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Technical Supplement
9-2
•
SpO
Analog Block Diagram (Figure 9-2) — Analog circuitry has high
2
signal sensitivity and reduced susceptibility to noise. Its design allows for a
wide range of input signal levels and a broad range of pulsatile modulation.
The SpO
analog circuit consists of four subsections:
2
1. Sensor output/LED control, where the CPU controls the gain of both LEDs
so that signals received at the input amplifier are in its acceptable dynamic
range
2. Input signal conditioning, where sensor output current is converted to
voltage
3. Signal gain, where the separated LED signals are amplified so their current
levels are within the A/D converter's acceptable range
Input Signal
Conditioning
Zeroing
and
DMUX
Current to
Voltage
Conversion
Sensor
RED
LED Drive
IR
Output
Figure 9-2: SpO
4. AC ranging, where DC offset is eliminated from each LED signal
•
Hardware Block Diagram (Figure 9-3) — This diagram shows the N-20PA
hardware and circuits, which include: the CPU and system memory, the
power supply and power control circuitry, user controls, display and ambient
light sensors, audio output, thermal printer and ambient temperature sensor,
and the real-time clock.
•
Power Supply Block Diagram (Figure 9-4) — Power supply circuitry is
located on the auxiliary PCB and consists of four subsections:
1. Four "C" size batteries that provide 4-6 VDC
2. Power control circuitry that senses a press of the On/Off button and switches
power on
3. Power shutoff circuit that controls power to all circuits except the power
control circuit
4. Power supply circuits include: a regulated power supply at 5 VDC,
unregulated power supplies of -5 VDC, 10 VDC, and 12 VDC, and a
high-voltage power supply of 70 VDC
SAMPIR
SAMPRED
Signal Gain
Variable Gain
Offset
and Filter
Subtraction
RED
IR
Variable Gain
Offset
and Filter
Subtraction
RSENS
LED DIS
PWM0, PMW1, PWM2
Analog Block Diagram
2
PWM1
AC + DC
AC Ranging
/ZERO
PWM0
Variable Gain
REDAC
and Filter
PWM2
PWM0
IRAC
Variable Gain
and Filter
AC + DC
/ZERO
IR/RED
OFF/ON
CPU
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