2 channel EMG (electromyograph) has weird behavior after instrumentation amp, INA2126

Discussion in 'The Projects Forum' started by BackyardBrains, Feb 5, 2014.

  1. BackyardBrains

    Thread Starter Member

    Aug 26, 2010
    I have a working 1 channel EMG (electromyograph) and laid out a new version, by essentially duplicating our 1 ch EMG on a single board, to make a device allows recordings from two different muscles. Both use a 9V battery power supply.

    This 2 channel version is not working properly, even though each individual stage has been tested and functions as it should. I noticed when I touch and hold the + terminal of the battery, it begins to work as intended. Any help why this would be and how I could fix it?

    Here's the schematic for the 1ch EMG we have working well:

    Here's the schematic for the 2ch EMG in question:

    My suspects so far:
    A difference between the two is on the EMG1ch our instrumentation op amp is the AD623, while on the EMG2ch we decided to use the INA2126 as it has a dual configuration. The INA2126 has a "Sense" pin that isn't seen on the AD623, it states in the datasheet that this must be connected to the output for proper operation. I am curious if possibly this INA2126 isn't the right choice and we should switch back to AD623. Does anyone have experience or advice with these two?

    Probing the signal (with an input into the device from gel pad electrodes on a forearm, same as we use on the 1ch) after the instrumentation amp shows where I assume the problem may lie. On the (working) 1ch EMG, the signal at AMP1 (after the AD623) has a ~4 DC offset (~Vcc/2). On the (broken) 2ch EMG, the signal at AMP1A and AMP1B (after the INA2126) has a ~7.75 DC offset (~Vcc). Since we are powering this off of a 9V battery, it seems as though the 2ch EMG is getting pulled up here and I'm not sure why. Does anyone have some insight on this and ideas on how to fix it?

    Thank you in advance for your time and help. :)