Nellcor N-180 Service Manual page 17

N-180 Pulse Oximeter Service Manual
The functional elements of the oximetry module are:
* LED driver
* Input amplifier and synchronous detector
+ Input source selection
* Demodulator filters and amplifiers
+ © Analog to digital converter
* Support circuits
JP4
ET- Extemal
Power Option Support Data and Control LED Pre-amp
Selector Circuits + Driver — Connector
JP3
JP5
Host Comm Lines
Input Amplifier
Power A:D and
Host Power and Coriverter Synchronous
Communications Detector
JPt
Red ADC Saturation Signal
Red
Filters Input Source Sensor
Configuration IR Selection
dumpers IR_ADC Connector
Figure 3-1: Oximetry Module Block Diagram
The patient signal (SAT for saturation), is conditioned and used to derive sat-
uration percentage and pulse rate values. This same patient information is
serial coupled to the CPU PCB and processed for display and for output
through the serial communication port.
Section 2, Principles of Pulse Oximetry, explains how Nellcor sensors operate.
An oxygen saturation signal is obtained from a Nellcor oxygen transducer at
'the selected patient measurement site such as a finger, nose or toe. Sensor
LEDs generate alternate infrared and red light pulses at this site. A photodi-
ode in the transducer responds to the modulated light energy transmitted
through the measurement site. The photodiode SAT signal is coupled into the
oximetry module via the monitor front-panel sensor connector.
LED Driver
The LED drive circuits control the current to, and the timing of, the sensor
LEDs. The current is controlled to maintain an optimal signal to noise ratio.
Input Amplifier and Synchronous Detector
The SAT signal is conditioned by the input amplifier and the synchronous
detector to provide signal gain and reduce or eliminate the effects of ambient
interference (such things as ambient light and spurious electrical noise). The
final analog conditioning of the SAT signal is accomplished by circuits in the
filter block.
3-2