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.

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  • \$\begingroup\$ What are your input output specs and tolerance? And temp. Range. \$\endgroup\$ – Tony Stewart EE75 Jun 24 '18 at 21:37
  • \$\begingroup\$ @TonyStewartolderthandirt Temperature range - around room temperature, but, because I need to keep the temperature stable using a heater, the target temperature is a bit hotter - around 40 degrees. The THAT2181 IC temperature range is 0-70 degrees. \$\endgroup\$ – sx107 Jun 24 '18 at 21:59
  • \$\begingroup\$ Yes for that circuit but we need to know how stable and how sensitive your VCO is. Can you define a proper spec? \$\endgroup\$ – Tony Stewart EE75 Jun 24 '18 at 22:05
  • \$\begingroup\$ @TonyStewartolderthandirt 0.036 voltage change on the input doubles the output frequency - this is the sensitivity and it works from around 10Hz up to 40kHz. The expected output stability needs to be at least as good as the solution with the 3300ppm NTC thermistor divider at the input, which provides around 0.2% stability, which is good enough, but not perfect. The VCO is used for musical purposes and even 6% change already changes the output note, say, from A to A#. \$\endgroup\$ – sx107 Jun 24 '18 at 22:10
  • \$\begingroup\$ is it a SIP or SOIC? The reason determines the size of the insulated oven heater and thermal sensor to regulate above self-heating \$\endgroup\$ – Tony Stewart EE75 Jun 24 '18 at 22:18

You could try something along these lines. Mechanical arrangement is up to you, but insulation will reduce maximum required power. Rt is a common 10K thermistor (eg. 0402 type) and R4 is an SMT power resistor. Pick R4 so that it is powerful enough to maintain the desired temperature at the minimum allowable ambient and supply voltage (preferably stick it in an environmental chamber to check it).

It's basically a bridge where the thermistor temperature causes balance at Rt= 4.99K which is about 44.5 degrees C for popular types. R6 and C1 should stabilize the controller with a sensible amount of gain so it doesn't oscillate and cause audible noise (total proportional band is about 1/3 Kelvin)


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Hm, maybe even make it without the LM358, just the transistor and a voltage divider with a thermistor? \$\endgroup\$ – sx107 Jun 25 '18 at 6:30
  • \$\begingroup\$ @sx107 Significantly worse performance to save a few pennies? \$\endgroup\$ – Spehro Pefhany Jun 25 '18 at 11:09
  • \$\begingroup\$ Having done this before with SMT heater on 2oz flex PCB. With several 1/10W to distribute heat AND 1cm blue foam insulation, you will get ambient influenced temperature errors, the error depends on Ohm’s Law for thermal Resistance of each layer Rcj/ (Rjc+Rca) \$\endgroup\$ – Tony Stewart EE75 Jun 25 '18 at 12:25
  • \$\begingroup\$ Therefore this design will fail for temp rise with insulation from overshoot, and temp sensitivity to 10’C change in ambient with uneven heating and sense errors \$\endgroup\$ – Tony Stewart EE75 Jun 25 '18 at 12:33
  • \$\begingroup\$ @TonyStewartolderthandirt maybe a peltier module will be a more stable solution then? \$\endgroup\$ – sx107 Jun 25 '18 at 13:10

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