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ts555 CMOS oscillator seems sentitive to temperature

Question asked by coppens.john on Mar 4, 2017

I'm using a simple capacitance to frequency converter with a ts555 to measure water level. After installation, I notice a large drift, so I returned the circuit to the lab to diagnose the problem. To eliminate the sensor as source of problem, I replaced it with a fixed (stable) value.

 

The drift seemed temperature related, so I tried to change each component's temperature separately, and was surprised that the power supply decoupling cap seemed the problem. I realized that probably the value was too low, so I changed with with several larger values, of different technologies, but the sensitivity didn't really go away.

 

I would like to simulate the circuit in Spice, but couldn't find a specific model for ST's TS555 version.

 

Questions:

  • Where can I find the Spice model for the TS555?
  • Any suggestions to reduce the temperature sensitivity?

Screenshot of the oscillator (C3 was changed to 22nF, then 100nF. T5/T6 are the terminals for the level sensor, replaced by a 560pF capacitor for the tests):

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