MPXM2102AS datasheet(498/670 Pages) MOTOROLA

498 / 670 page

AN1556

3–352

Motorola Sensor Device Data

www.motorola.com/semiconductors

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

implemented in a spreadsheet to efficiently perform the

required calculations.

Table 3. Design Example Using the MPX10

Given Parameters

Performance Parameters

Headroom Parameters

MaxFSS

(mV @ 25

°C)

[1]Resolution (% FSS)

4.5

[7]Maximum Temperature Effect on Offset

(V)

0.03

MinFSS

(mV @ 25

°C)

[2]Number of Steps

[8]Maximum Offset Variation

(V)

1.76

TCVFSS

(% FSS/

°C)

– 0.22

[3]Minimum Required Span

(V)

0.87

[9]Minimum Offset

(V)

– 0.03

MaxSensOff

(mV @ 25

°C)

[4]Gain

[10]Maximum Offset

(V)

1.73

MinSensOff

(mV @ 25

°C)

[5]Maximum Span

(V)

2.57

[13]VREF

(V)

0.23

TCVoff

(

µV/°C)

±15

(V)

[6]Calculated Headroom

(V)

1.78

[11]Required Headroom

(V)

1.75

Vhi

(V)

4.8

Vlo

(V)

0.2

[12]IS

Calculated Headroom

≥

Required Headroom ?

Vtol

(%)

Maxtemp

(

°C)

MinTemp

(

°C)

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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