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Technical Supplement
9.5.4
AC Ranging
9.5.4.1 Offset Subtraction Circuits
9.5.4.1 AC Variable Gain Control Circuits
9.6
DIGITAL CIRCUITRY
9-12
In order to achieve a specified level of oxygen saturation measurement and to
still use a standard-type combined CPU and A/D converter, the DC offset is
subtracted from each signal. Because the DC portion of the signal can be on the
order of one thousand times the AC modulation, 16 bits of A/D conversion
would otherwise be required to accurately compare the IR and red modulations
between the combined AC and DC signals. The DC offsets are subtracted by
using an analog switch to set the mean signal value to the mean of the range of
the A/D converter whenever necessary. The AC modulation is then
superimposed upon that DC level. This is known as AC ranging.
Each AC signal is subsequently amplified such that its peak-to-peak values span
one-fifth of the range of the A/D converter. The amplified AC signals are then
filtered to remove the residual effects of the PWM modulations and finally, are
input to the CPU. The combined AC and DC signals for both IR and red signals
are separately input to the A/D converter.
The AC variable gain control circuit is illustrated in Figure 9-13, at the end of
this section.
Voltage dividers R22 and R41 (red), and R31 and R5 (IR), which are located
between VREF and ground, establish a baseline voltage of 2.75 V at the input of
the unity gain amplifiers U7C (red) and U7D (IR).
Whenever SPST analog switches U11A and U11D are closed by HSO0 (active
low), the DC portions of the IR and red signals create a charge, which is stored
on C29 and C89, respectively. These capacitors hold this charge even after the
switches are opened and the resulting voltage is subtracted from the combined
signal—leaving only the AC modulation output. This AC signal is
superimposed on the baseline voltage output by U7C and U7D. The IRDC and
REDDC are then filtered and input to the CPU, and can be measured at TP58
and TP54, respectively.
The AC variable gain control circuit is illustrated in Figure 9-13, at the end of
section 9.
The AC modulations are amplified by U7A (red) and U7B (IR) and
superimposed on the baseline voltages present at the output of U7D (IR) and
U7C (red). The amplification is handled by means of the SPDT analog
multiplexing switch U3 within the feedback loop, which increases gain as
PWM0 is increased. The IRAC and REDAC are then filtered and input to the
CPU, and can be measured at TP55 and TP59, respectively.
The digital hardware and related circuitry, which is illustrated in Figure 9-14,
includes the following subsystems:
•
CPU — A 16-bit microcontroller that includes: a serial port, watchdog
timer, A/D converter with an 8-input analog multiplexer, 3-pulse width
modulators, and a high speed I/O subsystem.
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