MPXM2102AS datasheet(497/670 Pages) MOTOROLA

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AN1556

3–351

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.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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