Sensor-Emulator-EVM
System Reference Guide
by Art Kay
High-Precision Linear Products
SBOA102A
Sensor-Emulator-EVM
• Simplifies Development of Voltage
Excited Bridge Sensor Signal
Conditioning Systems
• Provides Eleven Different Emulated
Sensor Output Conditions
• Provides Three Different Emulated
Temperature Signals for Diode or
Series Resistor Methods of Bridge
Sensor Temperature Monitoring
• Emulates Bridge Outputs
- Cold Temp: 0%, 50%, 100%
- Room Temp: 0%, 25%, 50%,
75%, 100%
- Hot Temp: 0%, 50%, 100%
• LED Indicators for Emulation at a
Glance
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System Reference Guide
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Sensor Emulator EVM
Table of Contents
Pages
1.0 Introduction to the Sensor-Emulator-EVM………………………….………. 4-5
2.0 Overview Hardware Emulation of a Real World Sensor……………….……6-15
3.0 Changing the Programmable Range of the Emulator…………………….…17
4.0 Required Electrical connections……………………………………………….18-25
5.0 Configuring the Sensor-Emulator-EVM to Emulate a Real World Sensor…26-43
6.0 Schematic of Sensor-Emulator-EVM……………………..……………………44
7.0 Parts List for Sensor-Emulator-EVM………………………………………...…45-46
Note: Some sections of this user’s guide reference use of the PGA309EVM. This
is done for ease of documenting features available on the Sensor-Emulator-EVM
which will work with any bridge sensor signal conditioning chip which uses voltage
bridge excitation.
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1.0 Introduction to
The Sensor Emulator
Q:
A:
Q:
A:
What is the sensor emulator?
The sensor emulator is a design that uses rotary switches and
potentiometers to emulate the operation of a resistive bridge sensor at
discrete operating points, for voltage excitation applications.
Why use the sensor emulator?
Once the sensor emulator has been programmed, it allows the user to
cycle through a set of sensor output conditions very quickly. Doing this
with a real sensor can be extremely time consuming because it can take
several hours to cycle through various temperatures. Also, some sensors
have non-repeatability issues. For example, pressure sensors can have
pressure hysteresis and temperature hysteresis. The emulator does not
have non-repeatability issues (repeatability errors are typically less then
0.03%). This approach allows the user to program the sensor signal
conditioning chip many different ways to quickly and easily assess the
optimal calibration settings for a given application.
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1.0 Introduction to
The Sensor Emulator
Q:
A:
Why not just use a precision voltage source to emulate a
sensor?
A precision voltage source is not affected by changing sensor
excitation. Many sensor signal conditioning chips modulate the
sensor excitation to compensate for sensor nonlinearity. In this
case (and in the case of a ratiometric system), a precision voltage
source would not work. Also, it is much more convenient to have
all the different sensor conditions pre-programmed so that you
can quickly transition from one condition to another without having
to reprogram the source at each different condition.
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2.0 Overview of Hardware Emulation
of a Real World Sensor
• Description of real world bridge sensors
– Temperature Drift and Nonlinearity versus applied stimulus
– Measurement of the sensor temperature
• Description of how Sensor-Emulator-EVM circuitry
produces signals equivalent to real world sensors
– Emulates four different real world configurations
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Bridge Sensor Output
Figure 2.1
Figure 2.2
Figure 2.1 is an example of a typical resistive bridge sensor with no applied stimulus. With no stimulus applied, the
resistors ideally would be perfectly matched and the sensors’ output (VIN_DIF ) would be zero. Most practical sensors,
however, will have some output resulting from resistor mismatch. The output signal with no applied stimulus is called
offset. Figure 2.2 is an example of a typical resistive bridge sensor with full scale stimulus applied. For the example sensor,
the offset is 5mV (Figure 2.1) and the full scale output is 32mV (Figure 2.2). Span is defined as the difference between the
full scale stimulus and the offset (Span = Full Scale Output – Offset).
