2/Theory of Operation
2-20 1503-0151-000 3/27/97
Signals that are monitored by the ADC system include:
• Flow DAC output
• Flow Current sense
• Inlet Valve Current sense
• Patient Pressure
• Manifold Pressure
• Inspiratory Flow Sensor
• Expiratory Flow Sensor
•O
2
Concentration
• Power Supplies
• Battery Backup system
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 sys-
tem 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
(AD820AR).
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 1.048MHz. It is derived
from the microprocessor system clock.
With a 1.048MHz clock, the conversion time of the A/D converter is 13 clock peri-
ods or 12.4 µ sec, plus the 3 µ sec required for the track/hold amplifier. The re-
sulting conversion time for the A/D converter is 15.4 µ sec.
The MAX191BCWG has the following unadjusted dc accuracy specifications:
• Integral Nonlinearity ± 1 LSB max
• Differential Nonlinearity ± 1 LSB max (guaranteed monotonic)
• Full-Scale Error ± 3 LSB max
• Unipolar Offset Error ± 2 LSB max
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