ECG Recording Unit
The block diagram of ECG recording unit consists of following blocks, defibrillator protection circuit, lead selector, power supply unit, Amplifier unit and Output unit.
Connecting leads to the patient electrodes originate at the end of a patient cable and other end plugs into the ECG recorder.
The patient cable connects different leads from the limbs and chest to the defibrillator protection circuit.
Defibrillator circuit consists of buffer amplifier and voltage protection circuit.
The leads are connected with the buffer amplifier such that one buffer amplifier for each patient lead ( totally 4 leads)
The overvoltage protection circuit is necessary to avoid any damage to the bio amplifiers in recorder.
It consists of network of resistors and neon lamps.
Lamp fires when a pulse from a defibrillator is present during firing of the neon lamp. There is no output to the preamplifiers of the recorder.
It is used to feed the input voltage from the appropriate electrode to the preamplifier.
It also includes the resistors necessary for the unipolar leads.
A push button allows the insertion to calibrate the recorder.
Modern recorders are stable and their sensitivity does not change with time.
A special contact on the lead selector switch turns off the amplifier momentarily and turns it on again after the artifact is passed.
From the lead selector switch, the ECG signal goes to bio-amplifier.
Bio-amplifier consists of
Preamplifier and
Power amplifier
The preamplifier is a differential amplifier with CMRR ( Common Mode Rejection Ratio)
The pre-amplifier also provides a switch to set gain (or) sensitivity.
Pre-amplifier also avoid problems with small dc voltage from the polarization of the electrodes.
Pre-amplifier is followed by an ac amplifier called “pen amplifier” which provides power to drive the pen motor that records the actual ECG trace.
Input of pen amplifier is accessed separately with a special auxiliary input at the side of ECG recorder.