Physio Control LIFEPAK 9 - Page 37

Description 1
U34A PIN 4 | | | |
TRIGGER
|
>
PULSE
ト - ECHO PERIOD» | |
TP4
———
VOLTAGE
AT TP19
TIME En 거 he— 13 —». -一 i
RAMP TRACK
Figure 1-6 Echo Period
ECG conversion begins when the pulse train at the output of counter U4
passes to U32 pin 2. The rising edge of the first pulse clocks U31A, which
sends a trigger pulse every 1.8ms to U34A. The ECG signal is offset to
-6.3V at the output of U3SA, providing a signal range of -1V to -11V for
conversion.
The ECG signal from U35A passes to a track and ramp circuit consisting of
U35B, U35C, 07, and C76. This circuit controls the duration of the echo
period. The track phase begins when O7 conducts, which causes the ECG
signal at U3SB to charge sampling capacitor C76. The ramp phase starts
when comparator U34A receives the trigger pulse (every 1.8ms), driving the
output to -12V. This voltage passes to the gate of Q7 turning Q7 off. The
+5V line and R122 provide a constant current source to the inverting input of
U35C. When Q7 turns off, the constant current source causes a linear
discharge from C76, This discharge produces a ramp output from U35C (see
Figure 1-6).
While the track and ramp circuit is tracking, comparator U34A receives the
trigger pulse from U31A. This drives the output of U34A to -12V, turns off
Q6, and allows the +15V supply to charge C72 through voltage dividers
R100 and R99. The output of comparator U34C controls the on-time of Q6,
which discharges C72 through opto-isolator U6 and creates the echo signal.
The duration of the ramp output of the track and ramp circuit determines how
long the output of U34C is low and sets the duration of the echo period.
When the ramp output of U35C reaches OV, the output of comparator U34B
switches to -12V, presenting this voltage at the inverting input of U34C. This
Causes the output of U34C to switch to high impedance which turns on Q6
and allows the echo signal to be coupled through opto-isolator U6.
©
August
1993
Physio-Control
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