AC Ranging
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 super imposed upon that DC level. This is also
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.
Offset Subtraction Circuits
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 HSO 0 (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.
AC Variable Gain Control Circuits
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
RECAC are then filtered and input to the CPU, and can be measured at TP55 and TP59, respectively.
Digital Circuitry
The digital hardware and related circuitry, which is illustrated in the following block diagram, 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.
•
System Memory – External to the CPU and consists of an 8K x 8 static RAM and a 64K x 16 EPROM.
•
Real-Time Clock (RTC) – The RTC keeps track of date and time, which is printed on each printout. The
RTC is powered by a lithium battery designed to last up to 5 years before needing replacement.
•
Audio Output – A piezolectric ceramic beeper is used for audio output.
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