Physio Control LIFEPAK 9 - Page 148

© November 1994 Physio-Control Corporation
Description 1
Impedance
is
determined
by
sending
a
drive
signal
to
the
patient
and
measuring
the
signal
drop
across
the
patient.
U19
produces
this
signal
as
a
14kHz
sinewave
at
the
ZDRV
output
(pin
4).
The
drive
signal
returns
to
U19
at
the
ZIN
input
(pin
7).
A
synchronous
demodulator
within
U19
detects
the
returning
drive
signal.
An
internal
comparator
uses
this
signal
to
detect
a
loose
lead
condition.
If
a
lead
becomes
disconnected,
the
voltage
on
one
input
of
the
comparator
exceeds
the
threshold
set
at
U19
pin
21,
driving
the
output
at
pin
22
low.
This
low
output
activates
the
microcontroller
high
speed
input
(HSI.3).
The same U19 demodulator output is also directed to an internal
amplifier similar to U18 (see the following paragraph) between pins 8
and 9. Hard-wired control signals G1 through G3 fix the gain at 2V/mV.
The microcontroller then evaluates the impedance signal output at pin 15
for excessive motion.
Hybrid U18 amplifies the ECG signal in two stages: first with a fixed
gain setting, then with a variable gain setting. During the first stage, gain
is fixed as 120 + 18% with a ImV input signal. U18 also contains a
60Hz notch filter and a low pass filter for the ECG signal input.
capacitor C23 and U19 output-coupling capacitor C19 form a discrete
2—22Hz bandpass filter.
During the second stage, the microcontroller varies the ECG gain to meet
input reguirements of its internal A/D converter (pin 5). The
microcontroller selects the second-stage gain by supplying a three-bit
code to the GI through G3 inputs of hybrid U18. The four selectable
gain factors listed below are referenced to a ImV ECG input.
Gain Factor Inputs
G1 G2 G3
0.5V/mV High High High
1V/mV Low High High
2V/mV Low Low High
4V/mV Low Low Low
Op amps U20A and U20B buffer the output signals at pin 15 of both
hybrids. Buffering is necessary to match the high impedance outputs of
the hybrids to the low impedance inputs of the microcontroller A/D
converters. This compensates for any signal attenuation that may occur
when the hybrid output signals are transmitted to the microcontroller.
Hybrids U19 and U18 require a stable voltage reference (VREF) for
precise measurement of patient signals. The +8VA line provides this
reference signal at pin 18 of both hybrids. Resistors R33 and R34 divide
the +8VA line to approximately +4V; C26 filters VREF at power-on.
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