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8.2 QA-1290 Pressure
Measurement System
Refer to Appendix C, Drawing No. 1001-01-02, containing eight
schematic diagrams.
1. Pressure Sensor and Amplifier . Refer to Schematic Diagrams
1, 5 and 6 (Top Level, A/D Converter and Pressure Sensor and
Amplifier.
The pressure sensor used is the Honeywell (gauge
type) 136PC15G2. The sensor is based on a piezo-resistive
measuring bridge. It is temperature-compensated internally, but
has no built-in amplifier. It is powered with 12V, which is a ref-
erence voltage that is internally generated from the 5V-reference
voltage on the A/D-converter. The voltage is generated by U102-
C. The sensitivity of the pressure sensor is 128.92 µV/mmHg
(6,67mV/psi). The pressure range is 0 to 775.7 mmHg (0 – 15
psi). This gives a full-scale output (FSO) of 100 mV. Initial tol-
erance on FSO is ± 1.5%, and for offset voltage it is max ±
1.0mV
The amplifier takes the signal from the pressure sensor to a
maximum output voltage of 5.00 V at maximum pressure.
Maximum pressure is defined to be 512.0 mmHg. This gives a
nominal amplification of 75.8 The amplifier used is a JFET OP-
AMP, LF 347. Two multi-turn potentiometers are used for ad-
justment of offset and gain for the pressure sensor circuit. For
offset adjustment the two reference voltages of 12.1 V and - 1.2
V is used. Offset is adjusted with R101. Gain is adjusted with
R100.
Refer to Schematic Diagram 8 (Component Notation, Bot-
tom Side).
The amplified signal passes through a second order low-pass fil-
ter . The roll-off frequency is about 15 Hz.
2. A/D-Converter . Refer to Schematic Diagrams 1, 2 and 5 (Top
Level, Power and A/D Converter. The A/D-converter used is the
National Semiconductor ADC12030. This is a 12 bit (plus sign)
A/D with SPI-bus interface to the micro controller. The hardware
SPI on the micro controller is not used, as the pins are used for
other purposes. Instead the SPI is implemented in QA-1290’s
firmware, using ordinary I/O-port pins. The communication uses
the signals
MOSI , MISO , SCKB (SPI-signals) and AD_CS
(chip select). For a description of the data format refer to the data
sheet for the A/D-converter. The signal
AD_EOC indicates
when a conversion has finished and the data are available for re-
trieval from by the µC. A measurement range of 5V and 12 bit
gives a resolution of 5/(2
12
) = 1.22mV per step. A 4 MHz oscilla-
tor generates the converter clock.
The Voltage Reference for the A/D-converter is 5V. This volt-
age defines the range for the A/D-converter. From this reference,
8-2