Physio Control LIFEPAK 9 - Page 36

LIFEPAK 9 defibrillator/monitor
This preserves the dc offset level required by the Echo circuit. DC Restore
controls any transient signals to prevent the high potential gain of the
Instrumentation Amplifier from falsely amplifying the dc level input for
conversion. Under normal operation, the A/D conversion dc level at TP19 is
from +1V to +11V.
Switch smoothing is the process of limiting the response of the preamp to dc
offset voltages when switching from one lead (I, Il, Ill, or STD) to another.
This function is initiated when the appropriate number of pulses (see
Table 1-3, pulses 12 thru 20) turns on Q8 or Q9 and switches U42 to the
selected lead. When Q8 switches on, C90 discharges from +12V, causing
Q11 to conduct. Q11 changes the high-pass filter by switching R161 in
parallel with R165. Because of R161, this filter limits the signal passband to
above 40Hz.
Transient signals associated with defibrillation are sensed by op amp U45C.
U45C functions as a window comparator and responds with a positive output
when the voltage at its input exceeds +2V. Resistors R172 and R147 apply a
negative bias to the noninverting input of U45C; R171 and R143 apply a
positive bias to the inverting input. C104 and R151 function as a low-pass
filter to prevent very short spikes (such an an internal pacing spike) from
inducing DC Restore. The output of U45C drives transistor Q10, routing the
signal through high-pass filter U44C and U45D. Resistor R160, with U44C
and U45D, limits the signal passband to 5Hz.
When PADDLES is selected, further passband limiting is required to
eliminate motion artifact introduced by the paddles when used in the
QUIK-LOOK mode. A command pulse selects paddles mode, (see
Table 1-3, pulse 20), turning Q9 on. When Q9 conducts, flip-flop U36B is
loaded so that the next clock pulse at pin 6 switches the Q output high.
Analog switch U43B then closes, bringing R155 in parallel with R165 in the
high-pass filter, and limiting the ECG signal passband to above 2Hz.
Echo. The ECG signal is converted to a pulse-delay period on command
from the System microprocessor. The time from when the convert command
from the System microprocessor is received until the “echo” pulse is sent
back to the System microprocessor is proportional to a sample of the ECG
signal (see Figure 1-6). When the echo pulse is sent back to the System
microprocessor (across an opto-isolator), the System microprocessor converts
the pulse-delay period to a number.
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