ZOLL PD 2000 - Page 78

FUNCTIONAL DESCRIPTION
The lead select relay is of a latching configuration with the relay having a
separate set and reset coil. These coils are driven by FET switch Q2 and
latch bit LEAD_DRV. The relay state selected is determined by latch bits
LEAD(0) and LEAD(1). Multiplexer select bits are provided by the output
of latch U4; bits are latched via LEAD (1:0) and FET switch Q2.
The multiplexer outputs are filtered prior to the first stage
instrumentation amp (U2). The instrumentation amp configuration
provides excellent common mode rejection by using precision matched
gain resistors. The first stage instrumentation amplifier has a gain of 14.
The second stage after the instrumentation amplifier (U3) provides a dual
function. First, it limits the slew rate of the signal before it can pass to
subsequent stages. This happens when the input signal exceeds 1 v/sec
due to events such as internal pace pulses or noise. The second function
is to provide a signal to the high slew detection circuit (U5) when high
slew signals are present.
Leads Off
Disconnected ECG leads on RA, LA, and LL leads are detected. The
paddle lead is terminated so a paddle leads off status cannot be detected.
For the three ECG inputs, the leads off status is detected when current
source resistors (R7, R8) drive the input voltage out of bounds. During
paddle monitoring, leads off is not detected on the three lead ECG. It is
also possible to get a leads off message when a signal larger than
approximately 0.5 volts is applied to either the three ECG leads or the
paddles input.
The leads off status is detected from each of the three op amps (U2, U3b)
in the instrumentation amp. This will cover all possible combinations of
removed leads. These three signals are diode OR'ed together and are the
input to the leads off detection circuit (US). When either of the two input
op amps drops below approximately -8.75 volts or if the differential op
amp stage output exceeds +/- 8.75 volts the leads off signal is sent to the
EPU by clamping the serial data signal (ISO4).
Hislew
The second stage amplifier of the ECG signal provides a slew limit of 1
volts/second by saturating the ECG signal above this slew rate, as for
example, when there is a pacing pulse.
Electrode Impedance Measurement
The electrode-to-electrode impedance is measured by injecting a low-
level, high-frequency AC current into the electrodes via transformer T2.
The induced voltage at the electrodes is sensed via a demodulator circuit
(U10 and peripheral components) and measured by the EPU onits A/D,
ACH.
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