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Drift and Nonlinearity
with a Bridge Sensor
Kbridge vs. pressure @Troom
Bridge sensitivity vs. temp
4.0E-01
Kbridge, V/V or Vbridge@Vexc=1V
3.0E-01
2.5E-01
2.0E-01
Offset
Span
1.5E-01
1.0E-01
Kbridge, V/V, or Vbridge@Vexc=1V
3.5E-01
3.5E-01
3.0E-01
2.5E-01
2.0E-01
1.5E-01
1.0E-01
5.0E-02
0.0E+00
5.0E-02
0
10
20
30
40
50
60
70
80
90
Pressure
0.0E+00
-50
0
50
100
150
Temp, degC
Figure 2.3
Figure 2.4
An important aspect of pressure sensors is how they drift with temperature. Figure 2.3 is an example of span and
offset drift with temperature for a typical resistive bridge sensor. Note that the drift is fairly large and nonlinear.
Figure 2.4 is an example of how a bridge sensor can be nonlinear with applied stimulus (in this example the
stimulus is pressure). The sensor emulator can be configured to reproduce these characteristics for most sensors.
Note that the graphs are shown in a normalized format. The normalized format allows the graph to be easily
scaled by multiplying by the bridge excitation voltage.
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100
5.0V
Emulating
Bridge Sensor Outputs
R8
R101
10k
10k
2.526V
R102
200 ohm
This is a simplified diagram of how the sensor emulator
generates a bridge output voltage. The potentiometer
R102 is adjusted to set the output voltage of the
emulated bridge (Vin+ with respect to Vin-). The
potentiometer R103 is used to make fine adjustments in
the output value. R8 and R9 set a common mode
voltage for the other leg of the bridge. R101 and R104
set the adjustable output range of the emulated bridge.
For the configuration shown, the output range is -25mV
to +26mV (Vdif = Vin- - Vin+ ). Selecting a different value
for R101 and R104 can expand this range. On the
Sensor-Emulator-EVM, 11 channels like this one are
selectable using a rotary switch.
3
R104 := 10⋅ 10
R102 := 200
3
R101 := 10⋅ 10
Vexc := 5
R9
10k
3
R104 := 10⋅ 10
R102 := 200
R104
10k
R103 := 10
R103 := 0
Pot set to Minimum Resistance
(
Vexc
Vexc⋅ R104 + R103 + R102a
Vdif :=
−
R101 + R104 + R103 + R102
2
Vdif = 0.026
Vdif = −0.025
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26mV
to
-25mV
Output of Sensor
Vin- to Signal
Conditioning Chip
Figure 2.5
3
)
Vexc
Vexc⋅ R104 + R103 + R102a
−
Vdif :=
R101 + R104 + R103 + R102
2
Positive full scale output of emulator
Vdif =
R101 := 10⋅ 10
Wiper position at bottom of POT
)
Wiper at bottom
Output of Sensor
Vin+ to Signal
Conditioning Chip
2.5V
R102a := 0
(
2.475V
to
R103
10 ohm
R102a := 200 Wiper position at top of POT
Pot set to maximum Resistance
Wiper at top
2.5V
Positive full scale of the emulator Negative full scale of the emulator
Vexc := 5
Input To Bridge
Vexc
(Excitation voltage)
Negative full scale output of emulator
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Emulating a Temperature Sensor
with a Series Bridge Resistor
(Rt- connected to
the bottom of
the bridge)
VT
Figure 2.6
V exc⋅ R T
R T + R Bridge
Figure 2.7
Figure 2.8
Figure 2.6 illustrates one method for monitoring the temperature of a real world bridge sensor. In this type of circuit the resistance of the bridge
has a strong temperature coefficient. The bridge resistance generates a bridge current with a strong temperature coefficient, which generates a
voltage across the temperature sensing resistor (Rt). Rt is typically located remotely from the bridge and should not have a strong temperature
coefficient. Note that Rt can be connected to the top or bottom of the bridge. This diagram illustrates the case where it is connected to the bottom
of the bridge. The sensor emulator circuit has three channels to emulate the Rt temperature signal that are selectable through a rotary switch.
An important aspect of the Rt method of temperature sensing is the reduction of the excitation voltage across the bridge by the series Rt
resistance. For example, if Vt = 1V and Vexc = 5V, then only 4V remains for the bridge excitation. This phenomena is modeled by the sensor
emulator and the detail of how this works are described in Figure 2.15.
