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I am currently using a Wheatstone bridge to determine the resistance of a NTC thermistor 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:

schematic

simulate this circuit – Schematic created using CircuitLab

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)

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  • \$\begingroup\$ you want to zero the intrumentaion amplifier else you you end up measurung the thermal effect on its gain. \$\endgroup\$ – Jasen Dec 19 '17 at 1:17
  • \$\begingroup\$ If Bill was here, he would point you to this: iopscience.iop.org/article/10.1088/0957-0233/7/11/015 I think there's a free version on the web somewhere. \$\endgroup\$ – Jasen Dec 19 '17 at 1:23
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Quote from the wikipedia article on wheatstone bridges:

Alternatively, if R1, R2, R3, are known but R 2 is not adjustable, the voltage difference across or current flow through the meter can be used to calculate the value of Rx, using Kirchhoff's circuit laws. This setup is frequently used in strain gauge and resistance thermometer measurements, as it is usually faster to read a voltage level off a meter than to adjust a resistance to zero the voltage.

So it sounds like your best bet is to just measure the current of your wheatstone bridge digitally. If you adjust R2 until the current is very small, then you can use a large value shunt resistor and detect extremely small changes in current.

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