Datex-Ohmeda Aestiva 7900 SmartVent, Service Manual

Page: 42 / 181

2/Theory of Operation
2-24 1503-0151-000 3/27/97
Sensor Interface Board (SIB)
The Sensor Interface Board, (SIB), is the interface for the ventilator control module
and the anesthesia machine. It is designed to be used in the Excel models 110
SE, 210 SE's, and Modulus SE.
The SIB is located in the anesthesia machine below the work surface table top. In-
put air hoses and signal cables are routed from the side panel of the machine to
the board. A 50 conductor SCSI II cable routes power and signals to and from the
Ventilator.
Respiratory gas flow to and from the patient is measured in the machine by creat-
ing a small differential pressure across a calibrated variable orifice. The transduc-
ers for measuring the differential pressure are on the SIB. The delivered patient
airway gas pressure is also measured by a pressure transducer on the SIB. In ad-
dition, conditioning circuitry is provided for these transducers and for the Oxygen
sensor. For noise immunity, the signal outputs from the board are transmitted as
20 mA current signals.
Inspiratory and Expiratory Flow Measurement
Differential pressure levels of up to
±
- 3.5 cmH
2
O are applied to the differential
pressure transducers by the flow of gasses past the variable orifice. This produces
dc output voltages in the range of 0.200 to 2.700 volts, with zero pressure produc-
ing 1.450 volts at the transducer, and 2.000 volts at the A/D controller in the Ven-
tilator. Calibration data is stored in an EEPROM in each of these sensors. When
directed by system software, a differential clock signal is sent to the SIB where it is
transformed into a single ended signal for the EEPROMS. An enable signal is also
sent that turns on 5 Vdc power. For noise immunity, when the data has been read
by the microcontroller board in the ventilator, these signals are turned off.
Patient Airway Pressure Measurement
When there is gas flow in the system, the voltage produced by the differential pres-
sure transducer is applied to a current transmitter IC. Used in the 0 to 20 mA mode,
its full scale input range is 2.500 Vdc. In order to exactly match the transducer out-
put range, the current transmitters are offset by 0.200 Vdc. This is done by using
the 2.000 volt reference generated by one of the IC's. The reference voltage is re-
duced through a voltage divider and applied as a low impedance source by using
a buffer amplifier stage. This arrangement allows a full scale output swing of 0 to
20 mA, for an input of 0.200 to 2.700 Vdc.
Patient Airway Pressure is measured off the high side of the inspiratory transducer.
A pressure range of -20 to + 120 cmH
2
O produces a proportional dc output voltage
in the range of 0.050 to 4.050 volts, with zero pressure producing 0.621 volts. This
voltage is reduced by a voltage divider and applied to a current transmitter as de-
scribed above, with the exception that no offset voltage is used.