Re: SD24 to sample a resistive bridge with pulsed bridge supply

I apologize for top posting, but in this case I don't want to
address specific points below but instead "just talk." Keep
in mind I'm more of a hobbyist here. But perhaps something I
say may help a little, just the same.

First off, the idea of periodically powering the bridge is
probably a good idea. So you are on the right track here.
However, I don't think sigma-delta converters are the right
approach when combined with the idea of powering and
unpowering the bridge circuit. So, if you can change to a SAR
type ADC, for example, that might help a lot in simplifying
what you do.

A 16-bit SAR would be nice and I've used one that was on a
SiLabs which could run quite fast, but the analog section is
over-kill for your needs (though the C8051 core probably
isn't over-kill.) I don't know of an MSP430 with a 16-bit SAR
on it. But there are 12-bit ones. You could plan your noise
level to give you about 10 bits and then over-sample enough
to make sure you get a good 12 bits. Your 10Hz rate wouldn't
be at all hard to achieve.

The 350 ohm impedance you are talking about is "normal" for
strain gauges. Typically, I seem to recall, you get about 1mV
to 3mV of output per drive-volt on the bridge. The first
thing that came to mind was to run the bridge on a low supply
rail. But at 1V of drive, you're still talking about 3mA of
supply current. Dropping that to 0.1V brings the current down
into "reasonable" but then you will have trouble with
accuracy and precision. So I think this is why your thoughts
about short, powered samples is the correct approach. I just
don't think that goes well with sigma-delta.

Use a standard differential amplifier arrangement (3 op-amps)
and set up a three-terminal regulator with a BJT (or two)
that you use to enable its output into the bridge. This way
you can have a precision supply rail that you easily control
and can actively turn off when you don't need it. That supply
will supply your bridge AND your opamps. You'll have to work
out the setup time before sampling starts. Also, you might
want to consider using a CD4016 switch that is constantly
powered to gate the final amp output to your micro input. I'd
expect something less than 150 microamps (regulator, mostly)
as achievable for the OFF current compliance figure. ON, of
course, will be lots more.

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


On Wed, 9 Sep 2015 10:10:11 +0000 (UTC), you wrote:

>I want to use the SD24 (24 bit ADC in the MSP430AFE2x3) to sample a resistive  
>strain gauge bridge (load cell) with 350ohms directly. This is no rocket 
>science in case the bridge supply is at a DC reference voltage. But I want to 
>save power! The bridge supply is power hungry (2.5V at 350 ohms).
>My idea is to pulse the bridge supply e.g. 10 times per second (the load is 
>stable) and sample it with the adc24 10 times per second. 
>I want to get a load resolution of at least 12 bits.
>(bridge sensivity is 2mV/V, in case of a 2.5V supply I have 1uV for the LSB)
>
>I have not worked with the ad24 yet. Will this approach save power? From the 
>datasheet I see that the reference needs at least 5ms turn on time. 
>But in case I use the ratiometric concept (ADC-reference = bridge supply), I 
>do not need the reference at all.
>How low can I get the supply current? How long does it take for the AD24 to 
>take a sample? (I do not see any timings in the datasheet?)
>
>Or should I go a different way (e.g. sample and hold circuit in front of high 
>impedance low power OPV-instrumenation amplifier) and then I use the 12bit 
>ADC to sample it. 
>
>Any of your ideas are valuable for me.
>
>Thanks!
>Matthias


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Posted by: Jon Kirwan <[email protected]>
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