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Noise Problems Encountered in Constructing a Circuit to measure EMG from forearm muscles

Ahmad Jazlan
Prodigy 80 points
Other Parts Discussed in Thread: UAF42

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Hello Everyone

For a university class project on biomedical instrumentation, I constructed the following circuit to measure electromyogram signals from the forearm muscles, namely the flexor digitorum superficialis muscle. I want to do an experiment to relate hand grip strength to electromyogram signals measured from the forearm muscles. However this circuit gives inconsistent results; it sometimes measures EMG signals very well, at other times there is allot of noise as shown in the figures in the attachment.

 

6765.forum_TI.docx

 

In this circuit schematic U1 is a Burr Brown INA 128 instrumentation amplifier with gain = 20. U2 is an active unity gain first order high pass filter with 50Hz cut off frequency constructed using Texas Instruments OPA2604 op amp. U3 is a low pass filter with cut off frequency of 500Hz. This low pass filter also amplifies the EMG signal, the gain of the amplification is set by R3/R4 = 165. Therefore the total gain of this circuit is 20x165 = 3300. All circuits are powered by +9V/-9V from batteries and each IC has a 0.1 microfarad ceramic capacitor.

 

Could you all please suggest any improvements I can add to this circuit to solve this noise problem and get consistent EMG recordings? I have read about the usage of the UAF42 as a 50Hz notch filter (Im from Malaysia where power line interference is 50Hz instead of 60Hz). If I add a notch filter, should I put it after the band pass filter or before the band pass filter? Should I replace the simple first order high pass and low pass filters with Sallen Key filter topology?

 

I also have another question regarding safety. It is common to put a series protection resistor (around 10K to 100K) between each of the recording electrodes and instrumentation amplifier inputs. Another protection resistor (around 1 Megaohm) is usually placed between the ground(elbow) electrode and ground of the EMG circuit. I have tried doing this but these resistors tend to be a cause for noise. Will it be okay to use  parallel diodes at the instrumentation amplifier inputs (instead of protection resistors) and do away with the 1 megaohm resistor between my elbow and ground?

  • 0
    Guru 21975 points

    Ahmad,

    It can be difficult to eliminate 50Hz noise in a prototypes in a laboratory environment. Lack of shielding and uncertain grounding configurations can be very troublesome.

    First, with regard to safety and protection. The 1M-ohm resistors in series with the inputs provide some protection for the subject being measured in case of some fault condition. The clamping protection diodes you propose may help protect the amplifier but they do not protect the subject. In fact, without the resistors, the diodes increase the chance of fault currents flowing through the subject.

    In your previous posting, Noise Problems Encountered in Constructing a Circuit to measure EMG from forearm muscles, you inquired about use of an isolation amplifier for safety. An isolation amplifier is also very helpful in reducing the noise that you are experiencing. When the input circuitry is allowed to "float" with respect to main-powered equipment, interfering noise currents are not allowed to flow.

    In general, it may help to unplug unused lab instruments and disconnect unnecessary equipment from your circuitry. Sometimes it helps to use a "ground buster" on mains connections so that the instrument is allowed to float. Be extremely cautious to avoid shock hazards. Galvanic isolation is the only sure way to avoid risk.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 80 points

    Thanks Mr Bruce.

    I will incorporate galvanic isolation and try passing the EMG circuit output through a notch filter constructed using a UAF42.