ARCHIVED 2005 AN1652 ASB202 - MPX2000 Series Sensor Module

MOTOROLA
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by AN1652/D
AN1652
ASB202 Ċ MPX2000 Series Sensor Module
Prepared by: Bill Lucas and Warren Schultz
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A plug–in module that is part of a systems development tool
set for pressure sensors is presented here. It provides an
analog signal from an MPX2000 series sensor to a Motorola
Sensor Development Controller, or can be used stand alone
to provide power and signal conditioning for the sensor.
PLUG–IN MODULE DESCRIPTION
A summary of information for using systems development
plug–in module ASB202 includes the schematic in Figure 2,
connector pinout in Figure 3, a pin by pin description of
functionality, specs in Tables 1–3, and a parts list in Table 4.
Figure 4 in the Applications section provides a quick reference
for making connections. A discussion of the design appears
under the heading Design Considerations.
Function
The plug–in module shown in Figure 1 is designed to supply
pressure and temperature inputs to a sensor development
controller. The sensor output is amplified, level shifted,
filtered, and converted to a single ended signal that fits within
a zero to 5 volt window. Connections are made through a
DB–9 connector, which allows this board to be plugged
directly into its controller. If physical separation is desired, a
standard 9 wire straight–through serial cable can be inserted
between the two boards. Alternately, connections for B+,
5 volts, ground, and the output signal can be made through
screw terminals at the top of the board. A socket for sensor
connections makes changing from one pressure range to
another relatively easy.
Figure 1. ASB202 — MPX2000 Series Sensor Module
REV 1
Motorola Sensor Device Data
 Motorola, Inc. 1998
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JT1
B+
JT2
2
+5
JT3
R5
300
VS1
JT4
2
R6
300
2
D1
CNTL
JT5
1
D2
+5
D3
RANGE
C2
0.01 mF
TP3
U3B
+
5
1
C1
0.33 mF
1
1N914
C4
0.1 mF
RT1
10 k
2
1
KGND
R8
1.2 k
D4
6
7
–
B+
MC33272
JT6
GND
R11
R12
2.0 k
1%
2.0 k
1%
U1
MPX2010
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3
P1
TP2
5
9
4
8
3
7
2
6
1
R9
2.0 k
1%
3
VS2
2
R7
10 k
1%
R10
499
1%
2
+
B+
U3A
+
1
–
4
GND
1
3
2
–
GND
MC33272
J1
U2A
+
5
1
6
–
MC33272
U2B
+
7 TP1
–
MC33272
R13
R1
R2
R3
R4
390
8.06 k
1%
102
1%
1.02 k
1%
80.6 k
1%
RG1
RG2
C3
OPEN
OPEN
0.001 mF
VS1
DB9
Figure 2. Schematic
Electrical Characteristics
Unless otherwise specified, the electrical characteristics in
Tables 1, 2, & 3, apply to operation at 25 degrees Celsius,
B+ = 12.0 volts, and a +5 volt input of 5.00 volts. The values
in Tables 2 and 3 are nominal values.
Table 1. Electrical Characteristics
Characteristic
Symbol
Min
Typ
Max
Units
+5
B+
4.75
11.6
5.0
12
5.25
15.8
Volts
Volts
Pressure Sensor Output Voltage
— Zero Pressure
— Full Scale (MPX2010)
— Full Scale (MPX2050–MPX2700)
VS1
—
—
—
—
—
—
1.0
3.0
4.2
—
—
—
Volt
Volts
Volts
Temp Sensor Output Voltage
VS2
—
2.5
—
Volts
Quiescent Current
ICC
—
25
—
mA
DC Supply Voltage
+5
B+
2
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Table 2. VS1 Versus Sensor Type
Full Scale
Pressure (kPa)
Sensitivity
(mV/kPa)
Zero Pressure
Offset (Volts)
Full Scale
Output Voltage
(Volts)
Full Scale
Span (Volts)
10
200
1.0
3.0
2.0
MPX2050
50
64
1.0
4.2
3.2
MPX2100*
100
32
1.0
4.2
3.2
MPX2200
200
16
1.0
4.2
3.2
Sensor
MPX2010*
*Included with ASB202 kit
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Table 3. VS2 Versus Temperature
Temperature °C
RT Ohms
VS2 Volts
Temperature °C
RT Ohms
VS2 Volts
0
32773
1.17
40
5323
3.26
5
25456
1.41
45
4365
3.48
10
19932
1.67
50
3599
3.68
15
15725
1.94
55
2983
3.85
20
12497
2.22
60
2486
4.00
25
10000
2.50
65
2082
4.14
30
8055
2.77
70
1753
4.25
35
6528
3.03
75
1482
4.35
Content
Pin by Pin Description
Board contents are described by the following parts list and
the schematic in Figure 2. A pin by pin circuit description
follows in the next section.
Inputs and outputs are grouped into two connectors. A
DB–9 connector provides a plug–in feature. If this connector
is used, no other connections are necessary. Alternately,
power, ground, and output connections can be made through
screw terminals at the top of the board. The screw terminals
and the DB–9 are wired in parallel. DB–9 connector pinouts
are shown in Figure 3.
