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On Sun, 13 Sep 2015 23:08:19 +0000 (UTC), you wrote:
>"Jon Kirwan [email protected] [msp430]" <[email protected]>:
>
>> You can look up ideas with google using "sampled strain gauge
>> signal conditioner" and see where that takes you. I'd avoid
>> the sigma-delta for a pulse-powered bridge, though. But
>> perhaps you can get enough experimental data from that one to
>> see if you can make it work okay. I've never attempted it so
>> I really don't have any experience to draw on, regarding real
>> results. I just avoid them when I know I will be powering up
>> and powering down an analog input system or using an analog
>> input multiplexer. (They generally "don't like" switched
>> inputs as it takes them time to "re-acquire" an input without
>> mixing it up into a mush from the other states the input goes
>> though.)
>
>Jon, thank you for your idea's.
Well, don't thank me too quickly. I haven't helped at all.
Just offered a few of my mostly ignorant thoughts.
>Isn't it possible to just "pause" the sampling of the delta sigma converter
>(let it sleeping, all registers keep their values) and awake it after 100ms
>with nearly the same input level as before the sleep?
I haven't seen one work like that. They are basically very
fast "1 bit" converters with a lot of state behind it. It
takes a long time for those "bits" to fill the state. Once
you get the pipeline filled up, you can get very precise
values at a fairly reasonable rate.... if it is still
attached to the same input which is "drifting" around the
nominal value at less than some specified rate. But the
sample rate is based upon an ALREADY FILLED pipeline. You
have to fill it before the first value is really any good.
You can find out how long it takes to fill the state and, in
my limited experience, that will take a LOT LONGER than the
specified sample rate for the unit.
Whether or not you are able to stop the filling process on a
dime is another question. I haven't spent the time on the
MSP430 24 bit sigma-delta module. Nor have I tested it. So I
can't offer any experience there. Perhaps if you shut down
the module, then shut off the power to the bridge, and before
starting the module back up again first re-power the bridge,
you might be okay. But one problem of several that comes to
mind is that the bridge value may have drifted too far away
from its earlier value and may require, therefore, a complete
"refill" in order to re-acquire the ADC value. Even with all
that. So you'd need to explore this, I think. The counter to
what I just wrote is that you aren't looking for 24 bits or
21 bits or even 16 bits... but more like 12? So.... maybe.
>The sd24_a input is
>just a 18pF capacitor. The 350ohms will charge it quickly...
Hehe. That's true. Have you computed your expected S/N for
the analog portion of the input system, yet? Just curious.
>Sure, I have to clock the adc as long as it is required to step the new
>reading completely through the full decimation filter.
>
>The PIR example from texas instrument slaa283 is quite interesting,
>It uses the sd16 to sample a big capacitor behind the PIR sensor. It is
>sampling the input for 1ms (4 samples each time) and sleep for 341ms. It
>seems interrupted sampling is possible with delta sigma too.
I've just used the ADC10 and ADC12 sections, never the SD16
section. But yes, if you find some good examples there then I
think that you may be able to cross-apply them to your case.
Again... test things out. But perhaps it will work out fine.
>To further reduce power consumtion they also switch the reference voltage
>off during sleep. It seems that the charge in the capacitor at the ref-pin
>will stay - accurate enough for this application - at ref-level for 341ms.
>
>https://www.olimex.com/Products/MSP430/Starter/MSP430-
>PIR/resources/slaa283.pdf
Yes. To save power you should switch off everything you can
think of and handle. That's a given.
I'm interested in what you find out. I think you already have
figured out enough to take a fair shot at it. Sounds like you
may be able to do just fine. [Personally? I'd have preferred
to use an ADC12 or ADC10 section and use over sampling to
achieve the desired resolution... after making sure that my
noise is where it needs to be (for an ADC12 the noise should
be about 1000 ppm or so, I think, and for an ADC10 perhaps
5000 ppm; then use averaging to squeeze back down.)]
Jon
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Posted by: Jon Kirwan <[email protected]>
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