Mallinckrodt NELLCOR NPB-195 Service Manual page 77

11.8.4 Input Conditioning
11.8.5 Signal Gain
11.8.6 Variable Gain Circuits
11.8.7 AC Ranging
11.8.8 Real-Time Clock (RTC)
Input to the SpO analog circuit is the current output of the sensor photodiode.
2
In order to condition the signal current, it is necessary to convert the current to
voltage.
Because the IR and red signals are absorbed differently by body tissue, their
received signal intensities are at different levels. Therefore, the IR and red
signals must be demodulated and then amplified separately in order to compare
them to each other. De-multiplexing is accomplished by means of two circuits
that alternately select the IR and red signal. Two switches that are coordinated
with the IR and red transmissions control selection of the circuits. A filter with a
large time-constant follows to smooth the signal and remove noise before
amplification.
The separated IR and red signals are amplified so that their DC values are within
the range of the A/D converter. Because the received IR and red signals are
typically at different current levels, the signal gain circuits provide independent
amplification for each signal as needed. The gain in these circuits is adjusted by
means of the PWM lines from the CPU.
After the IR and red signals are amplified, they are filtered to improve the
signal-to-noise ratio and clamped to a reference voltage to prevent the combined
AC and DC signal from exceeding an acceptable input voltage from the A/D
converter.
The two variable gain circuits are functionally equivalent. The gain of each
circuit is contingent upon the signal's received level and is controlled to bring
each signal to approximately 3.5 V. Each circuit uses an amplifier and one
switch in the triple SPDT analog-multiplexing unit.
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. 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 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.
Real time is tracked by the NPB-195. As long as battery power or AC power is
available, the instrument will keep time. If the battery is removed, the time clock
will have to be reset.
Section 11: Technical Supplement
11-7