So I have a voltage controlled oscillator with an exponential input made using the THAT2181 VCA IC. The schematic is shown below - the THAT2181 IC is used to change the C4 capacitor charging current and, therefore, the output frequency depending on the LOWCV input.
The problem is the thermal stability of such a solution - THAT2181 has a 3300ppm temperature dependance and for precise audio it is unacceptable - with the same input the schematic will output 807 and 837 Hz signal at 25 and 35 degrees Celsius respectively.
A typical solution for this problem is adding a 3300ppm NTC thermistor divider at the input, but those are hard to find, not exactly precise anyways and manufacturers today move towards "IC heating solutions" in transistor array ICs using one of the transistors in it as a heater.
So, my question is: what will be the most compact, yet cheap solution for THAT2181 IC heating in my schematic? I've looked at the PTC heaters, but the minimal transition temperature of them is about 60-70 degrees Celsius, which is way too hot (I am looking for about 40 degrees) and adding a microcontroller feels like an overkill.