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Emulating a Temperature Sensor
with a Series Bridge Resistor
(Rt+ connected to
the top of
the bridge)
VT
Figure 2.9
V exc⋅ R T
R T + R Bridge
Figure 2.10
Figure 2.11
The sensor emulator circuit can also emulate the case where the temperature sense resistor is connected to the top of the bridge. This is done
using an instrumentation amplifier to translate the voltage signal to be referenced to Vexc rather then ground. This mode of operation is
selected by a jumper (JUMP1). This circuit also feeds the temperature signal back to the bridge emulator to adjust the excitation across the
bridge, as in the real world case. The details of how this feedback works are described in Figure 2.15.
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Diode Temperature
Sensor Emulator
Figure 2.12
Figure 2.13
Figure 2.14
Figure 2.12 illustrates another method for monitoring the temperature of a real world sensor. In this type of circuit a diode is placed in close thermal
contact with the bridge and a constant current is driven through the diode. The diode voltage is a reasonably linear function with temperature (the
slope is approximately -2mV / °C). The emulator circuit shown in Figure 2.13 uses resistors to develop a voltage equivalent to the diode voltage.
Figure 2.14 shows how the emulator can be used to develop an equivalent diode drop if an external current source is not available.
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Emulation Case 1: Rt(Resistance in the Bottom of The Bridge)
Rt- Temperature
Signal Emulation
Bridge Voltage Emulation
Note that the output signals of the
real world sensor and the emulator
are the same.
1.101k
VINP
1.1023k
R2401
5.0V
U1A
+
Adjusted
to 1k
R2402
R101
10k
50
2k
R102
200 ohm
U1b
+
12.48mV
VDIF = 12.4mV
Adjusted
to 50
R2403
Temperature
Signal VT = 0.415V
R103
Adjusted
to 5
U1d
+
10 ohm
Output of Sensor
Vin- to Signal
Conditioning Chip
2.292V
100ohm
R9
0.415V
Temperature
Signal VT-
2.305V
Output of Sensor
Vin+ to Signal
Conditioning Chip
2.305V
150
2.292V
Input To Bridge
Vexc
(Excitation voltage)
10k
10k
1.1075k
VINN
Rt100
X1
U4
R8
Vexc = 5V
1.1078k
-
5.0V
Real World Sensor
10k
R104
R2404
1k
10k
0.415V
0.415V
Rt
U1c
+
X1
JUMP5
U5
GND
RtDiode
JUMP1
U2
+
Temperature Signal
Output referenced
0.415V
Rt+
INA
Figure 2.15
Rt+ Temperature
Signal Emulation
(Not used in this mode)
2
This diagram illustrates how the emulator generates the bridge output and temperature signals for the circuit where the sense resistor is connected to the bottom
of the bridge. Note how the output voltage of the temperature emulator (0.415V) is fed back to the bottom of the bridge emulator via the buffer circuit (U5 and
U1c), to emulate the bridge excitation change due to Rt in series with the bridge in the real world.
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Emulation Case 2: Rt+ (Resistance in the Top of The Bridge)
Figure 2.16
This diagram illustrates how the emulator generates the bridge output and temperature signal for the circuit where the sense resistor is connected to
the top of the bridge. Note how the output voltage of the temperature emulator (0.415V) is fed back to the bottom of the bridge emulator via the
buffer circuit (U5 and U1c) to emulate the bridge excitation change due to Rt in series with the bridge in the real world. Also, note how the
instrumentation amplifier (U2) is used to translate the temperature signal voltage so that it is referenced to the excitation voltage (Vexc).
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Emulation Case 3: Diode Temperature Sensor with External Current Source
Figure 2.17
This diagram illustrates how the emulator generates the bridge output and temperature signal for the diode temperature
measurement. Note that in this case JUMP1 is set so that the bottom of the bridge emulator is at ground potential.
Also note that this configuration requires an external current source to operate (in this example, 7µA).
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Emulation Case 4: Diode Temperature Sensor with
On-Board Voltage Reference
Figure 2.17
This diagram illustrates how the emulator generates the bridge output and temperature signal for the diode temperature
measurement. Note that in this case JUMP1 is set so that the bottom of the bridge emulator is at ground potential.
Also note that this uses an on-board voltage reference (REF102) to set the diode voltage (JUMP2 selects this option).
