Display and Analog Shield Assembly
– this assembly connects to the main PCB by flex circuits. A metal
shield shrouds the SpO2 analog circuits on the main PCB to protect them from EMI. An integrated
electroluminescent backlight illuminates the display under low light conditions.
The N-20P has an additional printer control board (printer flex circuit), and printer hardware. The following
block diagram shows the relationship between these components.
SpO
2
Analog Circuitry
This subsection describes the SpO
2
analog hardware. The 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
2
analog block diagram (Figure 12.3) consists of four subsections:
Sensor/output LED control
– The CPU controls the gain of both LEDs so that signals received at the input
amplifier are within an acceptable dynamic range. Signal channel gain may also need to be increased. The CPU
uses PWM lines to control LED current level or to amplify the signal channel.
Input conditioning
– Sensor output current is converted to voltage. A demodulation circuit minimizes the effects
of other light sources and stray frequency inputs. Because the IR and RED signals are at different current levels,
the two LED signals are demultiplexed and separately amplified, so they can be compared with each other. Two
circuits handle the demultiplexing by alternately selecting LED signals using switches. Filters then remove noise and
smooth the signals before sending them to the amplifiers.
Patient
Sensor
Main PCB
SpO2 analog
Microprocessor
Memory
Display control
Sensors:
temperature
ambient Lght
battery voltage
PROM
Measure
button
Batteries
Auxiliary PCB
Power supply
Printer interface
Display control
Audio beeper
Real-
time
clock
Check
battery
button
20-Pin headers
Display
backlight
Batteries
Printer
N-20P only
Flex
Connectors
4-6 VDC
Figure 12.2 Overall Block Diagram
12-8