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From: "Spehro Pefhany"
Subject: Re: Strain gauge measurement circuit design
User-Agent: tin/pre-1.4-19990216 ("Styrofoam") (UNIX) (Linux/2.2.14 (i586))
Date: Sun, 06 Oct 2002 23:25:13 GMT
NNTP-Posting-Date: Sun, 06 Oct 2002 19:25:13 EDT
The renowned Simon Price wrote:
> I have built a circuit that uses a digital pot to initially 'balance' the
> bridge to give a zero output. The signal from the strain gauge is then fed
> into a high gain amplifier stage and sampled by the ADC on a PIC MCU. The
> PIC also controls the digital pot in order to balance the strain gauge. The
> problem that I have is that the digital POT only has 1024 tap off points
> (The highest I've been able to find) on it and this is causing me issues
> when I try and balance the output as the step size is too large and the
> output from the amplifier stage just swings from one extreme to the other.
> Anybody out there got any thoughts / ideas on possible solutions? Anybody
> tried to do this before?
Why don't you leave the bridge unbalanced and just subtract the signal
digitally? Alternatively (let's say you are only looking at a tiny part
of the signal so your A/D resolution/range would be inappropriate), you
could use a second digital pot as a "fine" adjustment (you can probably
daisy chain the SPI ports so it doesn't take any more port pins). You
bring it "close" (let's say within 1%) with the fine adjust in a fixed
position and using the coarse adjust, then bring it
in the rest of the way with the fine adjust (which has, say, a 2% range,
giving you steps of 0.002%) You might have to take a close look at the
stability of the various components.
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