Nellcor N-180, Manual

Page: 14 / 83

N-180 Pulse Oximeter Service Manual
3-10
Demodulator Filters and Amplifiers
There are three circuits in the demodulation and filtering block:
• Circuits that gate other logic
• IR Filter and amplifier
• Red Filter and amplifier
■
Gate logic:
Field effect (FET) switches U15A and U15B separate the
IR
information
in the SAT signal from the red information (figure FO10-1 3 of 7). Phase
one and phase three constitute the
IR
and red on-time segments,
respectively Phase two and phase four constitute the
IR
and the red off-
time segments, respectively. The gate control inputs (IRGATE* and
REDGATE*) to U15A and U15B are processor-controlled and operate in
time-sequence with the four-phase LED drive control. The switch pairs
in each gate operate exclusively so that the filter and amplifier input
does not encounter an open circuit when switches to the signal input
line are open.
During the time when phase one and phase two of the 2710.6 Hz
saturation signal (IR ON and IR OFF) follow one another on the signal
line, the processor strobes U15A twice. The first strobe pulse comes 114
microseconds after the beginning of phase 1 and continues for 70
microseconds, or to the end of phase 1. This strobe action gates the last
70 microseconds of IR ON signal level into the IR filter and amplifier.
The next gate strobe pulse comes 114 microseconds after the beginning
of phase 2 and continues for 70 microseconds, or to the end of phase 2.
This strobe action gates the last 70 microseconds of
IR OFF
signal level
into the
IR
filter and amplifier.
During the time that phases 3 and 4 of the 2710.6 Hz saturation signal
follow one another on the signal line, the microprocessor strobes U15B
twice. The first strobe pulse comes 114 microseconds after the beginning
of phase 3 and continues for 70 microseconds, or to the end of phase 3.
This strobe action gates the last 70 microseconds of RED ON signal level
into the red filter and amplifier. The second gate control pulse comes
114 microseconds after the beginning of phase 4 and continues for 70
microseconds or to the end of phase four. This strobe action gates the
last 70 microseconds of the RED OFF signal level into the red filter and
amplifier.
To avoid errors due to sensor photodiode settling time, only the last 70
microseconds of each phase gets gated into the filter and amplifier. This
scheme eliminates possible artifacts occurring during the first 114
microseconds of the phase. Photodiodes exhibit an exponential change
when the energy from a sudden LED state-change occurs. Using only
the last 70 microseconds of the photodiode output, after the diode has
settled, excludes this potential error from the measurement.
■
IR filter and amplifier:
Refer to figure FO10-1 (sheet 4 of 7). The IR filter and amplifier circuit
is an active low-pass type with a three-dB roll-off point at about ten
Hertz and an approximate gain of four. The filter cannot track the high-
frequency LED pulse input, but does respond to the low-frequency
patient pulse modulation, reproducing the patient’s pulse waveform at
the filter and amplifier output. The IR filter and amplifier pulse
waveform output is coupled to the analog to digital converter.
The input signal to the IR filter and amplifier, as explained above, is two
70-microsecond pulses separated by a 114 microsecond space (phases 1