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Introduction

Electrocardiogram
is one of the most frequently exercised diagnosis tool for problems related to
cardiac abnormality. Normally physicians or cardiologists go prefer
multi-channel, multi-lead ECG for diagnosis. Block diagram of a typical ECG unit
is shown below (fig 1). The most important part of the ECG unit is an ECG
amplifier, which is responsible for the faithful amplification of the signals
received from ECG electrode. A typical ECG (any biopotential) amplifier must
offer high signal to noise ratio, high input impedance, suitable gain,
isolation from body and to instruments used alongwith and high reliability.
Right leg drive technique is employed in order to reduce common mode signal
contribution to the output of the amplifier.

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The
study is around the single channel ECG unit, and, therefore, we will restrict
to single channel amplifier only. Such amplifier employs three electrodes, two
of them picking up the biological signal and the third providing the reference
potential, connect the subject to the amplifier. The input signal to the
amplifier consists of five components: (1) the desired biopotential, (2)
undesired biopotentials, (3) a power line interference signal of 50 Hz and its
harmonics, (4) interference signals generated by the tissue/electrode
interface, and (5) noise. An efficient design of such amplifier provides
rejection of majority of the signal interferences. The main task of the
differential amplifier is to reject the line frequency interference that is
electrostatically or magnetically coupled into the subject. As the line
interference to both inputs of a differential amplifier is almost same and in
same phase, it’s cancelled by common mode rejection. A good CMRR assures no
line and other ambient noise interference. The rejection of the common mode
signal in an amplifier is both a function of the amplifier CMRR and the source
impedances Z1 and Z2. For ideal conditions Z1
= Z2 and infinite CMRR the output of the amplifier is simply.

But
for a practical case of finite CMRR value the output of the amplifier can be
represented as:

Practically
the impedance unbalance can range up to 10000?. This makes it mandatory to have
CMRR of order 105 or more and input impedance be greater than 109,
in order to suppress the common mode signals significantly. Most amplifiers
used for ECG signal amplification offer CMRR 
120-140db. (i.e. 106-107). A typical signal from
ECG electrode is having amplitude of 0.01-1 mV and therefore a voltage gain of
100- 10000 is mostly significant for ECG amplifiers.

A
typical ECG amplifier must satisfy the following requirements:

·        
The amplifier must not interfere by any
means with the bio-physiological process under observation

·        
The amplifier should offer distortion free
amplification.

·        
The amplifier must offer a high signal to
noise ratio.

·        
The amplifier design should be such that
patient safety measures are considered well, and offer protection against electrical
shock hazard.

·        
The amplifier also has to be protected
from other sources like electrosurgery and defibrillation units.

Such
amplifiers can be designed using various components including general purpose
and specially designed amplifier ICs. Here is presented study of such few ECG
amplifier circuits and analysis of their performances.

Materials
and methods

As
part of the research work we have undertaken study of design and performance
analysis of ECG amplifiers with various components. After deliberate discussion
with the research group and guide we decided to use ICs LM741, LM324, TL084 and
__________to construct ECG amplifier. For analysis following parameters were
considered.

1.      Input
impedance

2.      Output
impedance

3.      Voltage
gain

4.      Signal
to Noise Ratio

5.      Operational
Voltage

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