Table 4. Parts List
Item
Quantity
Reference
Part
1
1
C1
.33 µF
2
1
C2
.01 µF
3
1
C3
.001 µF
4
1
C4
.1 µF
5
3
D1,D2,D3
LED (RED)
6
1
D4
IN914
7
1
P1
DB9
8
1
RT1
10K Thermistor
9
1
R1
8.06K 1%
10
1
R2
102 1%
11
1
R3
1.02K 1%
12
1
R4
80.6K 1%
13
2
R5,R6
300
14
1
R7
10K 1%
15
1
R8
1.2K
16
3
R9,R11,R12
2.00K 1%
17
1
R10
499 1%
18
1
R13
390
19
1
U1
MPX2010
20
2
U2,U3
MC33272
Motorola Sensor Device Data
5
9
4
8
3
7
2
6
1
+5
B+
VS2
GND
VS1
OPEN
KGND
CNTL
GND
Figure 3. DB–9 Pinout
DB–9 Connector
B+ :
Power for the sensor and op amps is supplied through pin 9
on the DB–9 connector. This voltage is labeled B+. It is
specified from 11.6 VDC min to 15.8 VDC max.
+5:
5 volt power is supplied through pin 5. It is specified from 4.75
VDC min to 5.25 VDC max.
GND:
The ground connection is on pin one. It connects the sensor’s
analog ground to the controller’s digital ground.
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KGND:
An additional ground connection, labeled KGND, is made on
pin 2. As shipped, KGND is tied to GND via jumper J1. If J1 is
opened, KGND provides a separate signal ground return that
does not carry the op amp and pressures sensor bias currents.
This feature can be helpful if a cable is used between the
sensor module and its controller.
VS2:
A temperature dependent output signal is connected to pin 4.
It is derived from a thermistor and has a nominal output
voltage of 2.5 volts at 25 degrees C. The thermistor’s output
is a function of both ambient air temperature, and temperature
rise on the board that is conducted through the leads. It will
typically read several degrees higher than the temperature of
still ambient air.
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VS1:
The pressure sensor output signal, VS1, is connected to pin 3.
It is the output of a two op amp discrete instrumentation
amplifier that has pressure sensor U1 as its input. Nominal
output voltage is 1.0 volt at zero pressure and 3.0 volts at full
scale with an MPX2010 sensor. With all of the other MPX2000
series sensors, nominal output voltage is 1.0 volts at zero
pressure and 4.2 volts at full scale.
CNTL:
A control signal is supplied on pin 6. It is normally high, and
switches low to light the RANGE light when the sensor’s full
scale pressure is exceeded. With code modifications, the
pressure at which this transition occurs can be changed, and
the signal used to control an external device.
Board Code:
A board code that lets the controller know that this is an
MPX2000 series module is supplied with a ground on pin 7
and an open on pin 8.
GND
B+
Screw Terminals
Connections for B+, +5, VS1, CNTL, KGND, & GND are
wired in parallel with the DB–9 connector. As shipped, KGND
and GND are tied together with Jumper J1.
Test Points TP1–TP3, & GND
Test points TP1, TP2, & TP3 provide access to output and
bias signals. TP1 is connected to the pressure output signal.
TP2 is connected to the thermistor output signal. The sensor’s
10 volt bias voltage, supplied from op amp U3B, appears on
TP3. A test point for ground is also provided.
Indicator Lights
B+ :
The B+ light is provided to indicate the presence of the B+
power supply.
+5:
The +5 light is provided to indicate the presence of 5 volt
power.
RANGE:
The RANGE indicator light turns on when the sensor’s full
scale pressure range is exceeded.
APPLICATION EXAMPLE
An application example shown in Figure 4 illustrates system
connections to an ASB200 sensor development controller and
a pressure source. This arrangement can be run stand alone,
or the ASB200 can be connected to an MMDS or MMEVS
system for code development. The two boards are designed
such that the DB–9 connectors plug into each other. Once they
are plugged in, it is only a matter of connecting a power supply
and a pressure source to get a system up and running. If
physical separation between the sensor location and the
controller is desired, a standard 9 wire straight–through serial
cable can be used between the two boards. Measuring
different pressure ranges is facilitated by using a socket for the
sensor that is supplied on the board.
11.6–15.8 VDC
LCD
MOTOROLA
ASB200
NC
+5
VS1 CNTL KGND GND
MOTOROLA
ASB202
PRESSURE
(TOP PORT)
OR
VACUUM
(BOTTOM PORT)
+5
VS2
VS1
KGND
GND
SENSOR DEVELOPMENT CONTROLLER
4
Figure 4. Application Example
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AN1652
across R1, generating 5V/8.06K = 620 µA of current.
Assuming that the current in R2 is equal to the current in R1,
620 µA X 102 ohms produces a 63.275 mV drop across R2,
which adds to the 5.0 volts at pin 2. The output voltage at pin
1 of U2A is, therefore, 5.063275 volts. This puts 5.063275
5.0 volts across R3, producing 63.275mV/1.02K = 62.035 µA.
