M

#### Marc

- Jan 1, 1970

- 0

I am designing a temperature servomechanism. The layout is as

follows: I am exciting a wheatstone bridge configuration (where one

of the resistors is replaced with my thermistor) using an AC sine wave

voltage source. The bridge difference is then amplified and then sent

to my demodulator chip. The ouput of that is low-pass filtered (which

gives me a DC signal proportional to the bridge voltage difference),

sent to the PID control, and then goes to the TEC driver which of

course provides the current for the TEC.

My problem is this: I need to monitor the signal somewhere in the

configuration so I can know what the achievable temperature stability

that my circuit provides. I thought about recording the amplified

bridge difference but this is then nonlinearly related to the change

in temperature. No problem, just take the derivative of a few

equations (the diffeerence between two resistor divider equations and

the Steinhart equation) and make a substitution. But you will find

that the change in the amplified voltage is related to the change in

temperature with the value of the thermistor resistance (Rt) in the

equation. Well heck, if I had and could monitor Rt then I wouldn't

need to go through all this other business. I also thought about just

monitoring the voltage change from one side of the bridge (the side

with the thermistor obviously) but again I think Rt will show up in

the equations plus I expect the change in this voltage to be extremely

small once my circuit is properly designed, tuned, and thermally

shielded.

Perhaps I am missing something obvious here. Does anyone have any

thoughts on the best way to monitor the temperature change and

stability from this circuit? Any help is greatly appreciated. If

more information or clarification is needed please let me know.

Thanks!

Marc Airola

JHU-APL

Electro-Optical Systems Group