Analog-to-Digital Converter - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 624

This program can be tested on the Dragon12-Plus demo board by rotating the potentiometer.
The potentiometer emulates the output from the signal conditioning circuit. Like Example 12.9,
the user can eliminate the signal conditioning circuit by setting the V RH input to a 3.9-V Zener
diode. The user will also need to modify the translation formula from the ADC result back to
the humidity. The modification of the program is straightforward and is left as an exercise.

12.8 Measuring Barometric Pressure

Barometric pressure refers to the air pressure existing at any point within the Earth's
atmosphere. This pressure can be measured as an absolute pressure (with reference to ab-
solute vacuum) or can be referenced to some other value or scale. The meteorology and
avionics industries traditionally measure the absolute pressure and then reference it to a
sea-level pressure value. This complicated process is used in generating maps of weather
systems.
Mathematically, atmospheric pressure is exponentially related to altitude. Once the pres-
sure at a particular location and altitude is measured, the pressure at any other altitude can be
calculated. Several units have been used to measure the barometric pressure: in 2 Hg , kPa , mbar ,
or psi . A comparison of barometric pressure using four different units at sea level up to 15,000
ft is shown in Table 12.11.

Altitude (ft) Pressure (in-Hg) Pressure (mbar) Pressure (kPa) Pressure (psi)

0
500
1000
6000
10,000
15,000

29.92
29.38
28.85
23.97
20.57
16.86

1013.4
995.1
977.2
811.9
696.7
571.1

101.4
99.5
97.7
81.2
69.7
57.1

14.70
14.43
14.17
11.78
10.11
8.28

Table 12.11 ■ Altitude versus pressure data

There are three forms of pressure transducer: gauge , differential , and absolute . Both the
gauge pressure ( psig ) and differential ( psid ) transducers measure pressure differentially. The ab-
breviation psi stands for “pounds per square inch”; the letters g and d stand for gauge and differ-
ential , respectively. A gauge pressure transducer measures pressure against ambient air, whereas
the differential transducer measures against a reference pressure. An absolute pressure trans-
ducer measures the pressure against a vacuum (0 psia) and hence it measures the barometric
pressure.
The SenSym ASCX30AN is a 0 to 30 psia (psi absolute) pressure transducer. The range of
barometric pressure is between 28 to 32 inches of mercury (in-Hg) or 13.75 to 15.72 psia or
948 to 1083.8 mbar. The transducer output is about 0.15 V/psi, which would translate to an
output voltage from 2.06 to 2.36 V. The complete specifications of the SenSym ASCX30AN
are shown in Table 12.12. Since the range of V OUT is very narrow, the designer will need to use
a level shifting and scaling circuit in order to take advantage of the available dynamic range.
The pin assignment of the SenSym ASCX30AN is shown in Figure 12.21.