I recently bought a board that consists of this circuitry. It came without an instruction whatsoever, so I have to figure out how to measure the temperature based on the output voltage at the To.
The circuit consists of a Wheatstone Bridge of R13-R15 and an NTC thermistor TH1. the voltage between point A and B Vd then go through a differential amplifier U3A and then further amplified by U3B. I don't have a volt meter or oscilloscope with me, what I did is read the To using an Arduino ADC and convert it to Voltage output. In order to calculate the temperature, I need to know the thermistor value based on To output voltage, I sort of know the basic theory on how this circuit works, but not quite able to figure out the relationship between To and the resistance of the thermistor yet.
Here are my questions:
1) What is the gain of this overall circuit? If I read it correct, it has a gain of 12, so Vd = To/12, is this correct?
2) Once I derive the Vd, I assumed that:
Vd = Vref * R14/(R14+R15) - Vref * R15/(R15+TH1)
and further derive from this formula to get the TH1 value. Again, what is the correct formula for getting the resistance value of TH1?
But so far what I get seems to be way off from the correct temperature.