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9.8.3
Display and Analog Shield Assembly
This assembly connects to the main PCB by flex circuits. A metal shield shrouds the SpO
circuits on the main PCB to protect them from EMI. An integrated electroluminescent backlight
illuminates the display under low light conditions.
The N-20P has an additional printer control board (printer flex circuit) and printer hardware. The
following block diagram shows the relationship between these components.
9.9
SpO
Analog Circuitry
2
This subsection describes the SpO
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
subsections:
9.9.1
Sensor Output/LED Control
The CPU controls the gain 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 LED current level or to amplify the signal channel.
9.9.2
Input Conditioning
Sensor output current is converted to voltage. A demodulation circuit minimizes the effects of other
light sources and stray frequency inputs. Because the 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.
9.9.3
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.
9.9.4
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.
The relationship between these subsections is shown in the following block diagram.
9.9.5
Sensor Output/LED Control
The SpO
analog circuitry provides control of the red and IR LEDs such that the received signals are
2
within the dynamic range of the input amplifier. Because excessive current to the LEDs will induce
changes in their spectral output, it is sometimes necessary to increase the received signal channel gain.
To that point, the CPU controls both the current to the LEDs, and the amplification in the signal
channel.
At initialization of transmission, the LEDs' intensity level is based on previous running conditions, and
the transmission intensity is adjusted until the received signals match the range of the A/D converter.
analog hardware. The analog circuitry has high signal sensitivity
2
analog block diagram (Figure 9-3) consists of four
2
Technical Supplement
analog
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9-7
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