I don't mind if it's a band-pass or low pass filter (although I think low-pass will give a better result). Input operating frequency range is 100Hz to 1kHz. Input amplitude will be about 1Vp-p and although it will be a bit noisy and not very square-wave-ish it'll be good enough to feed into a comparator to get a decent square-wave should that be required.

Basically I need a design that will take a noisy square-wave output from a strain gauge amplifier, determine the frequency of said square-wave and filter out the high frequency noise and harmonics leaving a sinewave.

EDIT - the end game is to measure the amplitude of the squarewave accurately - by filtering-in the fundamental frequency it becomes easier to measure because the squarewave amplitude is very accurately linked to the sine amplitude (4/Pi from memory)

This is for a test box where the strain gauge (connected to the amplifier) is supplemented with a FET switching a large value resistor connected across the strain gauge - this simulates a small but predictable amount of strain. The output from the amplifier needs to be converted to a sinewave for measurement purposes. I'm aware that this signal could be taken into a PC running a Fourier analysis on it but it's not always convenient to have a PC handy.

I suspect, for this tracking filter a switched-cap filter like the MF10 (or based on similar) will be required and a phase-locked-loop to track the actual input frequency to generate a clock at a much higher rate.

Currently, I have a 12th order low-pass filter that is good for a small range of frequencies around 600Hz (with minor adjustments) but this is no good for lower or higher frequencies.

Please let me know if there is information missing that would help answers. I am looking for quite high precision and if it needs a select-on-test components that's not a problem.

  • 3
    \$\begingroup\$ Read up on the lock-in amplifier, which sounds like exactly what you need. \$\endgroup\$ – Dave Tweed May 23 '13 at 13:48
  • 1
    \$\begingroup\$ have you peeked at the VFC32?ti.com/lit/ds/sbvs015/sbvs015.pdf \$\endgroup\$ – Scott Seidman May 23 '13 at 14:01
  • \$\begingroup\$ @DaveTweed interesting observation - I have the signal that inputs to the amp under test and this could work - OK I'd need to generate 0º and 90º reference signals to properly do the job. Good call. \$\endgroup\$ – Andy aka May 23 '13 at 14:01
  • \$\begingroup\$ @ScottSeidman are you suggesting using the VFC32 as some kind of signal demodulator? \$\endgroup\$ – Andy aka May 23 '13 at 14:08
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    \$\begingroup\$ @Andyaka Since you're considering the MF10, look at the LTC1068-25 as an option: 4 matched 2nd order filters, clock to corner ratio 25, can be cascaded for 8th order low-pass. Use a zero crossing or low pass filter plus comparator on the source to get a clock, divide it by 24 (not 25), and feed that clock in, for a sharp cutoff just above the source signal's primary frequency, that will track as the source frequency changes. That 24 --> 25 margin will give the filter the breathing room you might need. Would that work for you? \$\endgroup\$ – Anindo Ghosh May 25 '13 at 10:00

protected by W5VO May 23 '13 at 14:15

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