Mallinckrodt NELLCOR NPB-4000, Service Manual

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Section 11: Isolated Front End Functions -- Theory of Operation
11-4
11.3 INTERFACE CIRCUIT
Figure 11-4: Interface Circuit Block Diagram
The interface circuit shown in the highlighted portion of the expanded block
diagram, provides physical connection between patient connectors on the unit
and preamplifier and other signal conditioning circuitry of the isolated Front
End. It also incorporates ESD, defibrillator, and electrosurgery voltage
suppression.
The instrument incorporates the standard ECG 3-lead system (I, II, III). The
abbreviations RA, LA, and LL are the right arm, left arm, and left leg leads,
respectively, and I, II, III are the potential differences between the limbs. The
ECG signals through a patient cable are fed to a panel connector and through the
panel harness to the board connector J11. All leads are protected by voltage
suppressers. Suppressers D27, D25, and D24 are intended to clamp
differentially applied defibrillation and voltages to around 160 volts, with the
current limited to approximately 2.5Amps by two 1 kilo-ohm, 1W resistors
installed in a standard ECG cable. The same suppressers will clamp ESD
voltages close to 80 volts, relative to isolated ground. The R-C networks R278,
C162, R279, C164, R254, C150; R261, C157, R247, C148, R246, and C146
provide further voltage and current limiting to the levels safe for inputs of the
multiplexers having their own over-voltage protection of ±0.5 volts over supply
voltages. The same R-C networks will provide for ESU and RFI attenuation and
rejection of 100 kHz respiration signal in the ECG channel.
There are six other nets permanently connected to RA and LA leads. These are
used for respiratory measurements. Two of them are connected to a high
frequency source U81, two others to the inputs of signal followers U75C, and the
last two, R121, C68 and R257, C178, are balancing LL to the isolated ground.
The respiratory nets rely on the same suppression circuits as the main leads.
The temperature sensor signal is brought through a panel connector, and the
common lead is connected to the reference through ESD voltage suppresser D23.
Capacitors C131 and C136 are used to filter out RFI signals.