AN1316 Frequency Output Conversion for MPX2000 Series Pressure Sensors

Freescale Semiconductor
Application Note
AN1316
Rev 3, 05/2005
Frequency Output Conversion for
MPX2000 Series Pressure Sensors
by: Jeff Baum
Discrete Applications Engineering
INTRODUCTION
Typically, a semiconductor pressure transducer converts
applied pressure to a “low-level” voltage signal. Current
technology enables this sensor output to be temperature
compensated and amplified to higher voltage levels on a
single silicon integrated circuit (IC). While on-chip temperature
compensation and signal conditioning certainly provide a
significant amount of added value to the basic sensing device,
one must also consider how this final output will be used
and/or interfaced for further processing. In most sensing
systems, the sensor signal will be input to additional analog
circuitry, control logic, or a microcontroller unit (MCU).
MCU-based systems have become extremely cost
effective. The level of intelligence which can be obtained for
only a couple of dollars, or less, has made relatively simple
8-bit microcontrollers the partner of choice for semiconductor
pressure transducers. In order for the sensor to communicate
its pressure-dependent voltage signal to the microprocessor,
the MCU must have an analog-to-digital converter (A/D) as an
on-chip resource or an additional IC packaged A/D. In the
latter case, the A/D must have a communications interface
that is compatible with one of the MCU's communications
protocols. MCU's are adept at detecting logic-level transitions
that occur at input pins designated for screening such events.
As an alternative to the conventional A/D sensor/MCU
interface, one can measure either a period (frequency) or
pulse width of an incoming square or rectangular wave signal.
Common MCU timer subsystem clock frequencies permit
temporal measurements with resolution of hundreds of
nanoseconds. Thus, one is capable of accurately measuring
the frequency output of a device that is interfaced to such a
timer channel. If sensors can provide a frequency modulated
signal that is linearly proportional to the applied pressure
being measured, then an accurate, inexpensive (no A/D)
MCU-based sensor system is a viable solution to many
challenging sensing applications. Besides the inherent cost
savings of such a system, this design concept offers additional
benefits to remote sensing applications and sensing in
electrically noisy environments.
Figure 1. DEVB160 Frequency Output Sensor EVB
(Board No Longer Available)
© Freescale Semiconductor, Inc., 2005. All rights reserved.
The following sections will detail the design issues involved
in such a system architecture, and will provide an example
circuit which has been developed as an evaluation tool for
frequency output pressure sensor applications.
DESIGN CONSIDERATIONS
Signal Conditioning
The Freescale Semiconductor, Inc. MPX2000 Series
sensors are 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 from 10 kPa (1.5 psi) to 200 kPa (30 psi).
Although the specifications in the data sheets apply only to a
10 V supply voltage, the output of these devices is ratiometric
with the supply voltage. At the absolute maximum supply
voltage specified, 16 V, the sensor will 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 full-scale span
of the MPX2010 (10 kPa sensor) will be only 40 mV due to a
slightly lower sensitivity. Since the maximum supply voltage
produces the most output voltage, it is evident that even the
best case scenario will require some signal conditioning to
obtain a usable voltage level.
Many different “instrumentation-type” amplifier circuits can
satisfy the signal conditioning needs of these devices.
Depending on the precision and temperature performance
demanded by a given application, one can design an amplifier
circuit using a wide variety of operational amplifier (op amp) IC
packages with external resistors of various tolerances, or a
precision-trimmed integrated instrumentation amplifier IC. In
any case, the usual goal is to have a single-ended supply,
“rail-to-rail” output (i.e. use as much of the range from ground
to the supply voltage as possible, without saturating the op
amps). In addition, one may need the flexibility of performing
zero-pressure offset adjust and full-scale pressure calibration.
The circuitry or device used to accomplish the voltage-tofrequency conversion will determine if, how, and where
calibration adjustments are needed. See Evaluation Board
Circuit Description section for details.
Voltage-to-Frequency Conversion
Since most semiconductor pressure sensors provide a
voltage output, one must have a means of converting this
voltage signal to a frequency that is proportional to the sensor
output voltage. Assuming the analog voltage output of the
sensor is proportional to the applied pressure, the resultant
frequency will be linearly related to the pressure being
measured. There are many different timing circuits that can
perform voltage-to-frequency conversion. Most of the
“simple” (relatively low number of components) circuits do not
provide the accuracy or the stability needed for reliably
encoding a signal quantity. Fortunately, many voltage-tofrequency (V/F) converter IC's are commercially available that
will satisfy this function.
