Nellcor N-180, Руководство пользователя

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Section 3: Functional Block and Circuit Analysis
3-11
and 2). Likewise, the next two phases (3 and 4) are gated into the RED
filter and amplifier. This leaves a 483 mi crosecond period until the next
pair of IR pulses is gated into the IR filter and amplifier. The overall
pulse amplitudes are proportional to the transmitted light at the
measurement site. The individual pulse pair amplitudes are a function
of the low-frequency patient pulse modulation and artifacts at the
measurement site.
These pulse pairs are coupled to the first of two identical filter and
amplifier stages, each having a gain of approximately two. The signal is
then coupled to the last stage, which has a gain of one for IR, and two
for red. The DC offset of the resulting low-frequency patient pulse
waveform is proportional to the average light intensity at the
measurement site. The peak-to-peak amplitude of the patient pulse
waveform is a result of the factors expressed in the Beers-Lambert Law,
which is used to calculate oxygen saturation.
The patient pulse waveform at the IR filter and amplifier output labeled
IR, must al ways be a positive voltage, because the next step is to digitize
the waveform in the analog to digital circuits. To ensure that the
waveform does not move to a negative level, the final amplifier stage
input has a + 2.5 millivolts offset, which guarantees a minimum positive
offset.
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Red Filter and amplifier:
Refer to figure FO10-1 (sheet 4 of 7). The red filter and amplifier circuit
is an active low-pass type with a –three dB roll-off point at about ten
Hertz and a total gain of about 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 red filter and amplifier pulse
waveform output is coupled to the analog to digital converter.
The input signal to the red filter and amplifier is two 70-microsecond
pulses separated by a 112 microsecond space (phase 3 and phase 4). The
next two phases (1 and 2) are gated into the IR filter and amplifier the
same way. This leaves a 483 microsecond period until the next pair of
red pulses are gated into the red filter and amplifier. The overall pulse
amplitudes are proportional to the transmitted light at the measure-
ment site. The individual pulse-pair amplitudes are a function of the low
frequency patient pulse modulation and artifact at the measurement
site.
These pulse pairs are coupled to the first of two identical filter and
amplifier stages, each having a gain of approximately two. The signal is
then coupled to the last stage having a gain of two. The DC offset of the
resulting low frequency patient pulse waveform is proportional to the
ambient light intensity at the measurement site. The peak-to-peak
amplitude of the patient’s pulse waveform is a result of the factors
expressed in the Beers-Lambert Law is used to calculate oxygen
saturation.
The pulse waveform at the red filter and amplifier output labeled RED,
must be a positive voltage, because the next step is to digitize the
waveform in the measurement system. To ensure that the waveform
does not move into a negative voltage region, the final amplifier stage
input has a +2.5 millivolt input, which guarantees a minimum positive
offset.