Post by thread:[EE]: bridge circuit or equiv for PIC A/D input

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 -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

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 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body