# Need Help Generating Equation

#### cgha20@yahoo.com

Joined Oct 21, 2009
82
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

#### crutschow

Joined Mar 14, 2008
31,501
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.

#### cgha20@yahoo.com

Joined Oct 21, 2009
82
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?

#### crutschow

Joined Mar 14, 2008
31,501
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.

#### cgha20@yahoo.com

Joined Oct 21, 2009
82
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?

#### AnalogKid

Joined Aug 1, 2013
10,216
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