For removing the low frequency noises first a high pass
filter of 0.03 Hz cut off frequency is used after first stage
amplification. After second stage amplification, a band pass
filter is implemented using a high pass and a low pass filter in
cascade. For ECG, EOG and EMG cut off frequency of all high
pass filter is 0.03 Hz. For ECG cut off frequency of the low
pass filter is 159 Hz. For EMG it is 564 Hz and for EOG it is
15 Hz. Three switches S1, S2 and S3 are used for filtering path
selection of the ECG, EOG and EMG signals. All those filters
are passive First order filter. General expression for cut off
frequency of the filter is given in Eq. (2)