BANDPASS SELECTION FOR BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 81

identi cation and removal. A good example of this are the artifacts produced by cardiac

pacemakers on the ECG signal. The Association for the Advancement of Medical Instru-
mentation (AAMI) recommends designing ECG equipment assuming the ECG waveform
of Figure 2.30. The highest slew rate for an ECG can be estimated by dividing the maximum
peak amplitude within the AAMI range of 0.5 to 5 mV and dividing it by the minimum rise
time of the QR interval within the AAMI range of 17.5 to 52.5 ms. This gives a maximum
slew rate of 5 mV/17.5 ms

fi

0.28 V/s for a worst-case ECG pulse. The slew rate of a
pacing pulse is much higher than this, making it possible to design a slew-rate

fi filter that

limits the rate of change in the signal rather than a speci fi c frequency band.

filter technique is also useful in other applications that require limiting
artifacts from fast transients. For example, it can be used to

This slew-rate fi

filter large artifacts produced
by magnetic-resonance imaging (MRI) equipment, allowing a patient's vital signs to be
monitored during procedures involving this imaging technique. Removing “pop” artifacts
(fast transients caused by movement) from EEG recordings and limiting stimulus artifacts
in devices designed to measure nerve conduction are also possible through the use of slew-
rate limiters.
A simple slew-rate limiting

fi

fi filter is shown in Figure 2.31. This fi filter, designed by Williams

[1998], is a simple op-amp bu ed by the addition of a bidi-

rectional diode clipping network. Whenever the input voltage to op-amp IC1 di

ff ered RC low-pass fi filter modi fi

ers from its
output voltage by at least one forward diode drop, the forward-biased diodes will conduct.
Under these conditions, the voltage at the output and noninverting input of the op-amp are
equal, causing the voltage across R2 to remain approximately constant at one forward diode
drop, V F DIODE .
A constant voltage across R2 forces a constant current that charges C1 linearly rather than
exponentially. If R2

ff

R1, the maximum slope ( ∆ V out / ∆ time) of the signal at the output is

given by

∆ t V

im

V
R

F
2

D
)(

IO
C

D
1

E
)

t
(

ou
e

slope max ∆

Input signals with a slope higher than this cause a constant rate of change at the output
equal to this limit. A signal segment with a slope lower than this limit passes through the

0.5 mV max

a r

½ sinewave

a

T

d T

a s

d = 37.5 ms min

d QT

Figure 2.30 The AAMI stereotype ECG waveform has a maximum slew rate of 0.28 V/s, which allows separating the ECG's signal
components from artifacts with higher slew rates, such as those from pacing and MRI.