MPXM2102AS datasheet(495/670 Pages) MOTOROLA

495 / 670 page

AN1556

3–349

Motorola Sensor Device Data

www.motorola.com/semiconductors

THE METHODOLOGY TO OPTIMIZE

PERFORMANCE

The methodology starts with defining all the known

parameters. The parameters with an asterisk (*) are specified

at 25

°C.

• Resolution

• MaxFSS (*)

• MinFSS (*)

• TCVFSS (*)

• MaxSensOff (*)

• MinSensOff (*)

• TCVoff

• Vlo

• Vhi

• VREF

• Vtol

• MinTemp

• Maxtemp

= Desired system resolution

= Maximum full–scale voltage span of

= the pressure sensor

= Minimum full–scale voltage span of

= the pressure sensor

= The maximum temperature coefficient

= of the sensor’s full–scale voltage span

= The maximum zero pressure offset

= voltage of the pressure sensor

= The minimum zero pressure offset

= voltage of the pressure sensor

= The sensor’s maximum temperature

= coefficient of offset voltage

= The low saturation level of the amplifier

= or low reference voltage of an A/D

= (whichever is most limiting case)

= The high saturation level of the

= amplifier or the high reference voltage

= of an A/D (whichever is most limiting

= case)

= The reference voltage for positive dc

= voltage level shifting

= The voltage regulator tolerance

= The application’s minimum operating

= temperature

= The application’s maximum operating

= temperature

These parameters are either chosen for the application

(e.g., system resolution) or can be determined from the

sensor’s data sheet. Tables 1 and 2 provide the necessary

information for the design examples presented here.

Note: The data in Tables 1 and 2 are scaled for a 5 V supply

voltage, whereas the MPX10 and MPX2010 data sheets are

specified at a 3 V and 10 V supply voltage, respectively.

The following steps outline the methodology that will be

applied to the MPX10 in the first design example and then

applied to the MPX2010 in the second design example.

1. Determine/choose the required Resolution for the

system.

2. Calculate the number of steps required for the chosen

resolution. The resolution determines the number of

steps into which the pressure signal needs to be broken

[see Figure 3 where an 8–bit A/D (255 steps of

resolution) is assumed]. A conservative approach to

determining this number of steps is to assume that with

an A/D, the digital quantization of the pressure signal can

be plus or minus one step. Therefore, assume that it

takes twice the number of steps previously determined to

resolve a given minimum incremental pressure. The

number of steps for the chosen resolution is

Number of Steps

· 100

Resolution

The scaling factor of 100 in the numerator converts the

resolution from a percentage to a decimal fraction.

A/D HIGH REFERENCE

A/D LOW REFERENCE

STEP 255

STEP 0

STEP 127

Figure 3. The 255 Digital Steps of an 8–Bit A/D

3. Calculate the minimum amplified sensor span (defined as

the Minimum Required Span — see Figure 4) required for

this resolution requirement. Using an 8–bit A/D with a 5 V

window where one step equals 19.6 mV (for the nominal

regulator voltage), the minimum amplified sensor span is

Minimum Required Span

+ (Number of Steps)

·(19.6 mV)

A/D HIGH REFERENCE

A/D LOW REFERENCE

ZERO PRESSURE

OFFSET VOLTAGE

FULL–SCALE

OUTPUT VOLTAGE

MINIMUM

REQUIRED

SPAN

MAXIMUM

SPAN

A/D’S

DYNAMIC

RANGE

Figure 4. The Minimum Required Span for the

Required Resolution and the Maximum Span

Due to Sensor Span Variations

4. Calculate the amplifier’s gain. The gain must be large

enough to achieve, over the entire distribution of sensor

spans, the Minimum Required Span. Therefore, this gain

is calculated using the smallest pressure sensor voltage

span, MinFSS. By using the worst case smallest pres-

sure sensor voltage span to calculate the gain, the

Minimum Required Span (the minimum span that will

achieve the resolution requirement) is guaranteed for the

entire distribution of sensor spans. The worst case

minimum full–scale sensor span will occur at the hottest

temperature, Maxtemp, in the application (not exceeding

the operating temperature of the sensor), since the span

decreases with increasing temperature (TCVFSS is

negative).

Gain

Minimum Required Span

[MinFSS]

· [1)TCVFSS · (Maxtemp–25)]

The term [1 + TCVFSS • (Maxtemp – 25)] is the temperature

effect on the span.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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