Smiths Medical 1818R, Operation And Service Manual

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Chapter 9: Service Maintenance and Repair
9-4 Oximeter Operation and Service Manual
The differential photo amplifier formed by U1 and U3A, converts the pho-
todetector’s current output to a voltage at T10 (VAMB). Amplifier U3B
offsets the signal at T10 allowing a wider signal range. Comparator U9B is
used to inform the microprocessor if U3A is saturated by an excess of
ambient light. VAMB is passed through blocking capacitor C77 to remove
the signal’s DC component. The signal is then buffered and amplified by
U27A. Gain is defined by the first channel of digital potentiometer U5. The
output of U27A is routed to the integrator-filter-offset circuitry U27B, con-
trolled by analog switch S53. The output of the integrator is then routed to
the ADC for measurement.
LED Controller
LED controller is built around U22 and U23 buffers. Data is latched into
buffers by DSP-LD signal. Signal/DSP-EN is used to disable LED’s at
power up to limit initial surge current.
Signal Dictionary
This section lists, in alphabetical order, the signal names used on the sche-
matics. The signal’s origin, destination, and purpose are described.
Signal Description
ADCLK
ADOUT
/ADCS 3 wire serial interface for analog to digital converter U6.
/AMB-
FAULT Signal informing processor about excess of ambient light.
ANA+5 Filtered VCC. ANA+5 powers the analog circuitry.
ANA-5 ANA-5 powers the analog circuitry.
BASE Keeps integrator U27B fro saturating during data acquisitions.
BATT Signal from watchdog timer U18 to inform processor of low bat-
tery condition.
CAP-
GND Signal used to short blocking capacitor C77 to ground.
CTS0 Serial Port Clear To Send handshake signal from external RS-232
communication device.