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Motorola Sensor Device Data
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After the discussion of the circuit components, the following
system–related issues will be discussed simultaneously using
the design example:
How the system works
Defining and designing the digital output for a desired sig-
nal resolution
A step–by–step procedure that shows you how to digitize
the signal
A procedure to show you how to software calibrate the digi-
tal output
Related software examples
This system, in addition to the benefits of a digital output
(noise immunity, etc.), also has the following additional
inherent benefits. These benefits will be addressed in more
detail in the systems topics.
The circuit topology and method of “digitizing” the sensor’s
analog output is very stable and accurate. The system uses
the microcontroller’s precise, internal, digital time base to
digitize the analog signal.
The signal resolution is user–programmable via software
— i.e. the user can program whether the resolution is 8–bit,
10–bit, etc.
The digital output is calibrated in software so that compo-
nent tolerances can be nullified.
The software required to digitize the signal requires very
little CPU time and overhead.
The required circuitry is minimal, simple, and cost–effec-
tive.
THE PRESSURE SENSOR
Motorola’s MPX2000 series sensors are temperature
compensated and calibrated (i.e. offset and span are
precision trimmed) pressure transducers. These sensors are
available in full scale pressure ranges from 10 kPa (1.5 psi) to
700 kPa (100 psi). Although the specifications (see Table 1)
in the data sheets apply to a 10 V supply voltage, the output
of these devices is ratiometric with the supply voltage. For
example, at the absolute maximum supply voltage rating,
16 V, the sensor will typically 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 span of the MPX2010
(10 kPa sensor) will be only 40 mV due to the device’s slightly
lower sensitivity. Since the maximum supply voltage produces
the largest output signal, it is evident that even the best case
scenario will require some signal conditioning to obtain a
usable signal (input to an A/D, etc.). For this specific design,
an MPX2100 and 5.0 V supply are used, yielding a typical
maximum sensor output of 20 mV (typical zero pressure offset
is 0.0 mV and typical span is 20 mV). The sensor’s output is
then signal conditioned (amplified and level shifted) to provide
a four volt span with a zero pressure offset of 0.5 V.
Table 1. MPX2100 Electrical Characteristics for
VS = 10 V, TA = 25
°
C
Characteristic
Symbol
Min
Typ
Max
Unit
Pressure Range
Pop
VS
VFSS
Voff
0
100
kPa
Supply Voltage
10
16
Vdc
Full Scale Span
38.5
40
41.5
mV
Zero Pressure Off-
set
–1.0
1.0
mV
Sensitivity
V/
P
0.4
mV/kPa
Linearity
—
–0.25
0.25
%VFSS
%VFSS
Temperature Effect
on Span
TCVFSS
–1.0
1.0
Temperature Effect
on Offset
TCVoff
–1.0
1.0
mV
AMPLIFIER STAGE
The amplifier circuitry, shown in Figure 1. , is composed of
two op amps. This interface circuit has a much lower
component count than conventional quad op amp
instrumentation amplifiers. The two op amp design offers the
high input impedance, low output impedance, and high gain
desired for a transducer interface, while performing a
differential to single–ended conversion. The amplifier
incorporates level shifting capability. The amplifier has the
following transfer function:
Vo
1
R4
R3
(Vsensor) + V + shift
where R1 = R4, R2 = R3, the gain is 1
sensor’s differential output (S+ – S–), and V+shift is the
positive dc level shift voltage created by the resistor divider
comprised of R+shift1 and R+shift2. V+shift is used to position
the zero pressure offset at the desired level.
Table 2 summarizes the 1% resistor values used to obtain
a four volt span with a zero pressure offset of 0.5 V (assuming
the typical sensor offset and span values of 0.0 mV and 20 mV,
respectively).
R4
Table 2. Resistor Values for the MPX2100
Amplifier Design
R+shift1
R+shift2
R1
R2
R3
R4
4.99 k
549
20.0 k
100
100
20.0 k
F
Freescale Semiconductor, Inc.
n
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