Datex-Ohmeda Aestiva 7900 SmartVent Technical Reference Manual page 34

2 Theory of Operation
Flow valve control
Gas inlet valve drive circuit
2-18
The flow valve control circuit consists of a D/A converter and a voltage to
current conversion circuit.
D/A conversion
The D/A conversion for the flow valve drive circuit is based around the
MAX530 12-Bit DAC. The output of the DAC is fed to an input of the
A/D converter multiplexer allowing the microprocessor to monitor the DAC
output.
The MAX530 operates from the same 5V power supply as the A/D converter.
The output voltage range of the converter is 0 to 4.095V (1mV per LSB).
The MAX530BCWG has the following dc accuracy specifications:
Integral Nonlinearity
Differential Nonlinearity
Full-Scale Error
Zero-Scale Error
Flow valve drive circuit
The flow valve drive circuit does a voltage to current conversion of the DAC
output voltage signal FLW_DAC. With a 0 to 4.000 Volts input, the drive circuit
outputs 0 to 1.0 Amps typical into a 3 ohm load. This current is passed to the
flow valve and is used to proportionally control the flow valve during
mechanical ventilation. This circuit does not require adjustment and is
accurate within ± 2% of full scale.
This circuit also limits the flow valve output to less than 10VA under normal
operation and under a single fault condition. This is a requirement of the
international regulation IEC 601-2-13.
A signal proportional to the actual drive current is input to the A/D converter
to permit the processor to monitor the current and detect fault conditions.
This circuit consists of a low-dropout regulator providing a regulated 5 Volts to
the inlet valve when enabled by the microprocessor. The SHUTDOWN pin of
the regulator provides on/off control.
This regulator has an output current of 250mA. It has an internal current limit
of 530mA max. This keeps the output under 10VA in a single fault condition.
A current sensing circuit is included to let the processor monitor the inlet valve
current via the A/D converter system and detect fault conditions.
± 1 LSB max
± 1 LSB max (guaranteed monotonic)
± 1 LSB max
+ 4 LSB max
05/04 1006-0453-000