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Technical Supplement
9.4
OVERALL BLOCK DIAGRAM
9.5
SPO
ANALOG CIRCUITRY
2
9-6
Exclusive of covers, buttons, and external connectors, the N-20PA consists of
three main components: the main PCB, the auxiliary PCB, and the display
assembly and analog shield.
•
Main PCB — Contains the SpO
memory circuits; sensor circuits for ambient light, temperature, and battery
voltage; the check battery circuit; a serial data port; and some display control
circuits.
•
Auxiliary PCB — Contains the power supply circuitry; the display driver
circuits; the real-time clock; the interface circuitry for the printer flex circuit
board (which is not used unless a printer is present); and audio output
hardware.
•
Display and Analog Shield Assembly — This assembly connects to the
main PCB by flex circuits. A metal shield shrouds the SpO
on the main PCB to protect them from EMI. An integrated
electroluminescent backlight illuminates the display under low light
conditions.
The N-20PA has an additional printer control board (printer flex circuit), and
printer hardware. Figure 9-1 shows the relationship of these components.
This subsection describes the SpO
high 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 block diagram (Figure 9-2) consists of four subsections:
2
•
Sensor output/LED control — The CPU controls the drive level of both
LEDs so that signals received at the input amplifier are within an acceptable
dynamic range. Signal channel gain may also need to be increased. The
CPU uses PWM lines to control the LED current level or to amplify the
signal channel.
•
Input conditioning — Sensor detector current is converted to voltage. A
demodulation circuit minimizes the effects of other light sources and stray
frequency inputs. Because the infrared (IR) and red signals are at different
current levels, the two LED signals are demultiplexed and separately
amplified, so they can be compared with each other. Two circuits handle the
demultiplexing by alternately selecting LED signals using switches. Filters
then remove noise and smooth the signals before sending them to the
amplifiers.
•
Signal gain —The separated LED signals are amplified so that their current
levels are within the A/D converter's acceptable range. The signals are
filtered to improve the signal-to-noise ratio, and clamped to a reference
voltage.
•
AC ranging —DC offset is eliminated from each LED signal. An analog
switch sets the mean signal value to the mean of the A/D converter range,
and the AC modulation is superimposed on that DC level. Then, each AC
signal is amplified and filtered to eliminate residual effects of the PWM
modulations. Finally, these two signals are input to the CPU A/D converter.
analog circuitry; the CPU; support
2
analog hardware. The analog circuitry has
2
analog circuits
2
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