3.8 : Instrumentation Amplifier

An electrocardiography (ECG) machine is an essential piece of medical equipment used to monitor the electrical activity of the heart. It operates by detecting small electrical changes on the skin that result from the depolarization of the heart muscle during each heartbeat. However, these signals are in the microvolt range and can be easily overwhelmed by noise or interference.

To overcome this challenge, an ECG machine utilizes an instrumentation amplifier. This specialized amplifier is designed to amplify the ECG waveform while minimizing the impact of common-mode noise signals, such as electrical interference from other devices.

The instrumentation amplifier is a type of difference amplifier. Its circuitry includes six resistors, three terminals, and an external resistor connected between the gain set terminals. This amplifier functions by amplifying the small differences between input signals while rejecting signals common to both inputs.

The output voltage of the amplifier is determined by the product of the voltage gain and the input voltage difference. The gain can be easily adjusted by varying the value of the external resistor, providing flexibility and control over the amplification process.

Instrumentation amplifiers exhibit several key characteristics that make them ideal for use in ECG machines. They have high input impedance, low output impedance, high gain stability, and a high common mode rejection ratio (CMRR).

The high input impedance prevents signal distortion by avoiding signal loading, ensuring the integrity of the electrical signals from the heart. The high CMRR enables the amplifier to effectively reject noises picked up by the electrode leads, such as electrical interference from other equipment, thereby improving the quality of the ECG waveform.

Beyond their use in ECG machines, instrumentation amplifiers are commonly employed in a variety of biomedical instrumentation and telecommunication applications due to their unique properties.