GE Corometrics 126, Service Manual

Page: 131 / 440

Revision B 120 Series Maternal/Fetal Monitor 4-73
2015590-001
Theory of Operation: MSpO
2
Board
The red filter/amplifier circuit is an active low-pass filter (total gain = 8) which
responds to the patient’s pulse modulation — reproducing the patient’s pulse
waveform at the filter/amplifier output. The red pulses (RED) are coupled to the
first of two identical filter/amplifier stages (U22A and U22C), each having a gain of
approximately two. The signal is then coupled to the last stage (U23A) which also
has a gain of two. The pulse waveform (RED) at the output of the filter/amplifier
stages, must always be at a positive voltage level in order to be digitized by the
analog-to-digital conversion circuitry. The final amplifier stage has an input of +25
mV to ensure that the waveform does not move to a negative level. The output
(RED ADC) is then coupled to the analog-to-digital conversion section.
Analog-to-Digital Conversion Circuitry
The analog-to-digital (A/D) conversion block digitizes
the infrared and red pulse waveforms. Sensor calibration information is also
digitized for use in the saturation calculation algorithms. A/D converter U20 is a
dual-channel device which produces a multiplexed serial output. Channel 1 (AIN1)
accommodates either VCAL1 voltage or the analog pulse waveform (RED ADC)
from the red filter/amplifier channel. Channel 2 (AIN2) accommodates either
VCAL2 voltage or the analog pulse waveform (IR ADC) from the infrared filter/
amplifier channel. These selections are determined by the configuration of quad
FET switch U24. Switching is controlled by the signal RESISTORS. The output bit
serial stream multiplexing of U20 is determined by logic signal ADCCHN, which
determines the channel present on the ADCDATA line. The ADCDATA line is
coupled directly to microprocessor U5 for saturation value and pulse rate calculation
prior to being routed to the 120 Series Monitor display processing circuitry.
Communications
Four data signals are used to communicate between the Nellcor module and the
SpO
2
Interface Board in the Oximeter Carrier Board in the 120 Series Monitor: CTS,
RESET, RXD, and TXD.
Processor Circuitry
The module support circuitry consists of microprocessor U5, ROM U6, and RAM
U12, all served by latch U2. Serial-to-parallel shift register U16 converts serial data
to parallel data for the DACBUS. The system operates using an 11 MHz crystal-
controlled oscillator. Sections U11 and U17 perform reset and buffer functions for
the communications link.
Masimo Module
Principle of Operation
The Masimo SET® MS-5 pulse oximeter is based on three principles:
1. Oxyhemoglobin and deoxyhemoglobin differ in their absorption of red and
infrared light (spectrophotometry).
2. The volume of arterial blood in tissue and the light absorbed by the blood
changes during the pulse (plethysmography).
3. Arterio-venous shunting is highly variable and that fluctuating absorbance by
venous blood is a major component of noise during the pulse.