Biomedical Engineering Reference
In-Depth Information
+15V
C2
0.01uF
IC1
TL081
J1
1
2
R1
R2
7 1
J2
1
2
3
2
+
VIN
6
C1
1uF
10K
1.8M
VOUT
-
4 5
C3
0.01uF
D
1
-15V
1N4148
D
2
1N4148
Figure 2.31
A simple slew-rate-limiting filter is based on a simple op-amp buffered
RC
low-pass filter modified by the addition of a bidi-
rectional diode clipping network. Whenever the input voltage to op-amp IC1 differs from its output voltage by at least one forward diode
drop, the forward-biased diodes will conduct. In this region, R2 forces a constant current that charges C1 linearly rather than exponentially,
thus limiting the slope of signals at the output of the filter.
circuit without attenuation. With the values shown in the schematic diagram, signals with
a slope of less than 0.33 V/s pass una
ff
ected, allowing ECGs to be
fi
filtered from fast arti-
facts without distorting their worst-case 0.28-V/s slopes.
J. Moore has proposed a di
filter ECG sig-
nals from fast transients. The following circuit is described by Moore [1991] in U.S. Patent
4,991,580 as part of an ECG recorder that is immune to artifacts induced by MRI equip-
ment. When the patient is placed inside the bore of a magnetic-resonance imager, the
strong time-varying magnetic
ff
erent slew-rate limiter that can be used to
fi
fields produced by the MRI system can induce voltage
spikes on the ECG leads with an amplitude of 65 mV and a duration of 0.5 ms.
In the circuit of Figure 2.32, when the output of op-amp IC1A is positive, diodes D2
and D3 are forward biased and diodes D1 and D4 are reverse biased. Under these condi-
tions, zener diode D5 is in series with diodes D1 and D4, cathode positive and anode neg-
ative. If the op-amp's output voltage exceeds the D5's breakdown voltage plus two diode
forward voltage drops, the voltage at the junction between R1 and R2 will be limited to the
zener voltage plus the two diode drops. With a 6.2-V zener, the limiting voltage is approx-
imately 7.4 V. Negative voltage swings will have a similar e
fi
ect, placing the zener diode
in series with D1 and D4, limiting the negative swing at the junction between R1 and R2
to
ff
7.4 V.
In operation, the voltage past the recti
ed-zener limiting bridge is converted by R2 to a
current. Since the voltage at the input of the resistor is limited to
fi
7.4 V, the current
fl
A. This current charges
capacitor C3. The change in voltage across this capacitor is then limited by its capacitance
since
dV
C
/
dt
flowing through the resistor will be within the range
0.74
µ
I
C
/
C
. For a 1-
µ
F capacitor, and with a current of no more than
0.74
µ
A,
the slew rate of the signal bu
7 V/s. The slew-rate limiting
applied to the biopotential signal is calculated by dividing 7 V/s by the gain of the stages
preceding the slew-rate limiter. For example, if the front-end biopotential ampli
ff
ered by IC1B is limited to
fi
er has a
gain of 21, the slew-rate limiting referred back to the biopotential ampli
fi
er's input signal
is 0.33 V/s.
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