Switching Time Reduction
One limitation of some V/F converters is the less than
adequate switching transition times that effect the pulse or
square-wave frequency signal. The required switching speed
will be determined by the hardware used to detect the
switching edges. The Freescale family of microcontrollers
have input-capture functions that employ “Schmitt trigger-like”
inputs with hysteresis on the dedicated input pins. In this case,
slow rise and fall times will not cause an input capture pin to
be in an indeterminate state during a transition. Thus, CMOS
logic instability and significant timing errors will be prevented
during slow transitions. Since the sensor's frequency output
may be interfaced to other logic configurations, a designer's
main concern is to comply with a worst-case timing scenario.
For high-speed CMOS logic, the maximum rise and fall times
are typically specified at several hundreds of nanoseconds.
Thus, it is wise to speed up the switching edges at the output
of the V/F converter. A single small-signal FET and a resistor
are all that is required to obtain switching times below 100 ns.
APPLICATIONS
Besides eliminating the need for an A/D converter, a
frequency output is conducive to applications in which the
sensor output must be transmitted over long distances, or
when the presence of noise in the sensor environment is likely
to corrupt an otherwise healthy signal. For sensor outputs
encoded as a voltage, induced noise from electromagnetic
fields will contaminate the true voltage signal. A frequency
signal has greater immunity to these noise sources and can
be effectively filtered in proximity to the MCU input. In other
words, the frequency measured at the MCU will be the
frequency transmitted at the output of a sensor located
remotely. Since high-frequency noise and 50-60 Hz line noise
are the two most prominent sources for contamination of
instrumentation signals, a frequency signal with a range in the
low end of the kHz spectrum is capable of being well filtered
prior to being examined at the MCU.
AN1316
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Table 1. Specifications
Characteristics
Symbol
Min
Max
Units
30
Volts
- MPX2010
10
kPa
- MPX2050
50
kPa
- MPX2100
100
kPa
- MPX2200
200
+
Power Supply Voltage
B
Full Scale Pressure
PFS
Typ
10
kPa
Full Scale Output
fFS
10
kHz
Zero Pressure Offset
fOFF
1
kHz
SAOUT
9/PFS
kHz/kPa
ICC
55
mA
Sensitivity
Quiescent Current
EVALUATION BOARD
The following sections present an example of the signal
conditioning, including frequency conversion, that was
developed as an evaluation tool for Freescale’s MPX2000
series pressure sensors. A summary of the information
required to use evaluation board number DEVB160 is
presented as follows.
Description
The evaluation board shown in Figure 1 is designed to
transduce pressure, vacuum or differential pressure into a
single-ended, ground referenced voltage that is then input to
a voltage-to-frequency converter. It nominally provides a 1
kHz output at zero pressure and 10 kHz at full scale pressure.
Zero pressure calibration is made with a trimpot that is located
on the lower half of the left side of the board, while the full
scale output can be calibrated via another trimpot just above
the offset adjust. The board comes with an MPX2100DP
sensor installed, but will accommodate any MPX2000 series
sensor. One additional modification that may be required is
that the gain of the circuit must be increased slightly when
using an MPX2010 sensor. Specifically, the resistor R5 must
be increased from 7.5 kΩ to 12 kΩ.
pressure and 10 kHz at full scale pressure. Zero pressure
frequency is adjustable and set with R12. Full-scale frequency
is calibrated via R13. This output is designed to be directly
connected to a microcontroller timer system input-capture
channel.
GND
The ground terminal on connector CN1 is intended for use
as the power supply return and signal common. Test point
terminal TP3 is also connected to ground, for measurement
convenience.
TP1
Test point 1 is connected to the final frequency output, Fout.
TP2
Test point 2 is connected to the +5 V regulator output. It can
be used to verify that this supply voltage is within its tolerance.
TP3
Test point 3 is the additional ground point mentioned above
in the GND description.
TP4
Circuit Description
Test point 4 is connected to the +8 V regulator output. It can
be used to verify that this supply voltage is within its tolerance.
The following pin description and circuit operation
corresponds to the schematic shown in Figure 2.
P1, P2
Pin-by-Pin Description
B+
Input power is supplied at the B+ terminal of connector
CN1. Minimum input voltage is 10 V and maximum is 30 V.
Fout
A logic-level (5 V) frequency output is supplied at the OUT
terminal (CN1). The nominal signal it provides is 1 kHz at zero
Pressure and Vacuum ports P1 and P2 protrude from the
sensor on the right side of the board. Pressure port P1 is on
the top (marked side of package) and vacuum port P2, if
present, is on the bottom. When the board is set up with a dual
ported sensor (DP suffix), pressure applied to P1, vacuum
applied to P2 or a differential pressure applied between the
two all produce the same output voltage per kPa of input.
