
3–316
Motorola Sensor Device Data
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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
μ
s 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
μ
s 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
μ
s. 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
Mcrocontroller
Pulse Train
Exaggerated
Capacitor Discharge
Ramp
Waveform
Ramp Waveform
Offset (100 mV)
Sensor
Voltage
PWMOutput
Voltage
Figure 6. PWM Output Pressure Sensor Linearity Data
Pressure (kPa)
100
90
80
70
60
50
40
30
20
10
0
650
600
550
500
450
400
350
300
250
200
150
100
50
0
0
20
40
60
80
100
Duty Cycle
Pulse Width
D
P
μ
F
Freescale Semiconductor, Inc.
n
.