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'[PIC]: PIC16LF627 and a wheatstone bridge.'
2002\06\04@140121 by A.J. Tufgar

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Hello all,
         I was wondering how I'd go about using the ADC on the
PIC16LF627 to measure the output of a wheatstone bridge (from a pressure
transducer).  Do I need any support components to bring the differental
output back to ground?

Can I just use the one voltage divider in the wheatstone or do I lose
sensitivity or does the internal resistance of the pic (like on a DMM)
come into play?

Thanks,
Aaron

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2002\06\04@153536 by Dwayne Reid

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At 01:59 PM 6/4/02 -0400, A.J. Tufgar wrote:
>Hello all,
>           I was wondering how I'd go about using the ADC on the
>PIC16LF627 to measure the output of a wheatstone bridge (from a pressure
>transducer).  Do I need any support components to bring the differental
>output back to ground?

Umm . . . The '627 does NOT have an ADC.  You can fake one by making a dual
slope convertor using one of the comparitor inputs but since you need a
differential input, I'd seriously consider using an external dual op-amp
configured as a diff in, diff out amplifier (needs only 3 resistors)
followed by a LTC1298 12 bit a/d used in differential input mode.  This is
exactly what I use to measure strain guages and pressure sensors and it
works well.

You have one of the easiest types of sensor to measure - the output is
ratiometric and is biased at half the excitation voltage.

dwayne

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2002\06\04@154947 by Paul Hutchinson

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It all depends on:
What transducer make and model?
What range of pressures?
What accuracy you need?

Depending on the answers to these question you may need anywhere from no
additional circuitry to a bucket load of precision analog circuitry.

The transducer manufacturer may have some helpful applications information.

Paul

> {Original Message removed}

2002\06\04@162809 by A.J. Tufgar

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To answer pauls questions:
1. This is the sensor I'm using from digi-key:  MSP6800-ND
2. It's 0-5 psi guage.
3. All I need to detect is any negative pressure at all (so very low
res.)

Thanks for the advice dwayne, but I need really low res and sorry my
error on the '627 mistook the comparator.

My new ideas are:
1. Using the circuit from the following url and an 16C711.
http://www.linear-tech.com/pdf/2053i.pdf

2. Or using the 627 and a ADC0831.

Opinions?

Thanks,
Aaron

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2002\06\04@173751 by Dwayne Reid
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At 04:23 PM 6/4/02 -0400, A.J. Tufgar wrote:
>To answer pauls questions:
>1. This is the sensor I'm using from digi-key:  MSP6800-ND
>2. It's 0-5 psi guage.
>3. All I need to detect is any negative pressure at all (so very low
>res.)

If all you need to determine is a negative pressure, I think the '627 may
be all you need.  It just depends upon how large the input offset voltage
is compared to the minimum negative voltage you need to see.

Connect the sensor to the '627's comparitor in differential mode.  You may
also want to add an offset pot to one input to trim out any offset or to
ensure that the output is in the state it is supposed to be with zero
pressure applied.

dwayne

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(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 18 years of Engineering Innovation (1984 - 2002)
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2002\06\04@181034 by Mark Perri

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A.J.,

That pressure sensor measures gauge pressure (0 psi = 1 atm),
not absolute pressure.  Does the voltage actually swing negative
when the pressure drops below atmospheric?  Usually the
pressure gauges I work with will just stop at 0 V.

Mark

>At 04:23 PM 6/4/02 -0400, A.J. Tufgar wrote:
>>To answer pauls questions:
>>1. This is the sensor I'm using from digi-key:  MSP6800-ND
>>2. It's 0-5 psi guage.
>>3. All I need to detect is any negative pressure at all (so very low
>>res.)

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2002\06\04@232510 by A.J. Tufgar

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I had a tough time actually getting this information out of the company
I'm
dealing with.  I believe the voltage actually does swing negative and
positive in relation to the pressure placed on it.  The engineer I taked
to
said it worked both ways so I'll take his word on it.

They sent me a couple of samples though.

Aaron

{Original Message removed}

2002\06\05@152406 by Peter L. Peres

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On Tue, 4 Jun 2002, A.J. Tufgar wrote:

>I had a tough time actually getting this information out of the company
>I'm dealing with.  I believe the voltage actually does swing negative and
>positive in relation to the pressure placed on it.  The engineer I taked
>to said it worked both ways so I'll take his word on it.

The voltage on all bridge pressure sensors changes sign when they are
driven with 'opposite' pressure. Usually there is a limit on the reverse
pressure (it is lower than the forward pressure). Yuo need to account for
the offset of the bridge to determine zero. The offset will change with
the bias voltage.

Peter

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