Spo 2 Analog Circuitry Block Diagram (Figure 9-3); Figure 9-2: Overall Block Diagram - Mallinckrodt Nellcor N-20 Service Manual

Patient
sensor
Batteries
4-6 VDC
9.6 SpO Analog Circuitry Block Diagram (Figure 9-3)
2
Analog circuitry has high signal sensitivity and reduced susceptibility to noise. Its design allows for a
wide range of input signal levels and a broad range of pulsatile modulation. The SpO
(Figure 9-3) consists of four subsections:
1. Sensor output/LED control, where the CPU controls the gain of both LEDs so that signals
received at the input amplifier are in its acceptable dynamic range
2. Input signal conditioning, where sensor output current is converted to voltage
3. Signal gain, where the separated LED signals are amplified so their current levels are within the
A/D converter's acceptable range; and
4. AC ranging, where DC offset is eliminated from each LED signal.
Patient
sensor
LEDs
Main PCB
SpO analog
2
Microprocessor
Memory
Display control
Sensors: temperature
ambient light
battery voltage
Measure
PROM
button
20-pin headers
Auxiliary PCB
Power supply
Printer interface
Flex
Display control
connectors
Audio beeper
Check
Real-
battery
time
button
clock

Figure 9-2: Overall Block Diagram

Input signal
conditioning
photocurrent
variable gain,
to voltage
conversion
demutiplexed
to 2 channels
Main PCB
Main PCB
LED drivers
(red & IR)
Figure 9-3: SpO
2
Display
backlight
Printer
flex circuit
N-20P only
Signal gain
AC Ranging
offset
filtered for
substraction;
each LED
additional gain
channel
and filtering
Main PCB Main PCB
Microprocessor
Main PCB
Control
Analog Circuitry Block Diagram
Technical Supplement
Printer
analog circuit
2
To digital section
9-3