Neither port is labeled. Absolute maximum differential
pressure is 700 kPa.
AN1316
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3
4
S1
C1
1 µF
1
3
ON/OFF
2
3
IN
GND
2
1
4
1
2
R4
1.5 kΩ
3
C2
0.1 µF
TP4
OFFSET
R12
200 Ω
OUT
U2
MC78L08ACP
X1
MPX2100DP
D1
MV57124A
R8
620 Ω
11
5
+
6 U1B
7
U1A
MC33274
1
R6
R5
120 Ω 7.5 Ω
3 +
2 -
4
R7
820 Ω
+
13 -
12 +
9
10
-
Figure 2. DEVB160 Frequency Output Sensor Evaluation Board
AN1316
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U1D
14
R10
C4
0.1 µF
R9
1 kΩ
8
R11
U1C
R13
1 kW
R3
4.3 kΩ
R2
1 kΩ
1
2
3
4
3
C6
0.1 µF
1
TP3
TANTALUM
+
C5
10 µF
FULL-SCALE
FOUT
LogCom
Rt
+VIN
OUT
C3
VCC 8 0.01
µF
7
Ct
Ct 6
VSS 5
GND
2
AD654
IN
U4
MC78L05ACP
TP1
1
2
3
CN1
FOUT
GND
B+
B+
U5
BS107A
R1
240 Ω
TP2
The following is a table of the components that are assembled on the DEVB160 Frequency Output Sensor Evaluation Board.
Table 2. Parts List
Designators
Quantity
Description
C1
1
1 µF Capacitor
C2
1
0.1 µF Capacitor
Manufacturer
Part Number
C3
1
0.01 µF Capacitor
C4
1
0.1 µF Capacitor
C5
1
10 µF Cap+
C6
1
0.1 µF Capacitor
CN1
1
.15LS 3 Term
PHX Contact
1727023
D1
1
RED LED
Quality Tech.
MV57124A
R1
1
240 Ω resistor
R2, R9
2
1 kΩ resistor
R3
1
4.3 kΩ resistor
R4
1
1.5 kΩ resistor
R5
1
7.5 kΩ resistor
R6
1
120 Ω resistor
tantalum
R7
1
820 Ω resistor
R8
1
620 Ω resistor
R10, R11
2
2 kΩ resistor
R12
1
200 Ω Trimpot
Bourns
3386P-1-201
R13
1
1 kΩ Trimpot
Bourns
3386P-1-102
S1
1
SPDT miniature switch
NKK
SS-12SDP2
TP1
1
YELLOW Testpoint
Control Design
TP-104-01-04
TP2
1
BLUE Testpoint
Control Design
TP-104-01-06
TP3
1
BLACK Testpoint
Control Design
TP-104-01-00
TP4
1
GREEN Testpoint
Control Design
TP-104-01-05
U1
1
Quad Op Amp
Freescale
MC33274
U2
1
8 V Regulator
Freescale
MC78L08ACP
U3
1
AD654
Analog Devices
AD654
U4
1
5 V Regulator
Freescale
MC78L05ACP
U5
1
Small-Signal FET
Freescale
BS107A
X1
1
Pressure Sensor
Freescale
MPX2100DP
NOTE: All resistors are 1/4 watt, 5% tolerance values. All capacitors are 50 V rated, ±20% tolerance values.
AN1316
Sensors
Freescale Semiconductor
5
Circuit Operation
The voltage signal conditioning portion of this circuit is a
variation on the classic instrumentation amplifier
configuration. It is capable of providing high differential gain
and good common-mode rejection with very high input
impedance; however, it provides a more user friendly method
of performing the offset/bias point adjustment. It uses four op
amps and several resistors to amplify and level shift the
sensor's output. Most of the amplification is done in U1A which
is configured as a differential amplifier. Unwanted current flow
through the sensor is prevented by buffer U1B. At zero
pressure the differential voltage from pin 2 to pin 4 on the
sensor has been precision trimmed to essentially zero volts.
The common-mode voltage on each of these nodes is 4 V
(one-half the sensor supply voltage). The zero pressure
output voltage at pin 1 of U1A is then 4.0 V, since any other
voltage would be coupled back to pin 2 via R5 and create a
non-zero bias across U1A's differential inputs. This 4.0 V zero
pressure DC output voltage is then level translated to the
desired zero pressure offset voltage by U1C and U1D. The
offset voltage is produced by R4 and adjustment trimpot R12.
