ECG Acquisition in a Computer - ECG Acquisition and Automated Remote Processing - page 54

3.3.3 ECG Amplifier and their Design Enhancements

The first block of the analog front end is essentially an AC-coupled low-offset
high-CMRR INA. The desirable characteristics of an ECG amplifier are given [ 9 ]
as

1. Gain: The gain of a biopotential amplifier (BPA) is expressed in decibels as

gain (dB) ¼ 20log 10 ð linear gain) ð 3 : 1 Þ

The nominal gain of ECG amplifier is around 750.

2. Bandwidth: The frequency bandwidth of the amplifier should be designed so as
to amplify, without attenuation, all frequency components of the ECG.
3. Common-mode rejection (CMR): Some of the artifacts picked up from
external sources appear as 'common-mode signal' (i.e., at both input leads) at
the input of the amplifier. Thus, CMR of the BPA qualitatively describes its
capability to reject these low-amplitude signals at the input stage and is mea-
sured by common-mode rejection ratio (CMRR), which is defined as

A d
A cm

CMRR (dB) ¼ 20 log 10 ð 3 : 2 Þ

where A cm stands for gain from common-mode input and A d stands for gain from
differential input. The effect of common-mode impedance unbalance and elec-
trode-skin impedance on CMRR is analyzed in [ 10 ] using four-terminal method.
In [ 11 ], a novel, balanced, AC-coupled, differential amplifier with AC-coupled
input stage and a third stage providing ground path using a third (common)
electrode is described. The CMRR of ECG amplifier greatly depends on
matching of the resistance pairs in the INA block, which in practical circuits is
difficult to achieve. This results in an amplification of the DC offset voltage,
mainly caused due to electrode-skin impedance and PLI. In [ 12 ], a design
approach is proposed, which does not rely on matching of resistors. It includes a
fully differential DC suppression circuit in addition to AC coupling for full
restoration of DC biopotential signals. The power line frequency (and its har-
monics) induces stray capacitive coupling with the patient body and lead wires.
The order of these capacitances is 60 pfd, which corresponds to an impedance of
64 kX at 50 Hz. The leakage current produces a voltage drop w.r.t. ground,
which appears as common-mode signal to the first stage of the ECG amplifier.
To minimize this interference effect, the CMR plays a vital role in rejecting this
noise. The typical CMRR of around 1,000 can help to minimize the interference.
4. Recovery: Sudden movement of the patient during ECG procedure can saturate
the amplifier output due to high-amplitude input transient pulses. After a small
interval, called 'recovery time,' the amplifier slowly comes back to the normal
condition.