3–207
Motorola Sensor Device Data
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When the tolerance of the system has to be in the range of
±
1%, the designer should provide only one calibration
procedure which sets the Full Scale Output (counts) at 25
°
C
100 kPa or under the local atmospheric pressure conditions.
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1
2
4
3
PRESSURE
SENSOR
SYSTEM
MPX2100AP
MC74HC4053
1/3
MC74HC4053
V1
V2
Rf
Rg
Rf
+ 5 V
0 V
I/O
MC68HC05B6
P1
CH1
P2
VRL
VSS
VRH
VDD
+
Figure 11. One Channel Input and Two Output Ports are used by the MCU
Due to the high impedance input of the A/D converter of the
MC68HC05B6 MCU, another configuration may be
implemented which uses only one channel input as shown in
Figure 11. It is interesting to notice that practically any dual op
amp may be used to do the job but a global consideration must
be made to optimize the total cost of the system according the
the requested specification.
When the Full Scale Pressure has to be sent with accuracy,
the calibration procedure may be executed in different ways.
For instance, the module may be calibrated directly using
Up/Down push buttons.
The gain of the chain is set by changing the VRH voltage of
the ratiometric A/D converter with the R/2R ladder network
circuit which is directly drived by the ports of the MCU. (See
Figure 12.)
Using a communication bus, the calibration procedure may
be executed from a host computer. In both cases, the setting
value is stored in the EEROM of the MCU.
The gain may be also set using a potentiometer in place of
the resistor Rf. But, this component is expensive, taking into
account that it must be stable over the temperature range at
long term.
MC68HC05B6
VRH
P3
P2
P1
CH1
CH2
VDD
VRL
VSS
2R
RO
R/2R
LADDER
NETWORK
2R
2R
2R
2R
R
R
R
I/O
+ 5 V
0 V
UP
DOWN
+ 5 V
Figure 12.
P0
BUS
Table 1. Pressure Conversion Table
Unity
Pa
mbar
Torr
atm
at=kp/cm2
mWS
psi
1 N/m2 = 1 Pascal
1
0.01
7.5 10–3
—
—
—
—
1 mbar
100
1
0.75
—
—
0.0102
0.014
1 Torr = 1 mmHg
133.32
1.333
.1
—
—
—
0.019
1 atm (1)
1 at = 1 kp/cm2 (2)
101325
1013.2
760
1
1.033
10.33
14.69
98066.5
981
735.6
0.97
1
10
14.22
1 m of water
9806.65
98.1
73.56
0.097
0.1
1
1.422
1 lb/sqin = 1 psi
6894.8
68.95
51.71
0.068
—
—
1
(1) Normal atmosphere
(2) Technical atmosphere
F
Freescale Semiconductor, Inc.
n
.