Nellcor N-180 Service Manual page 18

CPU PCB
Section 3: Functional Block and Circuit Analysis
Input Source Selection
There are two possible inputs to the oximetry module. These inputs are
controlled via the LEMO signal. Only one input is used in the model N-180,
the differential input at JP1. If more than one input were used, the input
source selection logic detects which input connector is active.
Connector JP1 goes to an internal preamplifier. The signal at connector JP3
requires an external preamplifier.
Demodulator Filters and Amplifiers
The oximetry module includes two separate active filter channels, IR and red.
These low-pass filter and amplifier circuits and associated gate circuits
recover the patient's pulse waveform from the SAT signal. The next step is to
digitize these pulse waveforms in the analog to digital converter circuit block.
Analog to Digital Conversion
The analog to digital conversion block digitizes the demodulated IR and red
signals. Sensor calibration information is also digitized for use in the
saturation calculation algorithms.
Support Circuits
Support circuits include the following:
* Processor Circuits
* Power
Oximetry module operations are controlled by an 80C552 micro controller
supported by 512K electrically programmable read only memory (EPROM)
and 256K random access memory (RAM). Module power is supplied by the
CPU PCB via connector JP5.
- Besides acting as a central controller and processing data from the oximetry
'module for display on the front panel LEDs, the multipurpose CPU PCB
includes:
a speaker driver
an LED current check function
a battery charger
a battery condition monitor
a power control
a set of optical isolators for the RS-232 lines
an RS-232 connector
Front Panel LED Displays
Front panel displays consist of LEDs that show SpO2 (saturation), pulse rate
and relative pulse strength. The relative pulse strength indicator is a bar
graph arrangement that does not have a legend. Status indicators are:
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