Passport 2®/Passport 2 LT™ Service Manual
0070-10-0441
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Theory of Operation
Passport 2 NIBP Module (P/N 0670-00-0730 or 0670-00-0746-01)
2.15.2
Pressure Transducer, PT1
A Fujikura XFPM-050KPG-P5 transducer was selected for the measurement channel. It
provides a high level output signal which eliminates the need for an external amplifier circuit.
All that is required is a pull-up resistor and compensation capacitor. The PT1 is screened to
maximize performance in designs which will perform a zero cal before each measurement
and a yearly span cal at 150mmHg near room temperature. It also provides a nonlinearity
specification beyond the screening limits which enhances performance.
The transducer is not installed during the initial PCB manufacturing and cleaning processes,
avoiding exposure to moisture which may effect its performance, but will be soldered in
place during final module assembly.
The transducer has an output transfer characteristic of 11.667mV/mmHg. Adjusting trim pot
R81 changes the attenuation factor of the transducer output signal (span calibration), and
thus the slope of the transfer characteristic. The nominal slope will correspond to the
attenuation produced when the wiper is at the midpoint of its range. With a 28.7kW value
for R80 and a 2kW value for R81, the nominal slope to be used by the software for the
measurement channel is 11.29mV per mmHg. The adjustment range is wide enough to cover
the +/-50mV transducer set point tolerance at the 150mmHg calibration pressure.
2.15.3
ADC, U2
The pressure signal is sampled by an Analog Devices AD7714 24-bit ADC which interfaces
to the microcontroller’s QSPI port.
The device must first be configured by using the AD7714 zero-scale self-calibration mode
followed by the full-scale self-calibration mode. From this calibration process, offset and gain
coefficients are determined that will be applied automatically by the AD7714 hardware.
NOTE:
Several samples must be read before the new coefficients
are applied after a calibration.
The system zero cal is then performed. If in the span calibration diagnostic mode the system
zero cal is followed by the span cal.
NOTE:
A 90 second warm-up period shall elapse prior to span
calibration to account for transducer warm-up drift. It is also
necessary to calibrate the ADC prior to system span cal.,
otherwise the gain coefficient in the ADC will not be taken
into account during the span calibration.
The ADC is used in the bipolar mode which causes the ADC gain error to be of opposite sign
above and below the 2.5V nominal voltage appearing on AIN2. That is, the ADC gain error
pivots at 2.5V. The trim pot (R81), on the other hand, adjusts for gain error which pivots at
0V and will not correctly account for ADC gain error without a corresponding change in the
offset value. This leads to an iterative process for adjusting ADC gain error and offset.
The input buffer reduces the input range below what is required, so the BUFFER control pin is
tied low to short out the internal buffer.
