low esr or
pcb short tracking. With a big cap across the 9-volt battery, I
suppose you could pulse at 1 amp, and get 1 mohm, but even lower would
be nice. So variable gain, if it doesn't featurize and cost it out of
sight.

This was a first shot at an architecture.

It would be tempting to go whole-hog and do a real gadget that could
do things like wide-range diode curves and c-vs-v and stuff, but
that's not the issue here I guess.


Internal ADCs are typically rather noisy with no way of knowing what
mechanism is involved. I have seen hardcore noise that was
code-dependent, making it next to impossible to cure with the usual
averaging tricks. At least I'd spring for a cheap Codec. The one in my
newest laptop is 20 bits yet definitely not the latest and greatest. I
guess it was so cheap that they forgot to mention that it's more than
the usual 16 bits. But they also forgot to mention that there is a RS232
port ....

Don't know what you'll be using for the SSR but how 'bout a big old FET?

--
Regards, Joerg

http://www.analogconsultants.com