Nellcor N-180, Руководство пользователя

Страница: 10 / 83

N-180 Pulse Oximeter Service Manual
3-6
Phase 3 Red LED on
Phase 4 Both LED s off
The LED drive current switching is accomplished by Q1 through Q6 and the
control lines, IRLED and REDLED . The figure illustrates the relationship of
these components and their association with the red (R) and IR (I) LED s in
the sensor. The LEDs are shown as they appear electrically in the circuit
without the interconnection diagram. The numerals 2 and 3 on either side of
the back-to-back LED s indicate pin numbers in the sensor connector.
The four-phase LED drive signal goes to U3A. The resulting drive potential is
coupled equally to both sides of the bridge circuit via
R
31 to
Q
4 and via R36 to
Q3. Control inputs IRLED and REDLED are pulsed low, to light respective
LED s . When both lines are a logic high, both LED s are turned off because Q1
and Q2 are directly turned off by the control inputs via R5 and R6. Device Q3
and device Q4 are turned off by the conduction of Q5 and Q6, which are now
conducting.
The IR LED (I) lights when control signal IRLED * is pulsed low. Device Q5
turns off, allowing Q3 to respond to the drive level from U3A, and Q2 is
turned on. The resulting current flow is from ground through R1, Q3, IR LED,
Q2, and to Vcc. The red LED (R) lights when control signal REDLED * is
pulsed low. Device Q6 turns off, allowing Q4 to respond to the drive level from
U3A, and Q6 is turned on. The resulting current flow is from Vcc through Q1,
red LED , Q4 and R1 to ground. The LED back-to-back configuration ensures
that the proper LED lights.
LED intensity is critical. Intensity variations during LED On-time, caused by
any source other than blood oxygen levels, can distort the SAT signal. The
LED driver is a current regulator. Its purpose is to keep the voltage at TP2
exactly the same as the input voltage to the circuit (U3A, pin 3). This is
accomplished by using the voltage developed across R1 as a feedback to driver
U3A. This circuit has a very high rejection of power supply changes that could
cause intensity changes.
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.