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Thread: voltage divider for force sensitive resistor

  1. #1
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    voltage divider for force sensitive resistor

    Hello,

    I would like to connect a few force sensitive resistors to my Teensy3.2. I am basically going to follow these instructions https://learn.adafruit.com/force-sen...r/using-an-fsr

    I sometimes saw people using capacitors in parallel with Rfix when building voltage dividers. I guess it's to dampen any high frequency noise that's why I am wondering whether it would be advisable in my case since I plan to measure force every 10ms or even less if possible.
    Would a 10nF capacitor in parallel with the 10K resistor do ?

    Cheers

  2. #2
    Senior Member+ Theremingenieur's Avatar
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    Simple computation: The time constant tau is 10kOhm * 10nF = 0.1ms. The reading can be considered to be stable after an elapsed time of 5 * tau = 0.5ms. Thus, if you want to sample every 10ms which is 20 times longer than that, you might even increase the capacitance value to 180 or 220nF which will improve the filtering.

  3. #3
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    Great ! thanks.

    And do you think using a capacitance is helpful when working with FRS ? Should I be worried about noise at all ? Obviously I don't want to make the circuit any more complicated than needed...

    The other question is: with a capacitor only in parallel with one resistor element of the voltage divider, will I loose linearity ? (i.e. the voltage divider is no longer symmetrical, so should I add a capacitance in parallel with the FRS as well ?). Having said that the response curve of FRS is non-linear in the first place and I have to apply an exponential fit. Would the new fit correct for both all at once ? Again I am weighing pros and cons of adding capacitances to the circuit.

    Cheers

  4. #4
    Senior Member+ Theremingenieur's Avatar
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    And do you think using a capacitance is helpful when working with FRS ? Should I be worried about noise at all ?
    Yes, a capacitance is definitively not only helpful but required. As soon as someone touches the FSR, the human body acts as an antenna for lots of environmental noise which risk to be coupled capacitively into the FSR, similar to touching the input pin of an audio amplifier with a finger which lets you hear lots of hum, noise, and buzz. The worst case coupling capacitance between the human body and the FSR can be estimated to about 150pF. Having a 150nF filter capacitor, this will attenuate that induced noise by at least 1:1000 or 60dB.
    Obviously I don't want to make the circuit any more complicated than needed...
    One single additional capacitor does not make a circuit more complicated. Complicated is adding a stabilized bipolar power supply to feed a bipolar op-amp to buffer and stabilize the current through the FSR...

  5. #5
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    Brilliant ! I'll go for a 220nF then.

    Any objection to use a capacitor network ? (I am already using a resistor network). "Complicated" is perhaps a misnomer, I mean bulky (I am trying to fit the electronics into the smallest enclosure possible and I have 10 FRS. I guess I'll have to go for CMS at some point)

    Cheers

  6. #6
    Senior Member+ Theremingenieur's Avatar
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    No objection to the capacitor network since these will all have a common and grounded pin.

  7. #7
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    Thanks a lot !

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