Electronic – Is this an RC filter


I'm trying to understand this temperature sensor circuit I came across on a room Air-Conditioner controller (Want to build one myself).

The sensor is an NTC type thermistor (15k at 25 deg C). What I collect is that the NTC along with R2 form a voltage divider, the output of which is fed into the ADC input of the microcontroller.

  1. What are C1 and C2 for? Do they form RC filters along with the thermistor? If that is the case, then the cut-off frequencies are 106.1Hz and 1.06Hz? These appear to be very low.
  2. What is R3 for? Is it limiting the current? If ADC input pins are high impedance, why limit the current?
  3. Do R3 and C3 form another RC filter? Then again the cutoff frequency is 3.386kHz. And there's nothing apparently operating at that frequency on the board. (The micro controller is connected to a 4Mhz external crystal and the board is powered by 230V/50Hz mains).


Best Answer

Yes, the 10 µF gives a low cutoff frequency, but your calculation isn't right. First you don't calculate for C1 and C2 separately; if they're parallel they act as one single capacitor. For the calculation the 100 nF C1 can be ignored, it's only 1 % of C2 and that will have a tolerance of 20 % or so anyway. And due to Thévenin you have to see the NTC and R2 in parallel, so that's 7.5 kΩ. The cutoff frequency is then 2 Hz. The second section formed by R3 and C3 has a much higher cutoff frequency, but is your power supply that noisy?

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I would dump R3, C2 and C3, and just keep C1. Use the microcontroller to average a number of readings; a moving average filter is a low-pass filter too, is dead easy and it doesn't need any components. Olin would suggest an IIR filter (for Infinite Impulse Response), which uses less memory, but is less intuitive. Besides the moving average filter won't need 100 bytes; 5 or so is already fine.

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