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.
The relationship between these subsections is shown in the following block diagram.
Patient
Sensor
LEDs
Input signal
conditioning
Signal gain
AC ranging
Photocurrent
to voltage
conversion
demultiplexed
to 2 channels
Variable gain,
filtering for
each LED
channel
Offset
subtraction;
additional gain
and filtering
Main PCB
Main PCB
Main PCB
LED drivers
(red & IR)
Main PCB
Microprocessor
Main PCB
Control
To digital section
Figure 12.3 SpO
2
Analog Circuitry Block Diagram
12-9
Sensor Output/LED Control
The SpO
2
analog circuitry provides control of the red and IR LEDs such that the 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, 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. 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 build-up 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 HSO's of the CPU.