Novametrix Medical Systems MARS PO2 TECH 2001, Service Manual

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Section 4
Theory of Operation 2775 Main Board
14
Model 2001 Service Manual Rev. 01
alert, and during a system power up self-test. See Calibrating the 20-Bit Analog-to-Digital
Converters on page 9.
Figure 1. Front-End Timing
The data sampling sequence consists of:
a. turn on the Red LED (RDLED*) and the Analog Sample line (ASAMP*)
b. allow the Red LED time to reach full brightness (steady state light output)
c. sample the Red LED return signal (RDSMP*)
d. turn off the Red LED, the Analog Sample line, and stop sampling
e. turn on the Infrared LED (IRLED*) and the Analog Sample line (ASAMP*)
f. allow the Infrared LED time to reach full brightness (steady state light output)
g. sample the Infrared LED return signal (ISMP*)
h. turn off the Infrared LED, the Analog Sample line, and stop sampling
i. repeat the process starting at step a.
The Analog Sample (ASAMP*) line is used to nullify the effects of any ambient light signals
returning from the sensor.
The IC42 INSIG* and SIGND* outputs are used in conjunction with the SC1 and SC2 inputs.
The IC42 pin 14 external sequencer (SYNC) line is equivalent to the PEEL’s D1 input. It
provides a “sync” pulse to the pulse width modulator on the power supply board in order to keep
power supply switching spikes from interfering with data sampling operations.
Sensor LED Drive Circuits
The VLED line voltage is derived from IC30 pin 8 which is controlled by the Digital to Analog
Converter IC29. (See sheet 3 of 4 on schematic.) When address line A0 is high (IC29 pin 6)
and both WR* (IC29 pin 16) and DACCS* (IC29 pin 15) are low the D/A Converter is enabled.
The data on lines D0-D7 (IC30 pins 14-7) now control the output voltage of IC30 pin 8 (VLED)
based on the VREFB voltage on IC29 pin 18.
See sheet 2 of 4 on schematic.
When the RDLED* signal at IC42 pin 18 goes low (logic 0), Q14 turns off and the Red LED
signal (VLED from IC30 pin 8) at R37 is divided by R37 and R41, finally causing IC36 pin8 to
go high. This positive output turns Q11 on and current flows from the LED source (LEDSRC),
through the Red LED in the sensor (it turns on) returning as LED1SK (LED1 sink) across Q11
and the current limiting resistor R53 to ground.