John Larkin wrote:
[...]
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 ....
I was thinking I'd prefer a fast ADC, with fewer bits, so we could
take a lot of samples and do some waveform crunching. But an adc with
nasty stats wouldn't be good. I've been using some ADS7866's lately,
12 bits, SOT-23, SPI, and they work great.
I've used the AD7928 in a recent design. 8-ch 12-bits, very clean,
pleasantly surprised. The SPI didn't quite like to be handled per
datasheet, it wanted the opposite clock edge. Other than that AD sure
knows how to design quiet ADCs.
Don't know what you'll be using for the SSR but how 'bout a big old FET?
We have some cheap 4-ohm ssr's in stock, so I just assumed them. I use
them a lot in signal switching situations. My tendency lately is to
minimize parts count more than parts cost, since it costs us 20 cents
or so to place a part. A fet would need gate drive stuff, unless we
feed the current source from the same voltage that powers the uP.
20 cents? Ouch, that hurts. On my side that would require some
re-learning of design strategies ;-)
Does that 20c still apply once the stencil NRE is amortized?
--
Regards, Joerg
http://www.analogconsultants.com