ECG Acquisition in a Computer - ECG Acquisition and Automated Remote Processing - page 55

L ead 1

+

C2

R4

R3

R6

R2

R7

R1

C1

R2

V0

+

+

R5

L ead 2

+ R3

R4

Reference

Fig. 3.2 Instrumentation amplifier configuration

5. Input impedance: This should be sufficiently high so as to ensure that input
signal is not attenuated. Most common ECG amplifiers offer an input imped-
ance of 10 MX.
6. Electrode polarization: Different varieties of Ag-AgCl electrodes are used for
ECG acquisition. The small potential, generated at the electrode-electrolyte
interface, is known as half-cell potential. This small DC potential must be
considered since it can saturate the INA output, suppressing low-level ECG
signal itself. Association for the Advancement of Medical Instrumentation
(AAMI) specifies that ECG amplifiers must tolerate a DC component of up to
300 mV resulting in electrode-skin contact.

The popular three-OPAMP configuration is shown in Fig. 3.2 [ 13 ], which offers
a high CMRR by matched pair of resistors R 3 and R 4 . The input stage offers a gain
of (1 ? 2R 2 /R 1 ), followed by a differential amplifier with gain (R 4 /R 3 ). There is a
final stage, which offers a gain of (1 ? R 7 /R 6 ).
The third stage uses a filter with bandwidth

1
2pR 7 C 2

1
2pR 5 C 1

Df ¼ ð 3 : 3 Þ

Some more configurations of biopotential amplifier are available in [ 9 ]. An
important issue is power consumption of INA block. Some monolithic and low-
power designs include current balancing and transconductance stage amplifiers,
described in [ 14 - 17 ].
In addition to the certain desirable characteristics, specific design enhancements
are required, depending on the concerned biosignal:

1. Electrical interference reduction: This is mainly contributed by PLI, RF from
transmitters, and electrical appliances. Stray capacitances are formed between
the ECG lead wires and power lines. The displacement currents flow to ground