Overview
This section provides a detailed explanation of N-20P operation using block diagrams and circuit schematics.
The N-20P consists of three main components:
1. The N020P main PCB schematic diagram.
2. The N-20P auxiliary PCB schematic diagram.
3. The N-20P flex circuit schematic diagram.
The relationship between these components and their interconnection is illustrated in the overall block diagram.
The main component has been divided into the following subsections:
SpO
2
Analog Circuitry Block Diagram
(Figure 12.3)
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 circuit 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, AC ranging, where DC offset is eliminated from each LED signal.
Digital Circuitry Block Diagram
(Figure 12.4)
The diagram shows the N-20P hardware and circuits, which include: the CPU and system memory, the power
supply and power control circuitry, user controls, display and ambient light sensors, audio output, thermal printer
and ambient temperature sensor, and the real-time clock.
Display Control Block Diagram
(Figure 12.5)
The N-20P display is controlled by the display control circuitry. A sensor is used to measure ambient light.
During low light conditions, the display backlight, an electroluminescent device, is automatically switched on.
Power Supply Block Diagram
(Figure 12.6)
Power supply circuitry is located on the auxiliary PCB and consists of four subsections:
1. Four "C" size batteries that provide 4-6 VDC;
2. Power control circuitry that senses a press of the measure button and switches power on;
3. Power shut-off circuit that controls power to all circuits except the power control circuit; and
4. Power supply circuits include: a regulated power supply at 5 VDC, unregulated power supplies of -5 VDC,
10 VDC, and 12 VDC, and a high voltage power supply of 70 VDC.
12-6