Obtaining Electromyography (EMG) signals from muscles is challenging. The EMG signal amplitude typically varies between 50 µV and 30 mV, which requires amplifiers with high gain for accurate signal amplification. Due to the ultra-low voltage range of the input signals, significant noise is picked up during amplification. To address this, a series of filters is used with cutoff frequencies between 0 Hz and 500 Hz, aligning with the frequency range of EMG signals. The process of obtaining EMG signals from muscles involves several steps.
Electrodes
Copper (Cu) is a conductive material that is easily accessible, making it a suitable choice for constructing electrodes. The quality of the signal depends on the electrode's shape and contact area with the skin. Research on surface electrodes indicates that circular shapes are ideal, with an optimal diameter of 10 mm for effective signal capture.
Filters & Amplifiers
Three electrodes are used to obtain EMG signals. One electrode, placed on a non-muscular part of the body, serves as the reference (Ground/Passive Electrode) and is connected to the ground terminal. The other two electrodes (Active Electrodes) are positioned on the muscle, with one placed at the muscle's midpoint and the other near the edge. For this experiment, the target muscle is the calf.
Initially, high frequencies are filtered out before reaching the differential amplifier. To maintain high input impedance and stable gain, an instrumentation amplifier is used. In the first stage, a Low Pass Passive Filter with a 486 Hz cutoff frequency is applied to each active electrode’s input, ensuring input impedance is not reduced. The second stage employs the instrumentation amplifier configured as a High Pass Filter with a 16 Hz cutoff frequency, as frequencies below this threshold do not significantly affect the output signal.
The circuit's response characteristics depend on its frequency response, shown in the simulated frequency response below.
According to the frequency response curve, the circuit functions as a bandpass filter with approximately 16 Hz as the lower cutoff frequency and 500 Hz as the upper cutoff frequency. The prototype circuit, including additional components like a Notch Filter, Gain Amplifier, Rectification Circuit, and Pass Filters, is shown below.
The instrumentation amplifier used in this circuit is the INA128 (datasheet), known for its high Common Mode Rejection Ratio (CMRR) of 120 dB. Below is the EMG signal output for calf muscle contraction and relaxation, displaying both time-domain (blue) and frequency-domain (red) waveforms. A 50 Hz supply frequency is also visible in the output.
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