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Compensation for drift in accuracy of the system is achieved by the unit recalibrating itself on switch on and
after each measurement and at ten-minute intervals. Calibration is achieved by measuring two points on the
high precision resistor chain of RV3, R70, and R74-77 which equate to 10°C and 37°C. Any changes in
VREFI or voltage to frequency transfer characteristic can therefore be accommodated within reason.
The temperature circuit is powered from an isolated 5-volt and ? 9 volt supply (5 and ? 15V on revision 04
and earlier), these supplies are generated by the isolated power supply module PM100. Calibration of the
voltage and hence frequency in the temperature reference circuit is achieved by the use of resistor trees,
attached to a stable reference voltage. This reference, VREFI is 3.24 volts (nominal) and is generated from
the isolated 5-volt line by D2, adjustment provided by RV2. Thus RV2 will adjust the overall calibration of
voltage ranges X0, X1 and X2. Additional adjustment of the calibration points X0 and X1 is provided by RV3
in their resistor tree. Together RV2 and RV3 are used to calibrate the temperature circuit.
The VREFI may be measured across TP3 and TP2, and the PRED_TEMP_F signal may be measured
across TP9 and TP6.
2.4.3 NIBP System
The non-invasive blood pressure system measures blood pressure and pulse rate. The NIBP measurement
circuitry is located on the Main Board.
To measure blood pressure, a reference voltage generated by IC2 supplies a bridge pressure transducer
ML1 to enable it to convert air pressure from the pneumatics system into an analog voltage. The
instrumentation amplifier IC5 amplifies this voltage. The unfiltered signal PT1 is routed to into an eight
channel analog multiplexer, IC20. When selected, the output signal is buffered by IC21 as signal
BUF_ANA_MUX and enters the ADC IC23. The digitized output of the ADC is presented to IC24, an octal
buffer/line driver, from where it is transmitted to the data bus and the system processor. Pressure
transducer offset PT1_REF from the output of IC2 also enters the analog multiplexer IC20. When selected,
it provides an off-set value to the system processor. The processor then measures the offset value and
subtracts the offset from PT1 measurements. The control signals for the multiplexer (ASEL0 ~ ASEL2) are
generated by the system processor.
The pulse rate component (typically one percent) of the cuff pressure signal is filtered and separated from
the PT1 pressure signal. The low pass filter formed around IC9a allows the low frequency pulse signal
through whilst attenuating any higher frequency noise. The signal is then ac coupled by the high pass filter
of C20 & R41 in order to strip off the small pulse rate signal from the cuff pressure signal. IC9b then
amplifies the pulse signal before being applied to high pass filter IC12. The output from IC12 FPT1 is routed
to the analog multiplexer IC20 for selection by the system processor. The system processor controls the
analog switch IC7, it is used to clamp and apply offset signals into the filtered pressure measurement
channel.
The system software includes an internal on-line self-test feature for the FPT channel at power-up and after
each non-STAT mode determination.
2.4.4 Pneumatics System
The pneumatics system provides air pressure for the NIBP cuff, manifolding for control of cuff pressure, and
an overpressure signal to the system processor and pneumatic control logic GAL if the pressure exceeds
300 mmHg in adult mode or 158 mmHg in neonate mode. The system is comprised of an air pump, pressure
transducer, overpressure detection circuitry, power control circuit, and two solenoid-operated pneumatic
valves. Air from the pump is routed through the dump valve to the cuff output manifold.
The pneumatics system provides control of the pump by commands received from the system processor.
The PUMP_ON signal from latch IC3 on the Main Board is passed to the pneumatic control logic GAL IC1 on
the power supply board. The GAL produces a pump drive signal which turns on FET TR4b, TR13 and TR8,
pulling down the ground return side of the pump motor through current sensing resistor R31. Both the
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