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3.0 Changing the Programmable Range of the Emulator
Figure 3.1 illustrates how the range of the bridge emulator can be adjusted by putting a resistor in parallel with R101 and R104. This needs to be
done for each channel of the emulator (11 channels x 2 resistors = 22 resistors total). The examples illustrates how the range is increased using a
1kΩ parallel resistor. In general, it is best to select a parallel resistance value that scales your range so that the full scale output of the emulator is
slightly larger than what is required for your emulator. Scaling the emulator in this manner will provide the optimal resolution and lowest noise. Note
that holes are provided for a parallel through-hole resistor to simplify the process of adjusting the emulator scale.
5.0V
R8
Input To Bridge
Vexc
(Excitation voltage)
R101
10k
10k
2.526V
R102
200 ohm
Wiper at top
2.475V
to
Wiper at bottom
Output of Sensor
Vin+ to Signal
Conditioning Chip
2.5V
R103
10 ohm
R9
R104
10k
10k
VDIF = 26mV
to
-25mV
Output of Sensor
Vin- to Signal
Conditioning Chip
2.5V
Figure 3.0: Default Range
Figure 3.1: Adjusted Range
Positive full scale of the emulator Negative full scale of the emulator
Vexc := 5
3
R104 := 10⋅ 10
R102 := 200
R102a := 200 Wiper position at top of POT
R103 := 10
Pot set to maximum Resistance
(
3
R101 := 10⋅ 10
)
Vexc := 5
3
R104 := 10⋅ 10
R102 := 200
3
R101 := 10⋅ 10
Positive full scale of the emulator
Negative full scale of the emulator
Vexc := 5
Vexc := 5
R104 := 909.1
R102 := 200
R101 := 909.1
R104 := 909.1
R102 := 200
R102a := 0
R102a := 200 W iper position at to p of POT
R102a := 0
W iper positio n at bottom of POT
R103 := 0
Pot set to Minimum Resistance
R103 := 10
R103 := 0
Pot set to Minimum Resistance
(
)
Po t set to maximum Resistance
(
)
(
)
Vexc⋅ R104 + R103 + R102a
Vexc
Vdif :=
−
R101 + R104 + R103 + R102
2
Vexc⋅ R104 + R103 + R102a
Vexc
Vdif :=
−
R101 + R104 + R103 + R102
2
Vexc⋅ R104 + R103 + R102a
Vexc
Vdif :=
−
R101 + R104 + R103 + R102
2
Vexc⋅ R104 + R103 + R102a
Vexc
Vdif :=
−
R101 + R104 + R103 + R102
2
Vdif = 0.026
Vdif = −0.025
Vdif = 0.259
Vdif = −0.248
Positive full scale output of emulator
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R101 := 909.1
Wiper position at bottom of POT
Positive full scale output of emulator
Positive full scale output of emulator
Sensor-Emulator-EVM
System Reference Guide
N egative full scale output of emulator
17
4.0 Required Electrical Connections
to Sensor-Emulator-EVM
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Example of a Typical Engineering Bench Setup
Using the Sensor Emulator
This diagram illustrates an example of how the sensor emulator would be used in an engineering bench setup.
The PGA309 is a programmable sensor signal conditioning chip. The Sensor-Emulator-EVM can be used in
conjunction with the PGA309EVM to facilitate the development of a PGA309 application.
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Note on the Buf_Temp
Banana Jacks
The Buf_Temp banana jacks are used to monitor the temperature signal
with a DVM. It is important to monitor temperature at this point because
the non-buffered temperature signal is a high impedance output, and the
DVM can load this output.
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Note on the Vin- and Vin+
Banana Jacks
The Vin- and Vin+ banana jacks are used to monitor the sensor output signal with a DVM.
The Vin banana jacks are connected to the Vin signal through a standard RC filter. This filter
helps to reduce the coupling of noise (from ground loops) into the sensor output circuit, and
into high gain sensor signal conditioner inputs.
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These jumpers are
not used in this
mode. Any
position is ok.
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These three
channels are used
to set the
temperature output
signal in this mode.
The diode
channels are not
used in this mode.
Set the jumper
JUMP1 to the
position shown
to connect the
Rttemperature
emulation.
Sensor-Emulator-EVM
System Reference Guide
Set the jumper
JUMP5 to the
position shown
to connect the
Rt emulation.
22
These jumpers are
not used in this
mode. Any
position is ok.
