*4.1.3. Band pass filtering for EMG*

As mentioned previously, for the transmission of pure EMG, the high and low frequency noise should be deleted. For this purpose, only a specific band of frequency should be carried forward [20]. This can be made possible with the help of a band pass filter. A band pass filter response is shown in Figure 20.

**Figure 20.** Band Pass Filter Response

The frequency region where the response of the EMG signal is '1' is called the 'passband' and in the case of band pass filter, it is between *f1* and *f2.* 

A band pass filter can be designed by connecting a low pass and a high pass filter in series. By selecting proper values of R and C, we can develop a band pass filter which can carry forward the most effective component of the EMG signal. It is recommended that for EMG, *f1* should be 65-70 Hz and *f2* be 150-180 Hz.

#### **4.2. Amplification**

After the signal has been filtered properly and a suitable band of EMG frequency is obtained, the next stage is amplification. The EMG signal obtained has to be powered up to a suitable level. The amplification of the EMG signal can be easily carried out with the help of a non-inverting amplifier, shown in Figure 21.

The gain of the amplifier is provided in the figure as 'Av'. The non-inverting amplifier is only used when the signal is being received from a single wire referenced to ground. Amplification can be done in stages in order to cater for chip requirements, by cascading them in series.

The EMG signal, as mentioned before, is very weak i.e. only 1-10 mV. For certain muscles, for which the signal response is very strong e.g. Biceps Brachii, a gain of 500-1000 can be enough. But for muscles, whose EMG response is weak e.g. Flexor Palmaris Longus (ring finger muscle), the gain settings should be very high i.e. 10000.

The proper gain setting solely depends upon the signal response observed from the subject's target muscle. It is to be noted that every subject gives a separate signal response. Some subjects will give weak responses as compared to others. So, in that case, appropriate gain value should be set once the subject's EMG signal response is properly observed.

**Figure 21.** A Non-Inverting Amplifier
