BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 2

Gain

G

70.7% G

0

f 2

Frequency
(Hz)

f

1

Figure 1.1 Frequency response of a biopotential amplifier.

Po w e r L i n e s

Biopot ential
Amplifier

Ea r t h

Figure 1.2 Coupling of power line interference to a biopotential recording setup.

and so on. The resulting interference on a single-ended bioelectrode is so large that it often
obscures the underlying electrophysiological signals.
The common-mode rejection ratio (CMRR) of a biopotential ampli

er is measurement
of its capability to reject common-mode signals (e.g., power line interference), and it is
de

fi

ned as the ratio between the amplitude of the common-mode signal to the amplitude of
an equivalent di

fi

erential signal (the biopotential signal under investigation) that would
produce the same output from the ampli

ff

fi er. Common-mode rejection is often expressed in

decibels according to the relationship

Common-mode rejection (CMR) (dB) 20 log 10 CMRR