Discussion in 'The Projects Forum' started by Ubuntu, Oct 3, 2013.

  1. Ubuntu

    Thread Starter New Member

    Sep 30, 2013
    We are trying here to detect and amplify EMG-sginals 65-150 Hz, the detection and amplification is working good. The problem we have is envelope detector and schmitt trigger, to detect and A/D conversion.
    Our Lower threshold= 0.2V
    Upper threshold= 0.8V
    everything in between is none of interest. We are using uA741 OpAMp, diode: 1N4148.
    P.S. envelope detector is working as a standalone but not when it's connected to schmitt trigger. See the attachment pic. I also attached the whole circuit that you can simulate on www.docircuits.com.

    All help are appreciated.
    Thank you in advanced.
  2. MrChips


    Oct 2, 2009
    There are a number of problems with your design.

    The 741 opamp is way past its best before date. The 741 is used in almost every text book but should not be used in modern designs. There are newer and better opamps available.

    Do not put the diode rectifier in the front end.

    Use an instrumentation amplifier as the first stage which will provide proper Common Mode Rejection. You can follow this with another gain stage if desired.

    Then use an active rectifier circuit (search precision rectifier) for envelope detector.
  3. Ubuntu

    Thread Starter New Member

    Sep 30, 2013
    Thanks for your answer,
    O, i will change it will appreciate if you suggest any opAMP.
    Where should I relocate the diode, It should be in the beginning to cut off the negative voltage, right? i tried several location without success.
    "Use instrument amplifier..." where do you mean, before the envelope, we will get an amplified signal for sure?
    Thanks in advanced