The goal of this project is to create a circuit model with multiple components that can adequately amplify and filter an incoming ECG signal. Three components will be modeled individually: an instrumentation amplifier, an active notch filter, and a passive bandpass filter. They will be combined to create the final ECG circuit model. All circuit modeling and testing was conducting in LTspice, but other circuit simulation programs would also work.

Step 1: Instrumentation Amplifier

This will be the first component of the full ECG model. Its purpose is to amplify the incoming ECG signal, which will initially have a very low voltage. I chose to use combine op-amps and resistive components in a way that would produce a gain of 1000. The first image shows the instrumentation amplifier design modeled in LTspice. The second image shows relevant equations and performed calculations. Once fully modeled, transient analysis of a sinusoidal input signal of 1 mV at 75 Hz was performed in LTspice to confirm a gain of 1000. The third image shows the results of this analysis.

Step 2: Active Notch Filter

This will be the second component of the full ECG model. Its purpose is to attenuate signals with a frequency of 60 Hz, which is the frequency of AC line voltage interference. This distorts ECG signals, and is typically present in all clinical settings. I chose to use combine an op-amp with resistive and capacitive components in a twin-T notch filter configuration. The first image shows the notch filter design modeled in LTspice. The second image shows relevant equations and performed calculations. Once fully modeled, an AC sweep of a sinusoidal input signal of 1 V was performed from 1 Hz - 100 kHz in LTspice to confirm a notch at 60 Hz. The third image shows the results of this analysis. The slight variation in simulation results compared to anticipated results is likely due to rounding done when calculating the resistive and capacitive components of this circuit.

Step 3: Passive Bandpass Filter

This will be the third component of the full ECG model. Its purpose is to filter out signals that are not within the range 0.05 Hz - 250 Hz, as this is the range of a typical adult ECG. I chose to use combine resistive and capacitive components so the high pass cutoff would be 0.05 Hz and the low pass cutoff would be 250 Hz. The first image shows the passive bandpass filter design modeled in LTspice. The second image shows relevant equations and performed calculations. Once fully modeled, an AC sweep of a sinusoidal input signal of 1 V was performed from 0.01 Hz - 100 kHz in LTspice to confirm the high and low pass cutoff frequencies. The third image shows the results of this analysis. The slight variation in simulation results compared to anticipated results is likely due to rounding done when calculating the resistive and capacitive components of this circuit.

Step 4: Combining Circuit Components

Now that all components have been designed and tested individually, they can be combined in series in the order they were created. This results in a full ECG circuit model that first contains an instrumentation amplifier to amplify the signal 1000x. Then, a notch filter is used to eliminate 60 Hz AC line voltage noise. Lastly, the bandpass filter does not allow signal to pass through that is outside the range of a typical adult ECG (0.05 Hz - 250 Hz). Once combined, as shown in the first image, a transient analysis and full AC sweep can be conducted in LTspice with an input voltage of 1 mV (sinusoidal) to make sure the components work together as anticipated. The second image displays the transient analysis results, which show signal amplification from 1 mV to ~0.85 V. This means that either the notch or bandpass filter components slightly attenuate the signal after it has been initially amplified 1000x by the instrumentation amplifier. The third image displays the AC sweep results. This Bode plot shows high and low pass cutoffs that match those of the bandpass filter's Bode plot when tested individually. There is also a slight dip around 60 Hz, which is where the notch filter is working to remove unwanted noise.