MPXM2102AS datasheet(410/670 Pages) MOTOROLA

410 / 670 page

AN1316

3–264

Motorola Sensor Device Data

www.motorola.com/semiconductors

The following sections will detail the design issues involved

in such a system architecture, and will provide an example

circuit which has been developed as an evaluation tool for

frequency output pressure sensor applications.

DESIGN CONSIDERATIONS

Signal Conditioning

Motorola’s MPX2000 Series sensors are temperature

compensated and calibrated – i.e. – offset and full–scale span

are precision trimmed – pressure transducers. These sensors

are available in full–scale pressure ranges from 10 kPa (1.5

psi) to 200 kPa (30 psi). Although the specifications in the data

sheets apply only to a 10 V supply voltage, the output of these

devices is ratiometric with the supply voltage. At the absolute

maximum supply voltage specified, 16 V, the sensor will

produce a differential output voltage of 64 mV at the rated

full–scale pressure of the given sensor. One exception to this

is that the full–scale span of the MPX2010 (10 kPa sensor) will

be only 40 mV due to a slightly lower sensitivity. Since the

maximum supply voltage produces the most output voltage,

it is evident that even the best case scenario will require some

signal conditioning to obtain a usable voltage level.

Many different “instrumentation–type” amplifier circuits can

satisfy the signal conditioning needs of these devices.

Depending on the precision and temperature performance

demanded by a given application, one can design an amplifier

circuit using a wide variety of operational amplifier (op amp)

IC packages with external resistors of various tolerances, or

a precision–trimmed integrated instrumentation amplifier IC.

In any case, the usual goal is to have a single–ended supply,

“rail–to–rail” output (i.e. use as much of the range from ground

to the supply voltage as possible, without saturating the op

amps). In addition, one may need the flexibility of performing

zero–pressure

offset

adjust

and

full–scale

pressure

calibration. The circuitry or device used to accomplish the

voltage–to–frequency conversion will determine if, how, and

where calibration adjustments are needed. See Evaluation

Board Circuit Description section for details.

Voltage–to–Frequency Conversion

Since most semiconductor pressure sensors provide a

voltage output, one must have a means of converting this

voltage signal to a frequency that is proportional to the sensor

output voltage. Assuming the analog voltage output of the

sensor is proportional to the applied pressure, the resultant

frequency will

be linearly related to the pressure being

measured. There are many different timing circuits that can

perform voltage–to–frequency conversion. Most of the

“simple” (relatively low number of components) circuits do not

provide the accuracy or the stability needed for reliably

encoding

signal

quantity.

Fortunately,

many

voltage–to–frequency (V/F) converter IC’s are commercially

available that will satisfy this function.

Switching Time Reduction

One limitation of some V/F converters is the less than

adequate switching transition times that effect the pulse or

square–wave frequency signal. The required switching speed

will be determined by the hardware used to detect the

switching edges. The Motorola family of microcontrollers have

input–capture functions that employ “Schmitt trigger–like”

inputs with hysteresis on the dedicated input pins. In this case,

slow rise and fall times will not cause an input capture pin to

be in an indeterminate state during a transition. Thus, CMOS

logic instability and significant timing errors will be prevented

during slow transitions. Since the sensor’s frequency output

may be interfaced to other logic configurations, a designer’s

main concern is to comply with a worst–case timing scenario.

For high–speed CMOS logic, the maximum rise and fall times

are typically specified at several hundreds of nanoseconds.

Thus, it is wise to speed up the switching edges at the output

of the V/F converter. A single small–signal FET and a resistor

are all that is required to obtain switching times below 100 ns.

APPLICATIONS

Besides eliminating the need for an A/D converter, a

frequency output is conducive to applications in which the

sensor output must be transmitted over long distances, or

when the presence of noise in the sensor environment is likely

to corrupt an otherwise healthy signal. For sensor outputs

encoded as a voltage, induced noise from electromagnetic

fields will contaminate the true voltage signal. A frequency

signal has greater immunity to these noise sources and can

be effectively filtered in proximity to the MCU input. In other

words, the frequency measured at the MCU will be the

frequency transmitted at the output of a sensor located

remotely. Since high–frequency noise and 50–60 Hz line

noise are the two most prominent sources for contamination

of instrumentation signals, a frequency signal with a range in

the low end of the kHz spectrum is capable of being well

filtered prior to being examined at the MCU.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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