Smiths Medical 1818R, Руководство по эксплуатации и обслуживанию

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Chapter 9: Service Maintenance and Repair
Oximeter Operation and Service Manual 9-3
Internal timer 1 of U11 is used as a tone generator to drive piezo speaker
through connector J5.
Analog Section
Analog to Digital Converter (ADC) U6 is a 12 bit, successive approxima-
tion sampling ADC. The ADC+IN signal input comes from TP22. VREF is
used here as a pseudo ground. The ADC is configured as a differential
input converter with the –IN input connected to VREF. This allows the +IN
signal to range from 2.5V to 5V. The ADC transfers data with a 3 wire
serial interface. ADCLK synchronizes the data transfer with each bit being
transmitted on the falling ADCCLK edge and captured on the rising
ADCLK edge. /ADCS must beheld low when the ADC transmits data.
LED Drive circuitry is used to turn on and off the oximetry probe LED’s,
precisely controlling the current through them. The second half of the vari-
able potentiometer U5 is used to generate voltage between 0 and 2.5VDC.
U5 is controlled, serially, by the processor. Operational amplifier U2B
together with transistor Q7 create a constant current sink. The current is
proportional to the voltage generated by variable potentiometer. The H-
bridge Q8-Q9 is used to activate either LED, red or infrared, inside the
oximeter probe. The following table describes the states of the H-bridge:
In this table, 1 indicates logic 1, 0 indicates logic 0, and x indicates either
logic 0 or 1.
The “Open circuit” state is used to check for a possible probe cable fault. In
case of probe cable fault, one of the LED wires can short to the ground
shield causing high current through the probe LED. To prevent this, before
turning any LED on, the processor checks the cable by “floating” the H-
bridge. If the cable is shorted CON3 and CON2 connections are pulled low,
and comparator U9A generates /PRB_FAULT signal, which causes the
processor to shut down the LED excitation cycle and generate the appropri-
ate message.
RED-DRV IR-DRV FET-ON Function
1 0 1 Red LED is on
0 1 1 Infrared LED is
on
X X 0 “Open circuit”
state