The same current flowing through R4 produces a voltage drop
of (62.035 µA)X(80.6K) = 5.0 volts, which sets the output at
zero. Substituting a value for VREF other than zero into this
calculation reveals that the zero pressure output voltage
equals VREF. For this DC output voltage to be independent of
the sensor’s common mode voltage it is necessary to satisfy
the condition that R1/R2 = R4/R3.
DESIGN CONSIDERATIONS
When interfacing MPX2000 series pressure sensors to
microcomputers, the design challenge is how to take a
relatively small DC coupled differential signal and produce a
ground referenced output that is suitable for driving A/D
inputs. The circuit shown in Figure 2 provides a reference
design performing this task.
To see how this amplifier works, let’s simplify it in Figure 5,
assume that the voltage source labeled VREF is zero, and set
the differential input voltage to zero. If the common mode
voltage at sensor inputs S+ and S is 5.0 volts, then pin 2 of
U2A and pin 6 of U2B are also at 5.0 volts. This puts 5.0 volts
*
*
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S+
3
S–
CONSTRAINTS
R4/R3 = R1/R2
R4 = 10 k MIN
(R3⋅R4)/(R3+R4) = 2 k MAX
(R1⋅R2)/(R1+R2) = 2 k MAX
R3 ≅ R2 + SENSOR DELTA ZOUT
2
U2A
+
5
1
6
–
MC33272
VREF
U2B
+
7
VOUT
–
MC33272
R1
R2
R3
R4
8.06 k
1%
102
1%
1.02 k
1%
80.6 k
1%
COMMON MODE VOLTAGE = VCM
C3
0.001 mF
DIFFERENTIAL GAIN ≅ R4/R3 + 1
ZERO PRESSURE OFFSET ≅ VREF(R2⋅R4)/(R1⋅R3) – VCM((R2⋅R4)/(R1⋅R3) – 1)
Figure 5. Amplifier — Simplified Schematic
Signal gain can be determined by assuming a differential
output at the sensor and going through the same calculation.
To do this let’s assume 100 mV of differential output and
VREF = 0. These values put pin 3 of U2A at 4.95 volts, and pin
5 of U2B at 5.05 volts. Therefore, 4.95 volts is applied to R1,
generating 614 µA. This current flowing through R2 produces
62.643 mV, placing pin 1 of U2A at 4950 mV + 62.6 mV =
5012.6 mV. The voltage across R3 is then 5050mV 5012.6
mV = 37.4 mV, which produces a current of 37.4 mV/1.02K =
36.6 µA that flows into R4. The output voltage is then 5.05V
+ (36.6 µA 80.6K) = 8.0 volts. Dividing 8.0 volts by the 100 mV
input yields a gain of 80, which provides a 3.2 volt span for
40 mV of full scale sensor output.
The foregoing nodal analysis can be summarized by the
following two equations, which are first order approximations.
Equation (1) assumes that the differential input between S+
and S in Figure 5 is zero, and that VCM is the common mode
voltage at S+ and S .
*
@
*
*
Motorola Sensor Device Data
(1) ZERO PRESSURE OFFSET
@
@
*
@
@ *1)
1.0([email protected])/([email protected])
*5.0(([email protected])/([email protected]) *1)
1.0(8220K/8220K) *5.0((8220K/8220K)*1)
= 1.0 * 0 = 1 Volt
= VREF (R2 R4)/(R1 R3)
VCM((R2 R4)/(R1 R3)
=
=
(2) DIFFERENTIAL GAIN = R4/R3 +1
= (80.6K/1.02K +1) = 79 + 1 = 80
These equations are based upon the same assumptions as
the nodal analysis, namely high open loop gain, zero input
offset voltage, zero input bias current, and that the resistor
values are actual values as opposed to specified values. As
is typical in discrete instrumentation amplifiers, the most
troublesome assumption is the resistor values. A 1% variation
in the ratio (R4 R2)/(R1 R3) causes an error that is 1% of the
common mode voltage at the amplifier’s input.
@
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sensor and interface is +/– 5%, provided that a provision for
zero pressure offset calibration is made.
CONCLUSION
The ASB202 plug–in module is part of a systems
development tool set for pressure sensors. It provides
pressure and temperature input signals to a Motorola Sensor
Development Controller, or can be used stand alone to
provide a signal conditioned output from MPX2000 series
sensors.
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Returning to Figure 2, a 1.0 volt VREF is generated by the
divider consisting of R9 and R10. This divider is sourced from
the same 5 volts as the controller’s A/D converter reference,
thereby minimizing power supply tolerance as a source of
error. This divider is buffered by U3A in order to preserve the
ratio R4/R3 = R1/R2. The power supply to the sensor is
nominally 10.0 volts, and is also generated from the 5 volt
reference to minimize power supply errors.
The resulting 1.0 V to 4.2 V output from pin 7 of U2B is
compatible with microprocessor A/D inputs. Over a zero to 75
degree C temperature range combined accuracy for the
6
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NOTES
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