Nellcor N-180, Руководство пользователя

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Section 3: Functional Block and Circuit Analysis
3-9
SATIN
–
+
U14B
Full Gain Amp
6 7
–
+
1
3
U10B
9
REDGATE
COMPARE 2
IRGATE
I R I R
8.57V-
0V
U7A LED
Control
Compare 1
SAT Signal
0V
R8 SIP
R51
8
10
6
5
U7B Gain
Control
U14A
Sync Det.
2
3
0V
REDGATE
IRGATE
U15B
U15A
16
15
4
6
0V
8
0V
RED Filter Gain 8
IR Filter Gain 4
U22B/D,
U23B
U22A/C,
U23A
2
15
9
5
10
DAC U9
Programmable Gain Stage
Gain 0.004 to 1 in 255 Steps
Bandpass Filter
10Hz
0V
260 kHz 200 Hz
6 7
19
2
U13A
3
10 Hz
RED_ADC
IR_ADC
10 Hz
DATABUS (0..7)
RED and IR ADC waveforms
shown above are typical of
normal human waveforms
0V
0V
U13B
1
1
C55
–
Figure 3-4: Oximetry Module Input Amplifier, Synchronous Detector, and Filter and Amplifiers
The signal is then coupled to inverting amplifier U14B, which has a gain of
–51. The output of U14B is used by the microprocessor as the sense point to
determine input channel gain requirements. Device U7B monitors the
amplified SAT signal at the output of the input amplifier.
Device U7B is a positive peak detector that produces a DC output propor-
tional to the positive excursion of the amplified SAT signal pulses. If the posi -
tive excursions of the saturation signal 10 volt (indicating excessive
amplifier gain), the microprocessor, which is monitoring the output of U7B
(COMPARE2 ), acts to reduce the pro grammable stage gain.
The microprocessor controls U10B via the SATIN line, and closes the switch
during phases 1 and 3 of the four-phase clock mentioned in the LED drive
discussion. The result is that the voltage values represented by the IR and
red LED On-times are inverted by the detector. The voltage values repre-
sented by the LED Off-times (phases 2 and 4) pass through the detector at
their original voltage level and polarity. The output of U10B is conditioned by
U14A.
The synchronous detector is an operational amplifier configured to operate as
two dif ferent circuits, an inverting amplifier and a voltage follower. When the
positive input of U14A is grounded by U10B, the device is an inverting ampli-
fier that has a gain of one. When this input is not grounded, the device be-
comes a voltage follower that has a gain of one. The signal is still in the orig-
inal multiplexed format and essentially a square wave at 2710.6 Hz.
Synchronous detection conditions the SAT signal so that subsequent filtering
can reclaim the patient’s pulse waveform component free of the 2710.6 Hz
switching frequency. Synchronous demodulation is performed by analog
switch U15, which is controlled by the REDGATE* and IRGATE* signals.