Figure 4. Linearity Specification Comparison
R
LEAST SQUARE
DEVIATION
0
50
100
PRESSURE (% FULLSCALE)
STRAIGHT
LINE
DEVIATION
LEAST SQUARES FIT
EXAGGERATED
PERFORMANCE
CURVE
OFFSET
END POINT
STRAIGHT LINE FIT
3–9
Motorola Sensor Device Data
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LINEARITY
Linearity refers to how well a transducer’s output follows
the equation: Vout = Voff + sensitivity x P over the operating
pressure range. There are two basic methods for calculating
nonlinearity: (1) end point straight line fit (see Figure 4) or
(2) a least squares best line fit. While a least squares fit gives
the “best case” linearity error (lower numerical value), the
calculations 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. Motorola’s
specified pressure sensor linearities are based on the end
point straight line method measured at the midrange pressure.
OPERATION
Motorola pressure sensors provide three types
of pressure measurement: Absolute Pressure,
Differential Pressure and Gauge Pressure.
Absolute Pressure Sensors
measure an
external pressure relative to a zero–pressure
reference (vacuum) sealed inside the reference
chamber of the die during manufacture. This
corresponds to a deflection of the diaphragm
equal to approximately 14.5 psi (one atmo-
sphere), generating a quiescent full–scale output
for the MPXH6101A6T1 (14.5 psi) sensor, and a
half–scale output for the MPX4200A (29 psi)
device. Measurement of external pressure is
accomplished by applying a relative negative
pressure to the “Pressure” side of the sensor.
Differential Pressure Sensors
measure the
difference between pressures applied simulta-
neously to opposite sides of the diaphragm. A
positive pressure applied to the “Pressure” side
generates the same (positive) output as an equal
negative pressure applied to the “Vacuum” side.
Figure 5. Pressure Measurements
Gauge Pressure
readings are a special case of differential measure-
ments in which the pressure applied to the “Pressure” side is measured
against the ambient atmospheric pressure applied to the “Vacuum” side
through the vent hole in the chip of the differential pressure sensor
elements.
éééé
NEGATIVE PRESSURE
VOFF
1 ATMPMAX
INCREASING VACUUM
INCREASING PRESSURE
Motorola sensing elements can withstand pressure inputs as high as four times their rated
capacity, although accuracy at pressures exceeding the rated pressure will be reduced.
When excessive pressure is reduced, the previous linearity is immediately restored.
POSITIVE PRESSURE
NEGATIVE
PRESSURE
VOFF
DIFFERENTIAL PRESSURE
INCREASING
PMAX
Absolute
Sensor
Differential
Sensor
VACUUM
F
Freescale Semiconductor, Inc.
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