Experiments with Vref Ovens

[thermistor-guy]

What thermistor do you use?

Do "typical" NTC thermistors drift over time?

As others have noted, class-encapsulated thermistors are more stable than the epoxy versions. BIPM recommends
glass encapsulation:
    https://www.bipm.org/documents/20126/41773843/Guide-SecTh-Thermistor-Thermometry.pdf

NTC thermistors do drift a little when operated at 100 deg. C or so, and accelerates at temperatures above that. The drift
tends to be "thermometric", where the whole resistance/temperature curve shifts.

At operating temperatures below 75 deg. C, I'd expect a glass device to be stable within 1 or 2 mK over 1 year, with a good device
having less than that. From BIPM:

The most stable thermistors are bead thermistors encapsulated in
glass. Within the range –20 °C to 60 °C, selected and pre-aged thermistors may be
stable to better than a few tenths of a millikelvin per year.

I only use glass devices, typically in a DO-35 package. You might use an epoxy device in a cost-sensitive application.

[dietert1]

Yes, i am using glass thermistors with good results. They are pretty expensive now.
In my oven setups the thermistor serves to control the oven heater, providing very low temperature noise.
There is another thermometer for the critical device inside the oven, usually a pn junction (-2.1 mV/K) or a Pt resistor. An outer control loop fine-tunes the oven set temperature to bring the DUT to the desired temperature. Need to take a closer look at the temperature drift data.
The outer oven i made for our ADR1399 evaluation board also has a glass thermistor and it includes a SHT35 temperature/humidity sensor. After nearly a year the SHT35 temperature may have drifted +10 mK but that isn't a measurement, more like an upper limit. That oven isn't really hermetic and there has been a large drift of humidity. When talking about sensor drift, you want both constant temperature and humidity.

Regards, Dieter

[dietert1]

Meanwhile i learned about the TI ADS1235 that is a similar delta-sigma ADC as the ADS1256 but includes a bridge voltage reversal MUX. For the time being i will continue with the separate MUX as it gives more flexibility. The ADS1256 operates at 20 msec cycle time plus some msec pauses for switching inputs and voltage reversal MUX. With three thermistors i can use 16 measurements for each thermistor in one second: 4x thermistor, 4x reference resistor, 4x thermistor with voltage reversed and 4x reference resistor with voltage reversed. This gives 3 x 16 x 20 msec = 960 msec, so 40 msec remain for switching.

Until now i wired two thermistors, the second one on the oven lid. I calibrated the two thermistors against each other and their temperature difference was only 0.042 K. These are TE CONNECTIVITY GAG10K3976A1. When running the oven at 37 °C and 21 °C ambient, the temperature difference between bottom thermistor (near TEC) and top thermistor (on lid) is about 0.35 K - more than expected. The PID controller can steer the average of those two temperatures as well, with about 16 uK standard deviation. Next is a third thermistor on the TEC heatsink.

The DAC8551 with its 16 bit resolution was replaced by a PWM running at 80 MHz / 64 000 = 1250 Hz pulse frequency, plenty enough for < 0.1 Hz bandwidth. When combined with some sigma-delta type modulator implemented in firmware it gives another 10 bits resolution in 1 second. Accuracy doesn't really matter in this case, but resolution is important, as the TEC usually runs at low currents. Another solution would be two TEC current ranges, one for power-up and a second one for steady state operation.

Also i added a TFT for 8 minutes of real time log display which helps a lot for tuning the PID. I also implemented kind of a demodulator that determines period of (damped) oscillation and damping factor. This also helps with the PID configuration. So there is no autotune but it became rather easy to configure the PID correctly.

Regards, Dieter