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Low Pass filter and Schmitt trigger

Discussion in 'General Electronics Discussion' started by Alex_Bam, Sep 28, 2020.

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  1. Alex_Bam


    Sep 28, 2020
    I am recovering the PWM message signal at the demodulator side of the system. In the attached schematics you can see Low pass filter which filters high frequency, then the signal is fed to Schmitt trigger to retrieve the message signal. I aimed to design the whole system on IC 350nm technology, in which it's preferred to use small size and fewer components. I would like to know:


    1) Is there any alternative for LPF, which perform the same job, with a small and fewer component? Especially I am concerned about dealing with a large resistor while implementing LPF on the IC chip.

    2) Second, Is there any best alternative to my Schmitt trigger design, with fewer components?

    Thanks in advance.

    Moderatots note : inverted the colors for better readability and posted inline

    Attached Files:

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    • wave.JPG
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    Last edited by a moderator: Sep 28, 2020
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Implementing resistors and capacitors on an IC is prone to inaccuracy. Modern circuits use switched capacitor technology to emulate the resistor by one or more capacitors and switches. The advantage is that the filter frequency is defined by:
    1) A capacitor ratio, which can be adjusted and fabricated to a much higher precision than a resistor and a capacitor.
    2) The switching frequency. Thus you can adjust the cutoff frequency of the LP-filter by adjusting the switching frequency.

    What is best? I've seen designs using only 4 MOSFETs, but for very low voltage (0.4 V) only and without adjustable threshold. You 6 transistor circuit seems to be the common design scheme.
    Alex_Bam likes this.
  3. Alex_Bam


    Sep 28, 2020
    Thanks for giving me the Switched-Capacitor idea. I will study and try to implement it in my case.
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