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These three
channels are used
to set the
temperature output
signal in this mode.
The diode
channels are not
used in this mode.
Set the jumper
JUMP1 to the
position shown
to connect the
Rt+ temperature
emulation.
Sensor-Emulator-EVM
System Reference Guide
Set the jumper
JUMP5 to the
position shown
to connect the
Rt emulation.
23
These jumpers
must be set to the
position shown to
allow for external
current source
connection.
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These three
channels are used
to set the
temperature output
signal in this mode.
The Rt channels
are not used in this
mode.
Set the jumper
JUMP1 to the
position shown to
connect the Diode
temperature
emulation.
Sensor-Emulator-EVM
System Reference Guide
Set the jumper
JUMP5 to the
position shown
to connect GND
to the bottom of
the bridge
emulator.
24
These jumpers
must be set to the
position shown to
allow the on board
voltage reference
to generate the
emulated diode
voltages.
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These three
channels are used
to set the
temperature output
signal in this mode.
The Rt channels
are not used in this
mode.
Set the jumper
JUMP1 to the
position shown to
connect the Diode
temperature
emulation.
Sensor-Emulator-EVM
System Reference Guide
Set the jumper
JUMP5 to the
position shown
to connect GND
to the bottom of
the bridge
emulator.
25
5.0 Configuring the SensorEmulator-EVM to Emulate a
Real World Sensor
If the raw output of the sensor is not known, the generate_emu_values.xls
spreadsheet (SBOC065, available for download at www.ti.com) can be used to
translate the specifications of your bridge sensor and temperature sensor to system
voltage levels. The spreadsheet contains five sections:
1.
Offset and Span: Generates the bridge output voltages.
2.
Diode Vo: Generates the temperature sensor output voltages for the diode
method.
3.
Rt-: Generates the temperature sensor voltages for the Rt- method.
4.
Rt+: Generates the temperature sensor voltages for the Rt- method.
5.
PGA309_Error: Allows you to read the PGA309 via the ADS1100.
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Offset and Span: Generates the bridge
output voltages from sensor specifications
All the areas shown in light blue are either sensor specifications or system requirements. Enter these values and the
spreadsheet will generate output voltage settings for each channel on the sensor emulator. The next several pages will
show how the voltages listed in the spreadsheet are used to program the sensor emulator.
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The sensor output at cold temperature and 0%
of applied stimulus is emulated by this channel.
The rotary switch S1 is used to select this
channel. When the channel is selected, LED
D101 will light to indicate that the correct
channel is selected.
Bridge Sensitivity v s Temp
4.5E-03
4.0E-03
3.5E-03
Kbridge, V/V
3.0E-03
2.5E-03
2.0E-03
offset
1.5E-03
span
1.0E-03
5.0E-04
0.0E+00
-5.0E-04
-1.0E-03
-50
0
50
100
150
Tem p, degC
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The sensor output at cold temperature and
100% of applied stimulus is emulated by this
channel. The rotary switch S1 is used to
select this channel. When the channel is
selected, LED D103 will light to indicate that
the correct channel is selected.
Bridge Sensitivity v s Temp
4.5E-03
4.0E-03
3.5E-03
Kbridge, V/V
3.0E-03
2.5E-03
2.0E-03
offset
1.5E-03
span
1.0E-03
5.0E-04
0.0E+00
-5.0E-04
-1.0E-03
-50
0
50
100
150
Tem p, degC
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The sensor output at room temperature and
0% of applied stimulus is emulated by this
channel. The rotary switch S1 is used to
select this channel. When the channel is
selected, LED D104 will light to indicate that
the correct channel is selected.
Kbridge vs. pressure @Troom
3.5E-03
Kbridge, V/V, or Vbridge@Vexc=1V
3.0E-03
2.5E-03
2.0E-03
1.5E-03
1.0E-03
5.0E-04
0.0E+00
0
10
20
30
40
50
60
70
80
90
100
-5.0E-04
Pressure
Emulate the nonlinearity of the curve at
room temperature for four points.
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The sensor output at room temperature and
25% of applied stimulus is emulated by this
channel. The rotary switch S1 is used to select
this channel. When the channel is selected,
LED D105 will light to indicate that the correct
channel is selected.
