3–21
Motorola Sensor Device Data
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TEMPERATURE COMPENSATION
Figure 2 shows the typical output characteristics of the
MPX12 series over temperature.
Because this strain gauge is an integral part of the silicon
diaphragm, there are no temperature effects due to differ-
ences in the thermal expansion of the strain gauge and the
diaphragm, as are often encountered in bonded strain gauge
pressure sensors. However, the properties of the strain
gauge itself are temperature dependent, requiring that the
device be temperature compensated if it is to be used over
an extensive temperature range.
Temperature compensation and offset calibration can be
achieved rather simply with additional resistive components,
or by designing your system using the MPX2010D series
sensor.
Several approaches to external temperature compensa-
tion over both –40 to +125
°
C and 0 to +80
°
C ranges are
presented in Motorola Applications Note AN840.
LINEARITY
Linearity refers to how well a transducer’s output follows
the equation: Vout = Voff + sensitivity x P over the operating
pressure range (Figure 3). There are two basic methods for
calculating nonlinearity: (1) end point straight line fit or (2) a
least squares best line fit. While a least squares fit gives the
“best case” linearity error (lower numerical value), the cal-
culations required are burdensome.
Conversely, an end point fit will give the “worst case” error
(often more desirable in error budget calculations) and the
calculations are more straightforward for the user. Motoro-
la’s specified pressure sensor linearities are based on the
end point straight line method measured at the midrange
pressure.
Figure 2. Output versus Pressure Differential
Figure 3. Linearity Specification Comparison
Figure 4. Cross–Sectional Diagram (not to scale)
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SILICONE
DIE COAT
WIRE BOND
LEAD FRAME
DIE
STAINLESS STEEL
EPOXY
CASE
RTV DIE
BOND
P1
OFFSET
(VOFF)
MAX
70
O
60
50
40
30
20
10
0
0
POP
SPAN
(VFSS)
PRESSURE (kPA)
ACTUAL
THEORETICAL
LINEARITY
PRESSURE DIFFERENTIAL
O
80
70
60
50
40
30
20
10
0
0
0.3
2.0
0.6
4.0
0.9
6.0
1.2
8.0
10
1.5
PSI
kPa
SPAN
RANGE
(TYP)
OFFSET
(TYP)
VS = 3 Vdc
P1 > P2
–40
°
C
+25
°
C
+125
°
C
Figure 4 illustrates the differential or gauge configuration
in the basic chip carrier (Case 344). A silicone gel isolates
the die surface and wire bonds from the environment, while
allowing the pressure signal to be transmitted to the silicon
diaphragm.
The MPX12 series pressure sensor operating characteris-
tics and internal reliability and qualification tests are based
on use of dry air as the pressure media. Media other than
dry air may have adverse effects on sensor performance and
long term reliability. Contact the factory for information re-
garding media compatibility in your application.
F
Freescale Semiconductor, Inc.
n
.