Try to determine the resistance of an NTC thermistor

[Lessnan]

I am currently using a Wheatstone bridge to determine the resistance of a NTC thermistor(Futher information about thermistor: http://www.apogeeweb.net/article/38.html) and feed the difference to the setpoint (analog potentiometer) as the error signal to a PID controller, giving me a way to stabilize the temperature of a sample in conjunction with a TEC to a very good degree.

Now, I'd like to have a way to instead control the setpoint digitally using a microcontroller. The first way I thought of was to simply replace the analog potentiometer by a digital one:

However, the selection of suitable digital potentiometers (able to have 5V at one terminal and possibly a small, negative one at the other) is quite limited and the best one I could find (AD5292) has a temperature coefficient of ~30ppm/K, which is very high for the required accuracy. Additionally, the Digipot gives me a setpoint resolution of ~0.02°C, which I'd like to push down to 0.01°C.

What do you guys think of instead replacing the potentiometer by a fixed resistor, and comparing the voltage after the INA with the output of a DAC and using this difference as the error signal for the PID loop? Would this give me the best performance? (Noise/Stability of this circuit is required to be on the order of milli-Kelvins or even less)

 

[Bystander]

(Noise/Stability of this circuit is required to be on the order of milli-Kelvins or even less)

Over how long a time span?

 

[Baluncore]

1. You will need to place your entire analogue circuit and PID controller in the stabilised oven to eliminate thermal difference voltages. That includes the final voltage regulator for the analogue supply voltages.

2. A "digital pot" is a ratiometric potential divider, not a reference resistor. Use it as a divider, not as a resistor. Keep it in the oven.

3. If the microcontroller is in the loop, use a PWM output to control the TED.

4. If you use the controller's A to D converter you should keep the controller in the oven also, or amplify the error signal before it exits the oven.

5. There will be a long term drift. If possible monitor the drift rather than trying to correct the temperature setting. Any disturbance of settings will add confusion to the knowledge of the stability.