MPXM2102AS datasheet(494/670 Pages) MOTOROLA

494 / 670 page

AN1556

3–348

Motorola Sensor Device Data

www.motorola.com/semiconductors

Where the gain is

(R4

) 1

), the pressure sensor’s

differential output voltage is the quantity (S+ – S–), and the

positive dc voltage level shift, created by the voltage divider

comprised of RREF1 and RREF2, is VREF. In addition to using

the above resistor ratios to preserve the common mode

rejection, the effective resistance of the parallel combination

of RREF1 and RREF2 should be a low impedance to ground

relative to the resistance of R1.

RESOLUTION AND FACTORS THAT AFFECT IT

Performance of a pressure sensor system is directly related

to its resolution. Resolution is the smallest increment of

pressure that the system can resolve — e.g., a system that

measures pressure up to 10 kPa (full–scale) with a resolution

of 1% of full–scale can resolve pressure increments of

0.1 kPa. Similarly, the resolution (smallest increment of

voltage) of an 8–bit A/D converter with a 5 V window (a high

reference voltage of 5 V and a low reference voltage of 0 V) is

255 (8 bits)

+ 19.6 mV

Many pressure sensor systems interface an A/D converter.

If the above system example requires 1% resolution when

interfaced to an A/D, the pressure sensor signal’s span must

be at least

19.6 mV

+ 1.96 V

If the system resolution required is 0.5%, the pressure

sensor signal’s span must be at least

19.6 mV

0.5%

+ 3.92 V

From these examples, the greater the resolution required,

the greater the sensor’s amplified span must be to meet the

resolution requirement. Since a pressure sensor’s span

before amplification is only on the order of tens of millivolts, the

amplifier must be designed to provide the minimum span that

gives the desired resolution. If the amplifier has a fixed gain,

any device–to–device variation in the sensor’s unamplified

span will result in variation of the amplified span. If, for

example, the sensor’s span variation results in an amplified

span that is smaller than required, the resolution of the system

will not be as high as desired. Alternately, if the sensor’s span

variation results in an amplified span that is larger than

required, the resolution will be better than desired, BUT the

amplified span may also either saturate the amplifier near its

supply rails or extend outside the high and low reference

voltages of the A/D. Voltages above the high reference will be

digitally converted as 255 decimal (for 8–bit A/D), and voltages

below the low reference will be converted as 0. This creates

a non–linearity in the analog–to–digital conversion and in the

overall system transfer function.

As presented above, the variation of the sensor’s span

creates a dilemma: how does one design a fixed–gain

amplifier that gives the desired resolution, does not violate the

limits of the linear output ranges of the op–amps and A/D

converter, and also accommodates the complete distribution

of possible sensor spans? The same question is presented to

the additional sources of variation: device–to–device variation

in the zero–pressure offset voltage and temperature effects on

both the sensor’s span and zero–pressure offset voltage. Also

any component tolerances for the voltage regulator and

resistors must be considered.

Designing the system when only one source of variation is

involved is not difficult; however, when all of these variations

are interacting, the solution becomes complicated. The rest of

this paper describes a design methodology that considers all

of the above variations and their interactions. Worst case limits

will be used in designing the fixed–value system.

RESOLUTION vs. HEADROOM

As stated previously, the amplified span of the sensor must

“fit” within the high and low references of an A/D to avoid any

nonlinearity errors. And the span must also be large enough

to provide the resolution required for the application. Any part

of the A/D’s “window” that is not used for the sensor’s dynamic

signal range is called headroom. Headroom may be thought

of as a cushion between the high and low reference voltages

and the sensor’s dynamic output range. This “cushion” is used

to allow the sensor’s dynamic range to move and/or vary within

the A/D’s window. A general description is shown in Figure 2.

The total amount of sensor output signal variation (due to

temperature effects, device–to–device variation, and interface

circuit component tolerances) cannot exceed the headroom

that is available for the requisite amount of system resolution.

A larger sensor span (more bits used for signal resolution)

means a smaller amount of headroom available to

accommodate sensor

parameter

and

interface

circuit

variations. This makes the tradeoff between resolution and

variation obvious. The more variation in the system, the more

headroom that is required to allow for the variation and,

consequently, less of the A/D window is available for the

sensor’s “true–signal” span. Less span results in poorer

resolution (less bits used for resolving sensor output signal).

A/D HIGH REFERENCE OR

HIGH SAT. LEVEL

OF AMPLIFIER

FULL–SCALE

OUTPUT VOLTAGE

ZERO PRESSURE

OFFSET VOLTAGE

A/D LOW REFERENCE OR

LOW SAT. LEVEL

OF AMPLIFIER

HEADROOM

A/D’S OR

AMPLIFIER’S

DYNAMIC

RANGE

SENSOR’S

FULL–SCALE

VOLTAGE SPAN

Figure 2. Sensor’s Full–Scale Span vs. Headroom

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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