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Motorola Sensor Device Data
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Maximum Temperature Effect on Offset = (Gain)
(TCVoff)
(Maxtemp – MinTemp)
MAX. TEMPERATURE
EFFECT ON OFFSET
(POSITIVE TEMP. COEFF.)
MAX. TEMPERATURE
EFFECT ON OFFSET
(NEGATIVE TEMP. COEFF.)
REQUIRED
HEADROOM
Figure 6. The Maximum Temperature Effect on Offset
8. Calculate the Maximum Offset Variation. The Maximum
Offset Variation is the total amount of the Required
Headroom that must be reserved to account for the entire
distribution of sensor offsets (at room temperature —
refer to Figure 7).
where largest offset is
and the smallest offset is
Maximum Offset Variation = [Gain]
[MaxSensOff – MinSensOff]
[Gain]
[MaxSensOff]
[Gain]
[MinSensOff]
9. Calculate the worst case Minimum Offset. The worst case
Minimum Offset includes both temperature effects (from
Step 7) and device–to–device variations (from Step 8) to
determine the smallest possible offset over the entire
distribution of sensor offsets and over the operating
temperature range. This worst case Minimum Offset
occurs when a sensor has a nominal room temperature
offset of MinSensOff (smallest offset in the sensor offset
distribution) and a negative temperature coefficient so
that the offset decreases with increasing temperature.
Refer to Figure 7.
Minimum Offset = [Gain]
[MinSensOff] – Maximum Temperature Effect on Offset
10. Similar to Step 9, calculate the worst case Maximum
Offset. The worst case Maximum Offset includes both
temperature effects (from Step 7) and device–to–device
variations (from Step 8) to determine the largest possible
offset over the entire distribution of sensor offsets and
over the operating temperature range. This worst case
Maximum Offset occurs when a sensor has a nominal
room temperature offset of MaxSensOff (largest offset in
the sensor offset distribution) and a positive temperature
coefficient so that the offset increases with increasing
temperature. Refer to Figure 7.
Maximum Offset = [Gain]
[MaxSensOff] + Maximum Temperature Effect on Offset
MAXIMUM
OFFSET
MINIMUM
OFFSET
MAX. TEMPERATURE
EFFECT ON OFFSET
(POSITIVE TEMP. COEFF.)
MAX. TEMPERATURE
EFFECT ON OFFSET
(NEGATIVE TEMP. COEFF.)
REQUIRED
HEADROOM
MAX. OFFSET VARIATION
(BEFORE ADDING
TEMP. EFFECTS)
Figure 7. Calculating the Maximum and
Minimum Offsets
11. Calculate the Required Headroom. Referring to Figure 7,
the Required Headroom is the difference between the
Maximum Offset and Minimum Offset and is the amount
of voltage range (bits of the A/D) required to allow for
device–to–device and temperature variations of the
sensor’s offset.
Required Headroom = Maximum Offset – Minimum Offset
12. Compare the Required Headroom of Step 11 to the
Calculated Headroom of Step 6. The Calculated Head-
room is the absolute maximum amount of offset variation
(due to device–to–device variations and temperature
effects) that the system can allow for the desired
resolution. If the Required Headroom is greater than the
Calculated Headroom, the desired resolution is not
attainable for all worst case variations due to temperature
effects, component tolerances, and device–to–device
variations. Therefore, the requirement to attain the
desired system resolution is:
Calculated Headroom
≥
Required Headroom
If this requirement is not met, as stated previously, the
alternatives to meeting this requirement are the following:
Relax the Resolution requirement and repeat the meth-
odology.
Reduce (tighten) the span or offset (or both) variation and
repeat the methodology.
Reduce temperature coefficients.
Reduce the component tolerances and repeat the
methodology.
Repeat the methodology by performing a combination of
the above suggestions.
F
Freescale Semiconductor, Inc.
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