Datex-Ohmeda Aestiva 7900 SmartVent Technical Reference Manual Download

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2 Theory of Operation

1006-0453-000 05/04

2-21

Functional description

Power supply regulators

The SIB power supply is a 10VA l12V to +15V supply from the Aestiva
7900 Ventilator CPU board. The supply is filtered at the SIB.

There are two regulators on the SIB that are supplied by the 12V to 15V
supply. A d5V is supplied by a low dropout regulator. This supply is
used to power the flow sensor EEPROMs. The +5V to the EEPROMs can be
shut down by pulling the /SHUTDOWN control on the regulator low.

A +6.7V is supplied by another low dropout regulator. This voltage is used to
power all of the operational amplifiers and pressure transducers on the SIB.
The +6.7V is necessary for the AD707 bipolar OPAMP to allow for enough
overhead voltage. This supply is also used to power a switched mode
capacitor based inverter.

The voltage inverter supplies -6.7V for analog circuits. This supplies the
AD707 OPAMP and permits it to operate to ground. The ADM660 switches at
25 kHz.

Patient airway pressure measurement

Patient airway pressure is measured at the high side (patient side) of the
inspiratory pressure transducer. A pressure range of -20 to + 120 cm H2O
supplies a proportional DC output voltage in the range of 0.3 volts to 3.8V,
with zero pressure supplying 0.8V. The transducer is filtered by an RC filter
with a pole frequency of 1 kHz and buffered by an OPAMP. The output of the
OPAMP is driving a 0.001 mF capacitor. The capacitor is used to keep a low
output impedance at RF frequencies to improve EMI immunity.

Manifold pressure measurement

The manifold pressure is measured from a port on the ventilator pneumatic
engine. A pressure range of -20 to + 120 cm H2O produces a proportional DC
output voltage in the range of 0.3V to 3.8V, with zero pressure supplying 0.8V.
This pressure transducer is also filtered and buffered in the same manner as
the patient airway pressure transducer.

Oxygen concentration measurement

The oxygen sensor is connected to the SIB by a 6-pin, 4-conductor modular
jack. The oxygen sensor supplies a linear output voltage in proportion to the
O2 concentration being measured. The oxygen sensor supplies an output of 3
to 15 mV DC in air (21% O2). It supplies an output of 14 to 72 mV DC in 100%
oxygen concentration.

The oxygen sensor output is filtered for EMI, protected against ESD and
amplified by a factor of 35 by an OPAMP. The amplified signal is then sent to
the Aestiva 7900 Ventilator CPU board after being buffered by an OPAMP.

The O2 OPAMP is bias current compensated. The filter at its positive input is a
combination of a T-type EMI filter and RC filter. The RC filter has a pole
frequency of 2 kHz. The RC filter in the AD707 feedback has a pole frequency
of 47 Hz.

A “DISCONNECT” signal, in the form of a grounded wire, is sent through the
O2 cell and to the Aestiva 7900 Ventilator CPU board. This signal is used to
sense if a cell is connected to the cable. The CPU board has a resistor pull up
to the +5V (VDD) supply and monitors this signal on an I/O port.