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I am trying to measure the resistance of a nichrome wire that has a PWM (1000Hz and different duty cycles between 0 and 100%) voltage applied to it from a 5.0V regulator. The setup I have in mind looks like this:

5V PWM ---- Wire (~20 Ohm) ----X---- 1W Resistor (0.5 Ohm) --- Ground

I then measure the voltage at point labeled X with respect to ground, and amplify it using an instrumentation amplifier (TI INA122). As the nichrome wire is heating, I want to obtain its temperature by measuring its change in resistance which would correspond to change in measured voltage given by my amplifier. However, I expect noise so I want to filter it out. I don't know to go about doing this since the signal is PWM. If it was straight up DC then I'd use a low pass filter, but now I am stuck.

Here is a picture of what I'm trying to accomplish. Basically smooth out the noise like shown. enter image description here

Thank you in advance!

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1 Answer 1

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You can still use a low pass filter but you might need to use a higher order filter in order to reduce the switching noise of the PWM. I'd place a low pass filter between 1 ohm resistor and input to the amplifier just to kill-off the bulk of the PWM noise and then after the amplifier maybe add another low pass filter. I take it that this will then feed some micro controller - if so the MCU can perform digital filtering to reduce noise even further.

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  • \$\begingroup\$ This will make the output be a function of the duty cycle. I want the output to be a function of the temperature only. \$\endgroup\$
    – Nic
    Apr 28, 2015 at 20:30
  • \$\begingroup\$ You could gate the positive part of the waveform using an analog switch. The output of the switch would feed a capacitor. Then use an opamp filter afterwards. Sorry for originally misinterpreting your question. Btw this technique is called sample and hold. \$\endgroup\$
    – Andy aka
    Apr 29, 2015 at 11:44

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