Adaptive Filters and Applications - Digital Signal Processing: Fundamentals and Applications - page 477

To 60-Hz
wall outlet

x () y ()

Adaptive
FIR filter

Reference
input and
ADC

ECG recorder with
the removed 60 Hz
interference

Primary signal
with the
inteference

60-Hz
interference

dn sn nn () () ()

ECG
preamplifier
and ADC

FIGURE 10.19

Illustration of canceling 60-Hz interference in ECG.

from the power line, including effects from magnetic induction, displacement currents in leads or in
the body of the patient, and equipment interconnections and imperfections.
Figure 10.19 illustrates the application of adaptive noise canceling in ECG. The primary input
is taken from the ECG preamplifier, while a 60-Hz reference input is taken from a wall outlet
with proper attenuation. After proper signal conditioning, the digital interference xðnÞ is acquired
by the digital signal (DS) processor. The digital adaptive filter uses this reference input signal to
produce an estimate, which approximates the 60-Hz interference nðnÞ sensed from the ECG

amplifier:

yðnÞ z nðnÞ (10.18)

Here, an FIR adaptive filter with N taps and the LMS algorithm can be used for this application:

yðnÞ¼wð 0 ÞxðnÞþwð 1 Þxðn 1 Þþ / þ wðN 1 Þxðn N þ 1 Þ (10.19)

Then after convergence of the adaptive filter, the estimated interference is subtracted from the primary
signal of the ECG preamplifier to produce the output signal eðnÞ , in which the 60-Hz interference is
cancelled:

eðnÞ¼dðnÞyðnÞ¼sðnÞþnðnÞxðnÞ z sðnÞ

(10.20)

With enhanced ECG recording, doctors in clinics can give more accurate diagnoses for patients.
Canceling the maternal ECG in fetal monitoring is another important application. The block
diagram is shown in Figure 10.20 (a). Fetal ECG plays an important role in monitoring the
condition of the baby before or during birth. However, the ECG acquired from the mother's
abdomen is contaminated by noise such as muscle activity and fetal motion, as well as the
mother's own ECG. In order to reduce the effect of the mother's ECG, four (or more) chest leads