MPXM2102AS datasheet(462/670 Pages) MOTOROLA

462 / 670 page

AN1518

3–316

Motorola Sensor Device Data

www.motorola.com/semiconductors

CALIBRATION PROCEDURE AND RESULTS

The following calibration procedure will explain how to

systematically manipulate the pulse train to create a ramp that

meets the necessary design constraints. The numbers used

here are only for this design example. Figure 6 shows the

linearity performance achieved by following this calibration

procedure and setting up the ramp as indicated by Figures 4

and 5.

1. Start with a pulse train that has a pulse width and

frequency that creates a ramp with about 100 mV dc

offset and a span smaller than required. In this example

the initial pulse width is 84

µs and the initial frequency is

1.85 kHz.

2. Decrease the frequency of the pulse train until the ramp

span increases to approximately 2.4 V. The ramp span of

2.4 V will ensure that the maximum pulse width at full

scale pressure will be at least 60

µs less than the total

period. Note that by decreasing the frequency of the

pulse train, a dc offset will begin to appear. This may

result in the ramp looking nonlinear at the top.

3. If the ramp begins to become nonlinear, increase the

pulse width to decrease the dc offset.

4. Repeat steps 2 and 3 until the ramp spans 2.4 V and has

a dc offset of approximately 100 mV. The dc offset value

is not critical, but the bottom of the ramp should have a

“crisp” point at which the capacitor stops discharging and

begins charging. Simply make sure that the minimum

pulse width at zero pressure is at least 60

µs. Refer to

Figures 4 and 5 to determine if the ramp is sufficient for

the application.

Figure 5. Relationships Between the PWM Output Pressure Sensor Voltages

Microcontroller

Pulse Train

Exaggerated

Capacitor Discharge

Ramp

Waveform

Ramp Waveform

Offset (100 mV)

Sensor

Voltage

PWM Output

Voltage

Figure 6. PWM Output Pressure Sensor Linearity Data

Pressure (kPa)

100

650

600

550

500

450

400

350

300

250

200

150

100

204060

100

Duty Cycle

Pulse Width

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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