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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