MPXM2102AS datasheet(393/670 Pages) MOTOROLA

393 / 670 page

AN1315

3–247

Motorola Sensor Device Data

www.motorola.com/semiconductors

OPERATION

Connect the system to a 12 Vdc regulated power supply.

(Note the polarity marked on the power terminal P1.)

Depending on the particular pressure sensor being used with

the system, wire jumpers J1 through J3 and J8 must be

installed at board assembly time. If at some later time it is

desirable to change the type of sensor that is installed on the

board, jumpers J1 through J3 and J8, must be reconfigured for

the system to function properly (see Table 1). If an invalid J1

through J3 jumper combination (i.e., not listed in Table 1) is

used the LCD will display “SE” to indicate that condition.

These jumpers are read by the software and are used to

determine which sensor is installed on the board. Wire jumper

J8 is installed only when an MPX2010DP pressure sensor is

used on the system. The purpose of wire jumper J8 will be

explained later in the text. Jumpers J4 through J7 are read by

the software to allow the user to adjust the slope constant used

for the engineering units calculation (see Table 3). The

pressure and vacuum ports on the sensor must be left open

to atmosphere anytime the board is powered–up. This is

because the zero pressure offset voltage is computed at

power–up.

When you apply power to the system, the LCD will display

CAL for approximately 5 seconds. After that time, pressure or

vacuum may be applied to the sensor. The system will then

start displaying the applied pressure in PSI.

Table 3.

Action

Normal Slope

OUT

Decrease the Slope Approximately 7%

OUT

Decrease the Slope Approximately 6%

OUT OUT

Decrease the Slope Approximately 5%

OUT

Decrease the Slope Approximately 4%

OUT

Decrease the Slope Approximately 3%

OUT OUT

Decrease the Slope Approximately 2%

OUT OUT OUT

Decrease the Slope Approximately 1%

OUT

Increase the Slope Approximately 1%

OUT

Increase the Slope Approximately 2%

OUT

Increase the Slope Approximately 3%

OUT

OUT OUT

Increase the Slope Approximately 4%

OUT OUT

Increase the Slope Approximately 5%

OUT OUT

OUT

Increase the Slope Approximately 6%

OUT OUT OUT

Increase the Slope Approximately 7%

OUT OUT OUT OUT

Normal Slope

To improve the accuracy of the system, you can change the

constant used by the program that determines the span of the

sensor and amplifier. You will need an accurate test gauge

(using PSI as the reference) to measure the pressure applied

to the sensor. Anytime after the display has completed the

zero calculation, (after CAL is no longer displayed) apply the

sensor’s full scale pressure (see Table 1), to the sensor. Make

sure that jumpers J4 through J7 are in the “normal”

configuration (see Table 3). Referring to Table 3, you can

better “calibrate” the system by changing the configuration of

J4 through J7. To “calibrate” the system, compare the display

reading against that of the test gauge (with J4 through J7 in the

“normal slope” configuration). Change the configuration of J4

through J7 according to Table 3 to obtain the best results. The

calibration jumpers may be changed while the system is

powered up as they are read by the software before each

display update.

DESIGN CONSIDERATIONS

To build a system that will show how to interface an

MPX2000 series pressure sensor to a microprocessor, there

are two main challenges. The first is to take a small differential

signal produced by the sensor and produce a ground

referenced

signal

sufficient

amplitude

drive

microprocessor’s A/D input. The second challenge is to

understand the microprocessor’s operation and to write

software that makes the system function.

From a hardware point of view, the microprocessor portion

of the system is straight forward. The microprocessor needs

power, a clock source (crystal Y1, two capacitors and a

resistor), and a reset signal to make it function. As for the A/D

converter, external references are required to make it function.

In this case, the power source for the sensor is divided to

produce the voltage references for the A/D converter.

Accurate results will be achieved since the output from the

sensor and the A/D references are ratiometric to its power

supply voltage.

The liquid crystal display is driven by Ports A, B and C of the

microprocessor. There are enough I/O lines on these ports to

provide drive for three full digits, the backplane and two

decimal points. Software routines provide the AC waveform

necessary to drive the display.

The analog portion of the system consists of the pressure

sensor, a quad operational amplifier and the voltage

references for the microprocessor’s A/D converter and signal

conditioning circuitry. Figure 4 shows an interface circuit that

will provide a single ended signal with sufficient amplitude to

drive the microprocessor’s A/D input. It uses a quad

operational amplifier and several resistors to amplify and level

shift the sensor’s output. It is necessary to level shift the output

from the final amplifier into the A/D. Using single power

supplied op amps, the VCE saturation of the output from an op

amp cannot be guaranteed to pull down to zero volts. The

analog design shown here will provide a signal to the A/D

converter with a span of approximately 4 volts when zero to

full–scale pressure is applied to the sensor. The final

amplifier’s output is level shifted to approximately 0.7 volts.

This

will

provide

signal

that

will

swing

between

approximately 0.7 volts and 4.7 volts. The offset of 0.7 volts in

this implementation does not have to be trimmed to an exact

point. The software will sample the voltage applied to the A/D

converter at initial power up time and call that value “zero”. The

important thing to remember is that the span of the signal will

be approximately 4 volts when zero to full scale pressure is

applied to the sensor. The 4 volt swing in signal may vary

slightly from sensor to sensor and can also vary due to resistor

tolerances in the analog circuitry. Jumpers J4 through J7 may

be placed in various configurations to compensate for these

variations (see Table 3).

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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