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'[EE]: bridge circuit or equiv for PIC A/D input'
2001\02\28@015653 by Michael Jackson

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You guys were so good on the last question, I just have to ask another.
As a senior design project, I am making a temperature sensor for up to
500C range with SiC.  The SiC chunk will vary in resistance from 18k to
120k over the 0-500C range.  Now I need someway to turn the resistance
into voltage to input to my PIC A/D.  My first try was like this...

  Bridge Configuration
                       ------------------------------------------------

                               |             |                |
                       +      Rmin         Rmax              Rthree
              Vsource          |--Vref+      |--Vref-
|-------A/D in
                       -      Rmax         Rmax              Rfour
                               |             |                |
                       -----------------------------------------------

Rmin=18k
Rmax=120k
Rthree = variable;         %ohms - This is our SiC chunk
Rfour=120k
Vsource =5;          %volts


BUT I tested today with a variable in place of R3 and it didn't work
good at all.  I guess I need to use the entire 5V range to get all the
resolution I can get.  My question is, what is my best bet to do it with
minimum electronics?  I had hoped this would be good enough because I
could get away with only a 5V source.  I was thinking about ---

                       ------------------------------------------------

                                         |                |
                       +               SiC chunk         120k
                       5vdc              |---to +opamp    |---to-opamp
                       -               120k              18k
                                         |                |
                       -----------------------------------------------

and having the differential opamp for maybe 2x gain (should mean signal
out from 0 to about 4.6V)  But I was wondering if someone had an idea
for a better solution.   Thanks for any help guys..

MJ

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'[EE]: bridge circuit or equiv for PIC A/D input'
2001\03\01@174803 by Peter L. Peres
picon face
A satandard ohmmeter is built using a constant current source and the
resistor. You may have to add a buffer amplifier and/or heavy decoupling
across Rx (i.e. a good quality ~1uF capacitor) to satisfy the A/D input
impedance requirements.

18k to 120k wants a current source that gives 5V across 120k i.e. Ic =
5/120E3 ~= 41 uA. This is easily implemented with an opamp. It will not
work from 5V, you will need at least 6V for it using a rail to rail opamp
whose input range includes Vcc probably.

Peter

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