Question

A EEG signal of a patient has a peak voltage of 1mV and operates in the range 5Hz to 70 Hz. Design a bioinstrumentation system that would be able to measure this system . Do make good measurements the signal has to be in the range of 0.5V to 5V and the 50Hz line frequency needs to be eliminated. Draw Circuit diagram

Answer 1

The entire schematic circuit goes as:

V_in ------>Band Pass Filter ------> Band Stop Filter ------> Amplifier --------> V_out

One should need a Band pass filter of lower cutoff frequency of 5 Hz and a upper cutoff of 70 Hz.

Band Pass Filters can be used to isolate or filter out certain frequencies that lie within a particular band or range of frequencies.

Working of Band pass Filter

Lower cutoff:

If R₁ is taken to be 10 k Putting in the formula,

Similarly,

Upper cutoff:

If R₂ is taken to be 1 k Putting in the formula,

A Band Pass filter

To cut out 50 Hz signal, the signal output of Band pass filter must pass through a Band stop filter working around 50 Hz.

A band Stop Filter known also as a Notch Filter, blocks and rejects frequencies that lie between its two cut-off frequency points passes all those frequencies either side of this range..

I want the band stop filter to have a very small bandwidth with respect to the band width of the band pass filter. Lets say it works around 49 Hz and 51 Hz (bandwidth of 2 Hz).

Again,

If R₁ is taken to be 10 Putting in the formula,

If R₂ is taken to be 10 Putting in the formula,

Working of Band Stop filter

Schematic Circuit

Circuit Diagram of Band stop filter with unity Gain

Finally I want my filtered signal to be amplified such that 1 mV input signal will correspond to 5 V output signal. This can be achieved by passing the filtered signal through an inverting Amplifier (OPAmp).

Gain:

Say the OPAmp operates between -15V and 15V.

Gain is:

If R_in=5 k then R_f=5 M