Heart Monitor (ECG) System
Real-time analog signal acquisition and conditioning system for measuring cardiac biopotentials
Final breadboard implementation of the ECG signal acquisition and conditioning system
Overview
I designed and built a real-time ECG signal acquisition system capable of amplifying millivolt-level cardiac signals and conditioning them for waveform visualization. The project combined several analog circuit stages developed and validated individually before being integrated into the complete heart monitor.
The signal chain used a three op-amp instrumentation amplifier followed by analog filtering stages to amplify the differential biopotential signal while rejecting unwanted common-mode interference and out-of-band noise. The completed system achieved approximately 2000× overall differential gain with a signal bandwidth extending to roughly 40 Hz.
Analog Signal Chain
Instrumentation Amplifier
Amplified the small differential voltage produced by the electrodes while minimizing amplification of common-mode signals.
Bandpass Filtering
Conditioned the amplified signal to emphasize the frequency content associated with the ECG waveform and reject unwanted components.
Low-Pass Filter
Attenuated higher-frequency noise before the final stage of signal conditioning.
60 Hz Notch Filter
Suppressed power-line interference while preserving the cardiac waveform produced by the preceding stages.
Signal Acquisition & Amplification
The front end of the system used a three op-amp instrumentation amplifier to acquire the differential biopotential generated across the electrodes. Because the ECG signal is extremely small, the amplifier needed to provide substantial differential gain without also amplifying common-mode interference.
A major part of the design process involved tuning the instrumentation amplifier to achieve the required differential gain while keeping common-mode gain low. I characterized the circuit experimentally and debugged individual stages before integrating them into the final system.
Oscilloscope measurement of the cardiac signal after amplification and filtering
Comparison of the ECG signal before and after the 60 Hz notch filter
60 Hz Noise Rejection
Power-line interference was one of the primary noise sources affecting the acquired ECG signal. I designed and tuned an active notch filter centered around 60 Hz to attenuate this interference before displaying the final waveform.
The notch filter was especially sensitive to component tolerances. Small differences between resistor and capacitor values changed the attenuation near the target frequency, so I tuned and experimentally validated the circuit rather than relying only on nominal component values.
Frequency-Domain Validation
I validated the completed filter chain in both the time and frequency domains. Using the oscilloscope, I compared the low-pass filter output with the final notch-filter output to observe the effect of 60 Hz rejection on the measured ECG waveform.
I then captured the signal spectrum from 0 to 100 Hz before and after the notch filter. This provided a direct frequency-domain comparison of the signal chain and allowed the reduction in power-line interference to be evaluated alongside the heartbeat's fundamental frequency content and harmonics.
0–100 Hz spectrum comparison before and after notch filtering