Front End Module Theory of Operation
2 - 12 0070-10-0441 Passport 2®/Passport 2 LT™ Service Manual
2.2 Front End Module
Overview
This document describes the theory of operation of the ECM Frontend module. This module is
responsible for acquisition of most of the patient-safety isolated signals, namely the ECG,
Respiration (by impedance), Temperature, and two channels of Invasive Blood Pressure.
The Front End Module consists of a common isolated power supply, data isolation,
microcontroller, and A/D converter, shared between the various patient signals.
2.2.1 Power Supply
The power supply takes a raw +12v DC supply voltage and generates the highly isolated
operating voltages required by the front end module. Since the input voltage varies over a +/
-5% range, some form of regulation is required. To preserve efficiency, a modular switching
supply is used.
This power supply requires sufficient isolation between the input and output to withstand the
open circuit voltage of a defibrillator, up to 5 kV. Further, it requires low capacitance
between the input and output, to minimize leakage currents which may flow should the
patient accidently contact line voltage.
2.2.2 Communication Isolation
The communication between the front end module and the host must be isolated to the same
degree as the power supply. The communications consist of an asynchronous bidirectional
serial data stream at 500 k baud. Since these signals are all digital, the isolation is
performed with optocouplers. All these devices are a special type which feature very high
isolation voltage.
2.2.3 Data Acquisition
The various signal processing blocks, such as ECG, IBP, etc., generate both analog
(waveform) and digital (status) signals. The purpose of the data acquisition system is to
capture these signals and to format them into a data stream suitable for transmission through
the communication isolation circuits. The system consists of a multiplexed analog to digital
converter, a single-chip microcontroller, and some digital level shifters. The microcontroller
serves the additional purpose of interpreting commands received from the host.
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