Ecg - Mallinckrodt NELLCOR NPB-4000 Service Manual

11.4 ECG

11.4.1 Preamplifier, Amplifier and Filter.
Section 11: Isolated Front End Functions -- Theory of Operation
The SpO sensor signal is brought through a panel connector and a separate
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panel cable to the board connectors J101 and J100. The Front End also has an
SpO output connector J10, into which the SpO
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Figure 11-5: ECG Circuit Block Diagram
See Figure 11-5. As shown in the highlighted portion of the expanded block
diagram, the ECG circuitry consists of the lead selection, preamplifier, amplifier
and filter, R-wave detector, and pacemaker detector circuitry.
The ECG preamplifier connects to a patient via three leads. At any time, two of
the leads are selected by the multiplexer U79 and connected to the inputs of the
instrumentation amplifier U80 configured to have a gain of four. A common
mode signal at the amplifier input is measured by the amplifier U75B, and used
to drive the patient body via the amplifier U75A, resistor R291, and multiplexer
U78 so that the common mode signal on the body and the output of the
preamplifier would be eliminated. The amplifier U75B is also used for driving
the shield with the common mode signal to eliminate capacitance loading.
The output of the drive amplifier U75A is used for leads-off detection. To
implement this function, input leads of the differential amplifier are connected to
the VDD supply through 20 megohm resistors R252 and R266. If any of the
patient leads become disconnected from the body, the output of the drive
amplifier U75A will go to a rail. A window comparator U77D is used to detect
this event by producing a positive going signal sent to the A/D converter.
module (MP-205) is plugged.
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11-5