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INA828: EMG 50HZ Harmonics

Dave
Prodigy 90 points
Part Number: INA828
Other Parts Discussed in Thread: INA118

I have created an EMG Electrode circuit but I am having trouble with the 50Hz coupled noise from myself which is rather larger than I thought. Also the 50Hz has larger harmonics as shown below.

The signal to noise ratio has more noise than signal.

The Analogue Front End (AFE) passive components have been chosen to increase CMRR but I believe that it is not enough.

Resistors, 0.1% Tolerance

Capacitors, 1% tolerance

The circuit below has a notch filter giving good centre frequency of 50.1Hz and good attenuation.

However, I am still getting quite large harmonics.

Has anyone has similar issues like this?


Regards

  • 0
    TI__Guru* 93105 points

    Hi Dave,

    Is the 50 Hz from the AC line coupling where you are doing the testing? If so, 50/60 Hz coupling from the AC line is a common problem encountered when attempting to measure the very low level electrical signals generated in the human body with a very high gain amplifier circuit.

    The 50/60 Hz AC line is usually close to a pure sine wave and should have low harmonic content. The fact that you are observing harmonic amplitude levels that exceed the fundamental indicates that the 50 Hz signal is distorted. I don't really see anything about the EMG amplifier that would introduce the distortion unless it is just a matter of the 50 Hz being so large in amplitude that it is causing output clipping?

    How are you protecting the EMG amplifier circuit from AC pick-up? When we have experimented with similar ECG circuits we enclose the circuit board in a closed, shielded enclosure, power the circuit from batteries inside the enclosure and receive the low-level signals from the sensors using shielded cables and connectors connected at the enclosure. The amplifier output is a shielded cable as well. All of this much reduces the AC line pick-up and helps the intended signal to be observed.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0
    Guru 140200 points

    Hi Dave,

    the reason for the huge harmonics is, that the imbalance of components of the input filter of INA828 is ruining the overall common mode rejection.

    A good idea is to limit the components of the input filter to the absolute minimum, similar to what is shown below. When it comes to ECG amplification "is less more":

    Only add a current limiting resistor of 10...100k in series to each input of INA828. Take two identical, 0.1% toleranced resistors and mount them close to these inputs.

    Chose an INA with very small common mode input capacitances, like the INA118, for instance.

    Kai