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9.5.1
Sensor Output/LED Control
9.5.1.1 LED Drive Circuit
The relationship of these subsections is shown in Figure 9-7.
Input signal
Patient
conditioning
sensor
photocurrent
LEDs
demutiplexed
to 2 channels
Figure 9-7: SpO
The SpO
analog circuitry provides control of the red and IR LEDs such that the
2
received signals are 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, intensity level of the LEDs is based on previous
running conditions; and, the transmission intensity is adjusted until the received
signals match the range of the A/D converter. If the LEDs reach maximum
output without the necessary signal strength, the PWMs will increase the channel
gain. The PWM lines will select either a change in the LED current or signal
gain, but will not do both simultaneously.
The LED circuit switches between red and IR transmission and disables both for
a time between transmissions in order to provide a no-transmission reference.
To prevent excessive heat buildup and prolong battery life, each LED is on for
only a small portion of the duty cycle. Also, the frequency of switching is well
above that of motion artifact and not a harmonic of known AC transmissions.
The LED switching frequency is 1.485 kHz. The IR transmission alone, and the
red transmission alone will each be on for about one-fifth of the duty cycle; this
cycle is controlled by the HSOs of the CPU.
The LED drive circuit is illustrated in Figure 9-8, at the end of section 9.
Signal gain
variable gain,
to voltage
filtered for
conversion
each LED
channel
Main PCB
Main PCB
Main PCB
Control
LED drivers
(red & IR)
Analog Circuitry Block Diagram
2
Technical Supplement
AC Ranging
offset
substraction;
additional gain
and filtering
Main PCB
Microprocessor
Main PCB
To digital section
9-7
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