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Motorola Sensor Device Data
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Once the above headroom requirement is met, the final step
is to determine the proper value of VREF:
13. A dc offset, VREF, is required to position the sensor’s span
within the A/D window so that no device–to–device or
temperature variation nor component tolerances cause
the sensor’s output to be outside the A/D window.
Therefore, calculate the VREF required to ensure that the
sensor’s smallest zero–pressure offset voltage (Minimum
Offset) is greater than or equal to Vlo (refer to Figures 5
and 7). In other words, the sum of the reference voltage
and Minimum Offset must be greater than or equal to the
amplifier’s low saturation voltage:
VREF + Minimum Offset
≥
Vlo
Solving for VREF:
VREF
≥
Vlo – Minimum Offset
Note: The reference voltage, VREF, also will have variation due
to resistor tolerances in the resistor divider used to create
VREF. To ensure that the system variation due to resistor
tolerances is negligible when compared to other sources of
variation, the system should be designed using resistors with
tolerances of 1% or better.
The following design examples use the methodology.
DESIGN EXAMPLES WITH THE MPX10 AND
MPX2010
The following table lists the methodology’s steps. The table
entries (names) will correspond to the names used in the
methodology outlined above; additionally, the step number
(Step 1, etc.) is bracketed ( [
]
) and superscripted next to the
entry to which the step refers. The first column lists the given
parameters that should be available in or derived from the
appropriate component’s (sensor, amplifier, voltage regulator,
resistors) data sheet. The second column lists the
performance requirements of the sensor system (i.e., this
column lists all the calculations that relate to ensuring a
minimum sensor span to achieve the desired resolution
despite device–to–device variations, temperature effects and
component tolerances). The third column lists the calculations
that determine the headroom for the system given component
tolerances and the device–to–device variations and
temperature effects on the sensor’s offset. The table and
associated system design equations may easily be
implemented in a spreadsheet to efficiently perform the
required calculations.
Table 3. Design Example Using the MPX10
Given Parameters
MaxFSS
(mV @ 25
°
C)
83
MinFSS
(mV @ 25
°
C)
33
TCVFSS
(% FSS/
°
C)
–0.22
MaxSensOff
(mV @ 25
°
C)
58
MinSensOff
(mV @ 25
°
C)
0
TCVoff
(
μ
V/
°
C)
±
15
VS
(V)
5
Vhi
(V)
4.8
Vlo
(V)
0.2
Vtol
(%)
5
Maxtemp
(
°
C)
70
MinTemp
(
°
C)
0
Performance Parameters
[1]Resolution (% FSS)
4.5
Headroom Parameters
[7]Maximum Temperature Effect on Offset
(V)
0.03
[8]Maximum Offset Variation
(V)
1.76
[9]Minimum Offset
(V)
–0.03
[10]Maximum Offset
(V)
1.73
[13]VREF
(V)
0.23
[2]Number of Steps
44
[3]Minimum Required Span
(V)
0.87
[4]Gain
29
[5]Maximum Span
(V)
2.57
[6]Calculated Headroom
(V)
1.78
[11]Required Headroom
(V)
1.75
[12]IS
Calculated Headroom
≥
Required Headroom
F
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