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Motorola Sensor Device Data
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PRESSURE SENSOR
Motorola’s MPX5000 series sensors are signal conditioned
(amplified), 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 of 50 kPa (7.3 psi) and 100 kPa (14.7 psi).
With the recommended 5.0 V supply, the MPX5000 series
produces an output of 0.5 V at zero pressure to 4.5 V at full
scale pressure. Referring to the schematic of the system in
Figure 1, note that the output of the pressure sensor is
attenuated to one–half of its value by the resistor divider
comprised of resistors R1 and R2. This yields a span of 2.0 V
ranging from 0.25 V to 2.25 V at the non–inverting terminal of
the comparator. Table 1 shows the electrical characteristics of
the MPX5100.
Table 1. MPX5100DP Electrical Characteristics
Characteristic
Symbol
Min
Typ
Max
Unit
Pressure Range
POP
VS
VFSS
Voff
S
0
—
100
kPa
Supply Voltage
—
5.0
6.0
Vdc
Full Scale Span
3.9
4.0
4.1
V
Zero Pressure Offset
0.4
0.5
0.6
V
Sensitivity
—
40
—
mV/kPa
Linearity
—
– 0.5
—
0.5
%FSS
Temperature Effect on Span
—
–1.0
—
1.0
%FSS
Temperature Effect on Offset
—
– 50
0.2
50
mV
THE RAMP GENERATOR
The ramp generator is shown in the schematic in Figure 1.
A pulse train output from a microcontroller drives the ramp
generator at the base of transistor Q1. This pulse can be
accurately controlled in frequency as well as pulse duration via
software (to be explained in the microcontroller section).
The ramp generator uses a constant current source to
charge the capacitor. It is imperative to remember that this
current source generates a stable current only when it has
approximately 2.5 V or more across it. With less voltage
across the current source, insufficient voltage will cause the
current to fluctuate more than desired; thus, a design
constraint for the ramp generator will dictate that the capacitor
can be charged to only approximately 2.5 V, when using a
The constant current charges the capacitor linearly by the
following equation:
V
I t
C
(1)
where
t is the capacitor’s charging time and C is the
capacitance.
Referring to Figure 2, when the pulse train sent by the
microcontroller is low, the transistor is off, and the current
source charges the capacitor linearly. When the pulse sent by
the microcontroller is high, the transistor turns on into
saturation, discharging the capacitor. The duration of the high
part of the pulse train determines how long the capacitor
discharges, and thus to what voltage it discharges. This is how
the dc offset of the ramp waveform may be accurately
controlled. Since the transistor saturates at approximately
60 mV, very little offset is needed to keep the capacitor from
discharging completely.
Figure 2. Ideal Ramp Waveform for the PWM Output Pressure Sensor
Mcrocontroller
Pulse Train
Exaggerated
Capacitor Discharge
Ramp
Waveform
Ramp Waveform
Offset (100 mV)
F
Freescale Semiconductor, Inc.
n
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