Nellcor N-180, Manual

Page: 11 / 83

Section 3: Functional Block and Circuit Analysis
3-7
U21A
1
–
+
SAT Signal
to Inamp
I R I R
8.57V–
0V
U21B
1
–
+
6
–15V
LEMO
+15V
U4D
U4C
U10A
16 1
3
7
10
6
11
8
9
15
Ref 2.5V
4
3
2
R17
Ref 2.5V
R16
5
6
1
1
4
6
6
RCAL
RCAL
R38
8.57V
R44
JP1
2
31
JP3
JP3
SAT Signal from
External Preamp
and Sensor
From External
Preamp/Sensor
VCALIN1
VCALIN2
Sensor
U8
+
–
Figure 3-3: Oximetry Module Input Source Selection Circuits
An undisturbed saturation signal at the input amplifier is essentially a
square wave (the actual amplitudes may vary between the IR and red phases)
that has a frequency of 2710.6 Hz. The square wave peak-to-peak amplitude
is proportional to LED transmitted light intensity, plus any artifact. The DC
offset (negative from the +8.57 volt bias point) is dependent on steady-state
background light or energy. Peak-to-peak amplitude changes in the signal
depend on the modulation caused by pulsatile blood at the detector site.
The remainder of this description assumes that a Nellcor PT-2500 pulse
oximeter module tester (pocket tester) is connected to the module input in
place of a normal patient sensor. This establishes a reproducible set of values
for discussion and comparison. The PT-2500 conditions the LED drive voltage
and simulates a saturation of 81%
±
1 digit (80% to 82%) and a pulse rate of
40
±
1 bpm (39 to 41 bpm). Note that pocket tester pulse rate is dependent on
LED switching rate and may differ on other Nellcor pulse oximeter models.
The SAT signal from the pocket tester is coupled to the SAT conditioning
circuits via the sensor input to JP1 (pin 1 and pin 4) on the oximetry module
PCB. After condi tioning by current-to-voltage converter U8, the signal has
the following characteristics:
DC offset: approxi8.5 volt
Frequency: 2710.6 Hz
Modulation: maximum peak-to-peak amplitude, approximately 5 volt
The only additional oxygen saturation input requirement is the wavelength of
the red LED. This number is derived from the RCAL resistor value located in
the sensor or PT-2500. When a sensor is connected to the monitor, the RCAL
resistor connects between JP1 pin 6 and ground to become part of a voltage
divider with R16 on the PCB. Power for this divider is a 2.5 volt reference