Question

What are the characteristics of the EMG? Bandwidth (frequency content)? Magnitude (amplitude)? How

will determining the bandwidth guide your filter design? How will determining the amplitude guide your amplifier design? Using these criteria and support information, roughly design an amplifier (provide a circuit schematic) to accomplish these tasks. What will be the gain of your amplifier, what type of filter do you need, what are the cut off frequencies.....

Answer 1

THE CHARACTERISTICS OF THE EMG:

Advent of modern electronics and the process of differential amplification have enabled the measurement of EMG signals of low noise and high signal fidelity (i.e. high signal to noise ratio). With differential amplification, it is now possible to measure the full effective bandwidth of the EMG signal. Typical bandpass frequency ranges are from between 10 and 20Hz (high pass filtering) to between 500 and 1000Hz (low-pass filtering). High-pass filtering is necessary because movement artifacts are comprised of low frequency components (typically <10Hz). Low pass filtering is desirable to remove high-frequency components to avoid signal aliasing (see Gerdle et al., 1999). In the past, it was common to remove power-line (A/C) noise components (i.e. either 50 or 60Hz) by using a sharp notch filter. See Figure 1 below. There are problems with notch filtering because EMG has large signal contributions at these and neighboring frequencies. The result of notch filtering is the loss of important EMG signal information, so notch filtering should be avoided as a general rule.

Amplification is also necessary to optimize the resolution of the recording or digitizing equipment (for more information see Gerdle et al., 1999). Amplifiers of high quality have adjustable gains of between, at least, 100 and 10 000 to maximize the signal to noise ratio of the EMG signal during each recording. This range of gains provides the sufficient range of amplifications for surface EMG signals which can range typically from 0 to 6mV peak to peak (Basmajian & DeLuca, 1985). The AMT-8 EMG System from Bortec Biomedical Ltd. has a gain range of between 100 and 15 000. The quality of the EMG signal, in part, depends on the characteristics of the amplification process. While there may be several stages of amplification, the most important stage is often described as pre-amplification. Pre-amplification implies the first stage of amplification, close to the signal source. There are several important parameters in preamplifier signal conditioning of the EMG signal.

fig: characteristics of the EMG