1. ECG
ECG is a test that displays the electrical activity of heart muscles over a period of time.
Detected with the help of electrodes attached to the outer surface of the skin and a device placed external to the body
ECG reflects the electrical depolarization and repolarization of the heart muscles associated with the contractions of atria and ventricles.
The shape, time interval and amplitude of the ECG give the details of the state of the heart.
Disturbances in the heart rhythm(arrhythmia) can be easily diagnosed.

2. ECG Equipment

3. ECG Block Diagram

4. ECG Block Diagram…
The potentials picked up by the patient electrodes are taken to the lead selector switch.
In the lead selector, the electrodes are selected two by two according to the lead program.
By means of capacitive coupling, the signal is connected symmetrically to the long-tail pair differential preamplifier.
The preamplifier is a three or four stage differential amplifier gives high gain and high CMRR.
The amplified output signal is given to the power amplifier.

5. ECG Block Diagram…
The power amplifier is generally of the push-pull differential type.
The output of the power amplifier is fed to the pen motor, which deflects the writing arm on the paper.
The paper is a heat sensitive and coated with wax.
Frequency selective network is an R–C network, which provides necessary damping of the pen motor.
The pen is heated electrically and temperature can be adjusted with a heat control.

6. ECG Block Diagram… The auxiliary circuits provide
1 mV calibration signal to preamplifier
{to observe the output on the display unit and adjust the scale for a known deflection corresponds to 1mV input signal}
Automatic blocking of the amplifier during a change in the position of the lead switch.
It may include a speed control circuit for the chart drive motor.
ECGs are almost invariably recorded on graph paper with horizontal and vertical lines at 1 mm intervals with a thicker line at 5 mm intervals.
The paper recording speed is 25 mm/s or 50 mm/s.