Nellcor N-180 Service Manual page 21

N-180 Pulse Oximeter Service Manual
The frequency of each control signal (IRLED* and REDLED*) is 1365.3 Hz,
with a 25% duty cycle. When the DAC outputs are gated at U4 (pins 2 and 15)
a four-phase LED drive signal with a frequency of 2710.6 Hz is created. A
negative LED drive bias voltage applied to R2 ensures that the output of the
LED driver USA is zero during the times when an LED is not selected by
either control line. This scheme counters any normal offsets that may be
present in component USA.
Typically, the LED drive signal has a peak- to-peak value of 0.5 volts. The
lower boundary is 0 volts.
Initially, both LED drive levels are maximum (0.5 volts), but may be reduced
as the processor adjusts each of the individual LED intensities to compensate
for measurement site lighting variables. High background ambient light or
highly translucent measurement sites (such as found in neonates) may cause
a reduction in overall LED intensity.
' As described, the LEDs operate in a four-phase sequence. Each phase lasts
184.5 microseconds, nominal.
Phase 1 IR LED on
Phase2 Both LEDs off
Phase 3 Red LED on
Phase4 Both LEDs off
"The LED drive current switching is accomplished by Qi through Q6 andthe 000
control lines, IRLED and REDLED. The figure illustrates the relationship of
these components and their association with the red (R) and IR (1) LEDs 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 LEDs 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 R31 to G4 and via R36 to
03. Control inputs IRLED and REDLED are pulsed low, to light respective
LEDs . When both lines are a logic high, both LEDs are turned off because Q1
' and Q2 are directly turned off by the control inputs via R5 and R6. Device G3
and device Q4 are turned off by the conduction of Q5 and Q6, which are now
conducting.
The IR LED (1) lights when control signal IRLED* is pulsed low. Device Q5
turns off, allowing Q8 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,
02, 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
USA, and Q6 is turned on. The resulting current flow is from Vcc through 91,
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
USA. This circuit has a very high rejection of power supply changes that could
cause intensity changes.