Physio Control LIFEPAK 9 - Page 34

LIFEPAK 9 defibrillator/monitor
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PROPORTIONAL TO NEGATIVE VOLTAGE AT TP20
TP4
Figure 1-5 Preamp Timing
Instruction Decoding. The pulse train commands from the System
microprocessor are coded by the number of pulses as listed in Table 1-3.
The number of pulses represents a binary number which corresponds to a
particular command function. Each set of pulses performs two tasks:
application of a specific preamp setup, and initiation of ECG signal
conversion to a digital equivalent. The rising edge of the first pulse is
detected by U31A which responds by sending a pulse to U34A. This initiates
the ECG conversion. Monostable multivibrator U31B detects the falling
edge of the first pulse and clears U31A so that ECG conversion is not
initiated with every pulse on the pulse train.
Flip-flop U36A and counter U32 form a five-stage counter that responds to
the pulses from the pulse train (see TP17, Figure 1-5). The first rising edge
of the pulse train causes the five-stage counter to load from the preset inputs
(pins 6, 5, 3, and 4). Subsequent edges count up from the preset inputs.
Because of the timing of RC network R101 and C74, U31B times out at
about 1.5ms and sends a positive edge to U33, latching the decoded
command signals. These command signals select between leads and paddles.
Instrumentation Amplifier. The typical ECG signal at U41 and U42 is
16mV p-p maximum and requires amplification for further signal processing.
U44A and U44B together form a differential input amplifier with the gain set
by RN2 pins 1 and 2, R144, and RN2 pins 3 and 4. Op amp U44D amplifies
the differential signal and rejects the common-mode signal from U44A and
U44B. This common-mode signal (primarily 60Hz) is generated from the
outputs of U44A and U44B through voltage dividers R148 and R149. The
common-mode signal is then inverted and amplified by U35D and passes
through analog switch U41 and K1. Relay K1 routes the signal to the unused
patient lead (LL) cancelling common-mode signals at the patient source.
1-20 All Rights Reserved © August 1993 Physio-Control