Datex-Ohmeda Aestiva 7900 SmartVent, Service Manual

Page: 37 / 181

2/Theory of Operation
1503-0151-000 3/27/97 2-19
DATA Acquisition
The data acquisition system for the 7900 Ventilator consists of two major building
blocks. The first is an analog to digital converter (ADC) system and the second is
a digital to analog converter (DAC) system.
This portion of the 7900 Ventilator allows the Mercury microprocessor to interface
with valves and pressure transducers. The data acquisition system is also used
for internal monitoring of safety relevant signals.
The ADC system is designed to meet the following specifications:
The DAC system is designed to meet the following specifications:
Analog to Digital Converter System
The ADC system is based around the MAXIM MAX 191BCWG, a 12 Bit A/D con-
verter. It operates from
±
15 Volt power supplies except for the A/D converter. The
MAX191 is powered by a +5V supply and protected from over-voltage. The con-
verter has an internal 4.096 Volt reference which results in a 1mV/Bit resolution.
A 16 channel multiplexer and buffer amplifier precedes the A/D converter. The
manifold pressure, patient pressure, inspiratory flow and expiratory flow signal in-
puts to the multiplexer are filtered with an antialiasing filter. Other inputs are filtered
by low pass filters.
• Resolution 12 Bits
• Input Voltage Range 0 - 4.095 Volts (1mV/LSB)
• Number of Channels 16
• Total Conversion Time 20u sec max
• Integral Nonlinearity
±
1 LSB max
• Differential Nonlinearity
±
1 LSB max (guaranteed monotonic)
• Full-Scale Error
±
6 LSB max
• Unipolar Offset Error
±
3 LSB max
• Resolution 12 Bits
• Number of Channels 1
• Settling time to 0.01% 30u sec max
• Output Voltage Range 0 - 4.095 Volts (1mV/LSB)
• Integral Nonlinearity
±
1 LSB max
• Differential Nonlinearity
±
1 LSB max (guaranteed monotonic
• Full-Scale Error
±
9 LSB max
• Zero-Scale Error + 4 LSB max