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INA333: EMG circuit - Suggestions for improvement

adrian von
Prodigy 10 points
Part Number: INA333
Other Parts Discussed in Thread: TLV9062, LM324

Hi all,

I my free time, I designed the attached schematic. My intention is to monitor EMG signals from the arm muscles. The circuit is quite self explanatory, two electrodes connected to the INA333 IA, followed by an active low pass and high pass filters. Before proceeding with buying the components etc, I would like to know if there are ways to improve the performance of the circuit (example: change component values, include additional components, etc). Ideally, I get the highest CMMR and a low noise output. Also, when checking the frequency response at the output of the LPF, I got the plot shown below. Is the bump at the end of the freq response normal? are there any component values which I need to change to improve this?

I would highly appreciate and suggestions for improvement.

Thanks in advance.  

  • 0
    Guru 140200 points

    Hi Adrian,

    the bump in the frequency response at high frequencies is a well known phenomenon when using a too slow OPAmp for the active low pass filter.

    I would exchange the LM324 by the TLV9062 and would add a passive 1k + 10n low pass filter directly at the output of INA333. Insert the passive low pass filter between the output of INA333 and C9.

    By the way, the ratio of the components for the active low pass filter is a bit unsual. Is there a special reason for your choice of these components?

    Kai

  • 0 in reply to kai klaas69
    Guru 48395 points

    Kai is right, 

    The SKF LP you are using has a stopband rejection problem as the signal feeds straight through the feedback C and shows up supported by an increasing op amp output impedance at higher F.

    Faster op amp is one solution, using an MFB topology is another - it does not have that feedforward issue.

  • 0
    TI__Genius 10850 points

    Thank you Kai and Michael.

    Hi Adrian,

    TI doesn't have any specific EMG reference designs that I could refer to you, but we do have another E2E thread with EMG discussions which you may find useful: https://e2e.ti.com/support/amplifiers/f/14/t/196300?INA128-EMG-design-question

    Also, I noticed you didn't have a bias current return path on the inputs of your INA. You must provide a current path for the input bias current to both input terminals. Without the input bias current path, the inputs will float to one of the rails resulting in improper operation of the circuit. This can be done by connecting a large (100k-ohm) resistor to each input to VCM. 

    -Tamara

     

  • 0
    TI__Genius 10850 points

    Hello again,

    We haven't heard from you in a while. We hope the issue was resolved. I am going to close this thread. Please reply to re-open if you have any questions.

    -Tamara