Kbridge vs. pressure @Troom
3.5E-03
Kbridge, V/V, or Vbridge@Vexc=1V
3.0E-03
2.5E-03
2.0E-03
1.5E-03
1.0E-03
5.0E-04
0.0E+00
0
10
20
30
40
50
60
70
80
90
100
-5.0E-04
Pressure
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The sensor output at room temperature and
50% of applied stimulus is emulated by this
channel. The rotary switch S1 is used to select
this channel. When the channel is selected,
LED D106 will light to indicate that the correct
channel is selected.
Kbridge vs. pressure @Troom
3.5E-03
Kbridge, V/V, or Vbridge@Vexc=1V
3.0E-03
2.5E-03
2.0E-03
1.5E-03
1.0E-03
5.0E-04
0.0E+00
0
10
20
30
40
50
60
70
80
90
100
-5.0E-04
Pressure
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The sensor output at room temperature and
75% of applied stimulus is emulated by this
channel. The rotary switch S1 is used to select
this channel. When the channel is selected,
LED D107 will light to indicate that the correct
channel is selected.
Kbridge vs. pressure @Troom
3.5E-03
Kbridge, V/V, or Vbridge@Vexc=1V
3.0E-03
2.5E-03
2.0E-03
1.5E-03
1.0E-03
5.0E-04
0.0E+00
0
10
20
30
40
50
60
70
80
90
100
-5.0E-04
Pressure
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The sensor output at room temperature and
100% of applied stimulus is emulated by this
channel. The rotary switch S1 is used to select
this channel. When the channel is selected,
LED D108 will light to indicate that the correct
channel is selected.
Kbridge vs. pressure @Troom
3.5E-03
Kbridge, V/V, or Vbridge@Vexc=1V
3.0E-03
2.5E-03
2.0E-03
1.5E-03
1.0E-03
5.0E-04
0.0E+00
0
10
20
30
40
50
60
70
80
90
100
-5.0E-04
Pressure
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The other channels are set in a similar
manner and selected using S1.
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Diode Vo: Generate Diode Voltages
based on Operating Temperature Range
The second tab in the spreadsheet allows the user to enter the temperature range and room temperature
diode voltage (light blue areas). The spreadsheet calculates the diode voltages and displays the results in
the yellow areas. Note that the Temp ADC area is specific to the PGA309 sensor signal conditioning chip.
The Temp ADC values will be used in the computation of the Counts for the temp ADC. The next several
pages will show how the diode voltages are used to program the sensor emulator.
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The temperature output signal at cold
temperature (-45°C) is emulated by this
channel. The rotary switch S2 is used to
select this channel. When the channel is
selected, LED D201 will light to indicate that
the correct channel is selected.
NOTE: When emulating Diode
temperature control, the Rt
temperature section is not used.
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The temperature output signal at room
temperature (25°C) is emulated by this
channel. The rotary switch S2 is used to
select this channel. When the channel is
selected, LED D202 will light to indicate that
the correct channel is selected.
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The temperature output signal at hot
temperature (85°C) is emulated by this
channel. The rotary switch S2 is used to
select this channel. When the channel is
selected, LED D203 will light to indicate
that the correct channel is selected.
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Generate Rt Voltages based on Operating
Temperature Range and System Parameters
The third tab in the spreadsheet allows the user to enter the temperature range and other system parameters in the light blue areas. The
spreadsheet calculates the voltage level of the temperature signal and displays this in the yellow areas. Note that the Temp ADC area is specific to
the PGA309 sensor signal conditioning chip. The Temp ADC values will be used in the computation of the Counts for the Temp ADC. The next
several pages will show how the Rt voltages are used to program the sensor emulator.
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The temperature output signal at cold
temperature (-45°C) is emulated by this
channel. The rotary switch S2 is used to
select this channel. When the channel is
selected, LED D204 will light to indicate
that the correct channel is selected.
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The temperature output signal at room
temperature (20°C) is emulated by this
channel. The rotary switch S2 is used to
select this channel. When the channel
is selected, LED D204 will light to
indicate that the correct channel is
selected.
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The temperature output signal at hot
temperature (90°C) is emulated by this
channel. The rotary switch S2 is used to
select this channel. When the channel is
selected, LED D204 will light to indicate
that the correct channel is selected.
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6.0 Schematic
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7.0 Parts List
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Parts List, cont’d
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