# Need Help Generating Equation

Discussion in 'General Electronics Chat' started by cgha20@yahoo.com, Oct 15, 2014.

1. ### cgha20@yahoo.com Thread Starter Active Member

Oct 21, 2009
61
0
I really need help generating an equation for the attached circuit. KTY84 is a temperature sensor that changes resistance value over the course of the measured temp range. It looks like a noninverting amplifier w offset but not sure how to derive the equation. PLease help

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2. ### crutschow Expert

Mar 14, 2008
13,023
3,236
The DC gain for a noninverting amp is 1 + R30/R29.

It appears that the ADR127 is configured as a constant current supply source to the thermistor, generating a constant current of 1.25V/1k.

3. ### cgha20@yahoo.com Thread Starter Active Member

Oct 21, 2009
61
0
Ok. I see that all numbers on datasheet are reflected by a 2ma constant current. How do I adjust these number for 1.25ma constant current. Is there an equation we can derive for the input to noninverting input with the resistance changing temp sensor?

4. ### crutschow Expert

Mar 14, 2008
13,023
3,236
The sensor voltage is equal to the sensor resistance times the current. From that and the other information I gave, you can derive the output voltage from the sensor resistance.

5. ### cgha20@yahoo.com Thread Starter Active Member

Oct 21, 2009
61
0
So to verify, Vin=(1.25ma)*R, R is known at a particular temp so gives Vin to amplifier. Then, Vout=(1+R30/R29)*Vin so Vout=1+1*.00125*R so then Vout = .0025*R. Is this correct?

6. ### AnalogKid Distinguished Member

Aug 1, 2013
4,542
1,251
The non-inverting amplifier input is not loading the sensor you have, so your circuit is a variable voltage source followed by a gain-of-2 amplifier. The input voltage at any temperature is the value of the constant current source times the RTD resistance. What is the resistance range of the RTD for this application?

ak

Oct 21, 2009
61
0
25C to 180C