Working principle of ECG

Time : 2023-02-09

The body surface ECG is sent to the electrocardiograph through electrodes and lead wires.

The main body of the electrocardiograph can be divided into input circuit, lead selection, amplification circuit, stylus drive and paper feeding parts in principle. Modern electrocardiograph usually has a program-controlled part.


The input circuit should have a high input impedance, which is generally above 10M, and usually uses a buffer amplifier with emitter follower. In addition, there should also be overvoltage and overcurrent protection circuit, including right leg zone circuit or shield drive circuit to reduce 50Hz kilo disturbance and other measures.

The lead selection is usually composed of a selection switch and a resistance network (Wilson network).

Select different leads by selecting different resistance combinations through the selection switch. The lead selection can be manual selection or program-controlled selection. Amplifier can be divided into pre-amplifier, post-amplifier, power amplifier, etc. The preamplifier is generally composed of differential amplifiers. The components selected to obtain a higher CMRR must be low noise. From the perspective of electrical safety, it is often made of electrical isolation (floating ground). The 1mv calibration circuit is also connected to the preamplifier. The latter stage amplifier is mainly used to amplify the circuit and filter the signal to obtain specific frequency response characteristics, including resistance-capacitance coupling circuit, blocking circuit, gain selection, cutoff frequency and 50Hz notch. The power amplifier and recorder are often connected to form a negative feedback system to improve the frequency response of the tracer and provide sufficient power to drive the tracer.

Tracing pens can include ink pens, thermal pens, etc. In some digital systems, thermal element arrays are used.

The paper feeding part is composed of a motor drive mechanism and a control circuit. Its purpose is to record the linear speed of the paper at 25 mm/s or 50 mm/s. For a non-real-time system that is stored and then printed, the paper feeding speed only needs to match the sampling rate.