R7's value is such that the total source impedance into pin 13
is approximately 1 k. The gain is approximately (R5/R6)(1 +
R11/R10), which is 125 for the values shown in Figure 2. A
gain of 125 is selected to provide a 4 V span for 32 mV of fullscale sensor output (at a sensor supply voltage of 8 V).
The resulting 0.5 V to 4.5 V output from U1C is then
converted by the V/F converter to the nominal 1-10 kHz that
has been specified. The AD654 V/F converter receives the
amplified sensor output at pin 8 of op amp U1C. The full-scale
frequency is determined by R3, R13 and C3 according to the
following formula:
Fout (full-scale) =
Vin
(10V)(R3 + R13)C3
For best performance, R3 and R13 should be chosen to
provide 1 mA of drive current at the full-scale voltage
produced at pin 3 of the AD654 (U3). The input stage of the
AD654 is an op-amp; thus, it will work to make the voltage at
pin 3 of U3 equal to the voltage seen at pin 4 of U3 (pins 3 and
4 are the input terminals of the op amp). Since the amplified
sensor output will be 4.5 V at full-scale pressure, R3 + R13
should be approximately equal to 4.5 kΩ to have optimal
linearity performance. Once the total resistance from pin 3 of
U3 to ground is set, the value of C3 will determine the fullscale frequency output of the V/F. Trimpot R13 should be
sized (relative to R3 value) to provide the desired amount of
full-scale frequency adjustment. The zero-pressure frequency
is adjusted via the offset adjust provided for calibrating the
offset voltage of the signal conditioned sensor output. For
additional information on using this particular V/F converter,
see the applications information provided in the Analog
Devices Data Conversion Products Databook.
The frequency output has its edge transitions “sped” up by
a small-signal FET inverter. This final output is directly
compatible with microprocessor timer inputs, as well as any
other high-speed CMOS logic. The amplifier portion of this
circuit has been patented by Freescale Semiconductor, Inc.
and was introduced on evaluation board DEVB150A.
Additional information pertaining to this circuit and the
evaluation board DEVB150A is contained in Freescale
Application Note AN1313.1
TEST/CALIBRATION PROCEDURE
1. Connect a +12 V supply between B+ and GND terminals
on the connector CN1.
2. Connect a frequency counter or scope probe on the Fout
terminal of CN1 or on TP1 with the test instrumentation
ground clipped to TP3 or GND.
3. Turn the power switch, S1, to the on position. Power LED,
D1, should be illuminated. Verify that the voltage at TP2
and TP4 (relative to GND or TP3) is 5 V and 8 V,
respectively. While monitoring the frequency output by
whichever means one has chosen, one should see a 50%
duty cycle square wave signal.
4. Turn the wiper of the OFFSET adjust trimpot, R12, to the
approximate center of the pot.
5. Apply 100 kPa to pressure port P1 of the MPX2100DP
(topside port on marked side of the package) sensor, X1.
6. Adjust the FULL-SCALE trimpot, R13, until the output
frequency is 10 kHz. If 10 kHz is not within the trim range
of the full-scale adjustment trimpot, tweak the offset
adjust trimpot to obtain 10 kHz (remember, the offset pot
was at an arbitrary midrange setting as per step 4).
7. Apply zero pressure to the pressure port (i.e., both ports
at ambient pressure, no differential pressure applied).
Adjust OFFSET trimpot so frequency output is 1 kHz.
8. Verify that zero pressure and full-scale pressure
(100 kPa) produce 1 and 10 kHz respectively, at Fout
and/or TP1. A second iteration of adjustment on both fullscale and offset may be necessary to fine tune the 1-10
kHz range.
CONCLUSION
Transforming conventional analog voltage sensor outputs
to frequency has great utility for a variety of applications.
Sensing remotely and/or in noisy environments is particularly
challenging for low-level (mV) voltage output sensors such as
the MPX2000 Series pressure sensors. Converting the
MPX2000 sensor output to frequency is relatively easy to
accomplish, while providing the noise immunity required for
accurate pressure sensing. The evaluation board presented is
an excellent tool for either “stand-alone” evaluation of the
MPX2000 Series pressure sensors or as a building block for
system prototyping which can make use of DEVB160 as a
“drop-in” frequency output sensor solution. The output of the
DEVB160 circuit is ideally conditioned for interfacing to MCU
timer inputs that can measure the sensor frequency signal.
1. Schultz, Warren (Freescale Semiconductor, Inc.), “Sensor
Building Block Evaluation Board,” Freescale Application
Note AN1313.
AN1316
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NOTES
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AN1316
Rev. 3
05/2005
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