ECG Circuitry in LTspice

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Build an instrumentation amplifier using the figures provided. The gain of this amplifier will be V0 /Vi = R4/R3 (1 + 2R2 / R1). The current resistor values result in a gain of 1000, but can easily be changed to get a gain smaller or larger depending on the need.

The diagrams attached show how to build a bandpass filter, which is just a high pass filter followed by a low pass filter. The bandpass filter in the diagrams results in a bandpass from 0.5 Hz to 150 Hz. This can easily be changed by changing the resistor and capacitor values of the high and low pass filters based on the equation f = 1/(2*pi*RC), where f is the cutoff frequency. The high pass filter will change the lower bound of the bandpass and the low pass will change the higher bound.

A notch filter is needed to reduce noise from electrical equipment. The notch in this diagram is set for a notch at 60 Hz and this can be changed by varying the resistor and capacitor values based on the equation f = 1/(2*pi*RC), where f is the cutoff frequency.

Place the amplifier and the two filters in the same file with the output of the amplifier being the input of the bandpass and the output of the bandpass being the input of the notch filter. This results in the full circuitry of an ECG prior to the addition of an ADC. To test this circuit an AC sweep will show a bandpass from 0.5 Hz to 150 Hz with a notch at 60 Hz and a transient analysis will show a gain of 1000.