2 Theory of Operation
1006-0453-000 05/04
2-17
Analog to digital converter
System
A 24 channel multiplexer and buffer amplifier precedes the A/D converter.
The manifold pressure, patient pressure, inspiratory flow and expiratory flow
signal inputs to the multiplexer are filtered with an antialiasing filter. Other
inputs are filtered by low pass filters.
Signals that are monitored by the ADC system include:
• Patient Pressure
• Manifold Pressure
• Inspiratory Flow Sensor
• Expiratory Flow Sensor
• Flow Current sense
• Flow DAC output
• Inlet Valve Current sense
• O
2
Concentration
• Power Supplies
• Battery Backup system
The ADC system is based around a 12 bit A/D converter. It operates from ±
15V power supplies except for the A/D converter. The 12-bit converter is
powered by a fi5V supply and protected from over-voltage.
Multiplexer and buffer amplifier
The multiplexer settles quickly, within 8 µ sec to 0.01%, to be stable for the
ADC 12-Bit conversion and to maintain the overall 20 µ sec conversion time.
The system bandwidth requirement is from dc to 20Hz. The multiplexer is an 8
channel fault protected device and the amplifier used as a buffer to drive the
A/D converter (AD822AR).
A/D converter
The MAX191BCWG is a 12-Bit converter that operates from a single 5 Volt
power supply. The clock frequency to run the A/D converter is 1.507MHz. It is
derived from the microprocessor system clock.
With a 1.507MHz clock, the conversion time of the A/D converter is 13 clock
periods or 8.63 µ sec.
The MAX191BCWG has the following unadjusted dc accuracy specifications:
Voltage reference
The MAX191BCWG has an internal 4.096 Volt ±1 mV voltage reference that
can be adjusted with a potentiometer. This reference voltage is buffered and
used for the digital to analog converter reference.
Integral Nonlinearity
± 1 LSB max
Differential Nonlinearity
± 1 LSB max (guaranteed monotonic)
Full-Scale Error
± 3 LSB max
Unipolar Offset Error
± 2 LSB max
Summary of Contents for Aestiva 7900 SmartVent
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