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From: Roger Johansson
Subject: Re: Strain gauge measurement circuit design
Date: Mon, 07 Oct 2002 01:15:30 +0200
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"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.
Are you really using the whole range of that pot? Or are you just
using a narrow middle part of it? If so you can change the circuit to
adjust only in the middle with a lot better resolution.
I do not know what your bridge looks like, but I can use an example:
If you wanted to set a voltage of 5 Volt from a source of 10 Volt you
could connect the pot between the ground and +10Volt and adjust it to
the middle point, as close as the 1024 steps let you do it.
But that would be a very bad solution if you always want to set the
output to 5 Volt. Because most of that pot is never used, you only use
the very narrow range around the middle point.
If you use two other resistors at each side of the pot, to take care
of the useless ranges around the middle range, you get a much smaller
adjusting range, and much better precision.
You might get a range of 4.75-5.25Volt divided into 1024 steps.
You might be able to do something like this in your circuit too.
Otherwise you can use another similar pot to fine adjust the 1025
But remember that you might get problems with small errors in the pot
if you do it this way, the fine adjust pot might need individual
settings for each step of the first pot.
You should not use a pot with 1024 steps if you need better than one
in a thousand precision, it cannot deliver any better precision than
that. If that is not enough you probably have planned your circuit in
a less-than-optimal way.
We seldom need better than 3-figure precision in a component, or in a
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