Biomedical Engineering Reference
In-Depth Information
C
1
R1
47K
.001uF
R2
3.5K
IC2B
TL084
IC2A
TL084
1
R3
47K
4
1
C
2
3
2
6
+
-
1
7
OUTPUT
5
-
SETTING
R4
3.5K
C3
0.1uF
+
R6
1
50KHZ
R5
24K
1
4
.001uF
50K
R7
I
I
4.7K
I
R8
47K
IC2C
TL084
11
R9
47K
D1
9
R10
8
10
-
4.7K
1N914
+
R12
2.2K
C4
0.1uF
R13
24K
R11
47K
4
C
5
I
I
I
33pF
R14
-5Viso
150K
-5Viso
R15
350
50 KHz
AMPLITUDE
+
C6
10uF
C7
.
01uF
IC2D
TL084
SETTING
Q1
MPf102
11
D2
3.6V
1W
R17
47K
I
I
R16
10K
13
D
R18
G
14
-
12
S
10K
+
4
C8
.01uF
I
R19
24K
I
+
C9
10u
F
C10
.01uF
+5Viso
I
I
I
I
Figure 8.16
A 50-kHz sinusoidal oscillator used to drive the current source of the general-purpose impedance plethysmograph. The out-
put of the
RC
oscillator formed around IC2A is feedback-stabilized by JFET transistor Q1, which is controlled by the error signal between
the amplitude set point given by R16 and the average oscillator output signal envelope calculated by the detector formed by D1 and C4.
15 should be supplied to the circuit from a medical-grade power supply. The isolation
ampli
5V
used to power the 50-kHz oscillator, constant-current source, and synchronous demodula-
tor. Alternatively, a 9-V battery can be used to power these circuits via the
fi
er (IC7) generates isolated
15 V, which are regulated via IC10 and IC13 to
5-V linear
regulator IC10. If a battery is used,
5 V is generated using voltage pumps IC11 and IC12.
Measurements of ventricular volume using a “conductance catheter” are done in essen-
tially the same way as for impedance cardiography, but using catheter-borne electrodes
instead of surface electrodes. As shown in Figure 8.21, a multielectrode catheter is intro-
duced in the left ventricle. A small (e.g.,
A), high-frequency (e.g., 50 kHz) constant
current is injected via the two extreme electrodes, and voltages are measured by several
intermediate pairs of electrodes. The idea is to divide the ventricular cavity into a number
of cylindrical segments. The various voltage signals are each proportional to the volume
held by each segment. The sum yields the total ventricular volume.
10
µ
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