Help! EMG Circuit

Discussion in 'The Projects Forum' started by Ocelot, Jul 2, 2009.

  1. Ocelot

    Thread Starter New Member

    Jul 2, 2009
    Hi everyone,

    I am trying to design a EMG circuit as a sensor for a robotics project I'm working on. I have came up with a design based on design's I've found on the internet and my limited knowledge of op-amps and analog circuitry. (see attachment, sorry if its blurry - i dont have a scanner and my iphone camera sucks)

    I can get the diff. amplifier portion of the circuit working (MAX666CPA, LMC6041IN, INA106) by itself but when i try to connect the rectifier circuit (low pass filter, high pass filter, and TL084) not only does it not work but, even after I disconnect the rectifier circuit, the diff. amplifier portion stops working and only will output a constant ~ -1.17V. :confused: I checked to make sure the power supply circuit is still functioning and is in fact generating 5V with a 2.5V virtual ground. Usually if I just turn it off and let it sit overnight, it will start working again in the morning but that sometimes doesn't even work. :mad:

    What the heck is going on? I'm at wits end trying to debug it. I've tried creating a separate 5V power supply circuit with 2.5V virtual ground to power the second portion of the circuit and also have tried using a dual supply (+/-9V) to power the second portion.

    When the diff. amp portion of the circuit is working, it produces a signal around the magnitude of 20mV. The point of the rectifier circuit is to filter, rectify, envelope and amplify the signal to around 4V (or at least to a scale thats within the 5V detection range of my MCU).

    Does anyone know why this intermittent functionality is happening?

    here's the data sheets on the chips in the design:

    Thank you for your time!
    Last edited: Jul 2, 2009
  2. Audioguru


    Dec 20, 2007
    The minimum supply voltage for a TL084 is 7V because its input common-mode voltage range is from +3.5V above its negative supply to -3.5V below its positive supply.
    You have its + inputs at only +2.5V which is too low.

    The TL084 has the "Opamp Phase Inversion" problem when its input voltage is too low which causes its output to suddenly go high. With a 5V supply its max output is +3.8V.
    The rectifier opamp has a diode in series with its output which reduces its max output voltage to +3.1V.

    Your rectifier circuit has an extra 10k resistor from its input to the negative supply which causes its output to be positive all the time.
    I think your rectifier circuit is designed to use a dual-polarity supply so its output can be 0V without a signal.
  3. Ocelot

    Thread Starter New Member

    Jul 2, 2009
    Thanks for replying Audioguru!

    Do you know of any chips that I could replace the TL084 with that would be better suited for this application?

    Do you have any suggestions on how to improve the rectifier circuitry? I thought I designed this one so that it essentially takes a running average of the EMG signal since and EMG signal is symetrical. So only taking half of the signal gives me the running average. I then filter the signal so that only the envelope (aka outline) is passed. I do want the signal to always be positive since I'll be passing it to a MCU that has a ADC works in the 0-5V range.

    I thought a virtual ground makes it so you can use a single supply with a circuit that needs a dual-polarity supply. Can I not do this with the rectifier circuit?
  4. Ocelot

    Thread Starter New Member

    Jul 2, 2009
  5. NullPoint


    Jul 16, 2011