Nellcor N-180 Service Manual page 22

Section 3: Functional Block and Circuit Analysis
Input.Source Selection Circuits
The SAT signal is developed by the photodiode in the Nellcor sensor (figure 3-
3), responding to the transmitted red and IR light at the measurement site.
The transmitted light intensity depends on the LED drive level, the patient's
oxygen saturation, and the amplitude of the pulse at the site.
The photodiode current is coupled into the oximetry module via JP1 (pin 1
and pin 4) to the signal input of U8, which performs current-to-voltage
conversion for the photodiode signal. The non-inverting input of U8 is biased
by R38 and R44 to 8.57 volts. Accordingly, U8 has an 8.57 volt maximum
positive offset, in the absence of any light or energy on the photodiode. This
bias prevents an absence of light from clamping U8 at +15 volts. The presence
of light, or energy from any source, moves the output of U8 in a negative
direction.
SAT Signal
'to Inamp
16
U10A
4
408
+ 一
A o
1 ] SAT Signal from
sm
AP
External Preamp
°
4] ом 3
|_| and Sensor
OV
—15V
6
U4C 7
] JP1 Sensor
=
a
2 _
LOTTI
EMC
6
一
1
LEMO
Us
|
+ 3 | | |
+15V
11 U4D
10
4
.
|
9 | ] ー she |
Ref 2.5V
.
σον
一
一 一 一 一 |
|
R17
ти
JP3
ー VCALN1 LR Ft
i+ ——— _—_—_—————__————€——————
UZiA lo
From Extemal
Ref 2.5V
L_j
Preamp/Sensor
R16
4 一 一 4
VGALIN2 4 +E
U21Bl。
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 48.57 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