CN-0289: Flexible, 4 mA-to-20 mA, Loop-Powered Pressure Sensor Transmitter with

Circuit Note
CN-0289
Devices Connected/Referenced
Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0289.
AD8226
Wide Supply Range, Rail-to-Rail
Output Instrumentation Amplifier
ADR02
Ultracompact, Precision 5.0 V Voltage
Reference
ADA4091-2
Precision Micropower, OVP, RRIO
Dual Op Amp
Flexible, 4 mA-to-20 mA, Loop-Powered Pressure Sensor
Transmitter with Voltage or Current Drive
The design is optimized for a wide variety of bridge based voltage
or current driven pressure sensors, utilizes only four active
devices, and has a total unadjusted error of less than 1%. The
loop supply voltage can range from 12 V to 36 V.
EVALUATION AND DESIGN SUPPORT
Circuit Evaluation Boards
CN0289 Evaluation Board (EVAL-CN0289-EB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
The input of the circuit is protected for ESD and voltages
beyond the supply rail, making it ideal for industrial
applications.
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 is a robust and flexible looppowered current transmitter that converts the differential
voltage output from a pressure sensor to a 4 mA-to-20 mA
current output.
+5V
+VLOOP
+5V
VOUT
U2A
1/2
ICIRCUIT
VIN
J2-1
ILOOP
ADR02
ADA4091-2
39.60µA
I12
G = 50.00
+VLOOP
VDRIVE
J1-3
RBRIDGE
R5
10kΩ
U3
RBRIDGE
R6
10kΩ
1nF
RBRIDGE
R1
J1-2 4.02kΩ
10nF
R2
J1-4 4.02kΩ
RBRIDGE
R3
1.008kΩ
U1
AD8226
U2B
1/2
ADA4091-2
R9
31.56kΩ
I9
0µA,
158.42µA
I10
VIN: 0mV, 100mV
39.60µA,
198.02µA
VLOOP_SUPPLY
ILOOP – ICIRCUIT
12V
TO
36V
Q1
ZXT13N50DE6TA
30kΩ
ICIRCUIT
REF
0V
+VLOOP
+VLOOP
VOUT
0V, 5.00V
1nF
J1-1
R12
126.25kΩ
R10
1kΩ
J2-2
3.960mA,
19.802mA
PCB GROUND
–39.60mV, –198.02mV
R8
10Ω
I10
I8
LOOP
LOAD
–
R7
250Ω
+
ILOOP
ILOOP: 4.000mA, 20.000mA
LOOP GROUND
10947-001
NOTES
1. R8, R10 ARE STANDARD 0.1% VALUES. R5, R6 ARE STANDARD 1% VALUES.
R3, R9, R12 ARE CALCULATED VALUES (SEE TEXT).
2. VOLTAGES MEASURED WITH RESPECT TO PCB GROUND.
Figure 1. Robust Loop Powered Pressure Sensor Signal Conditioning Circuit with 4 mA-to-20 mA Output (Shown in Sensor Voltage Drive Mode)
Simplified Schematic: All Connections and Decoupling Not Shown
Rev. 0
Circuits from the Lab™ circuits from Analog Devices have been designed and built by Analog Devices
engineers. Standard engineering practices have been employed in the design and construction of
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room temperature. However, you are solely responsible for testing the circuit and determining its
suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices
be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause
whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page)
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©2013 Analog Devices, Inc. All rights reserved.
CN-0289
Circuit Note
CIRCUIT DESCRIPTION
Sensor Excitation Drive
The design provides a complete solution for the 4 mA-to-20 mA
transmitter for pressure sensor measurements where the supply
for the entire circuit is provided by the loop. The circuit has
three critical stages: the sensor excitation drive, sensor output
amplifier, and the voltage to current converter.
Depending on the pressure sensor selected, either a voltage or
current drive is required. The circuit uses one half of the
ADA4091-2 (U2A) with different configurations chosen by
switching S1 to support either option. The switch S1 provides
the selection for either drive.
The total current required by the circuit is 1.82 mA (maximum)
as shown in Table 1. Pressure sensors requiring bridge drive
currents of up to 2 mA can therefore be used without exceeding
the maximum available loop current of 4 mA.
Excitation: Voltage Drive Configuration
Figure 2 shows the configuration for the voltage drive with S1 in
the position labeled VOLTAGE DRIVE on the PCB (see
complete circuit layout and schematics in the CN0289 design
support package: http://www.analog.com/CN0289DesignSupport).
Table 1. Maximum Circuit Currents @ 25°C
Current (mA)
0.80
0.50
0.43
0.05
0.04
1.82
+VLOOP
5V
IDRIVE = 2 mA FOR
VDRIVE = 10V, RBRIDGE = 5kΩ
1/2
ADA4091-2
VDRIVE
IDRIVE
U2A
J1-3
PCB
GROUND
R5
10kΩ
R6
10kΩ
PCB GROUND
RBRIDGE
RBRIDGE
RBRIDGE
RBRIDGE
R1
J1-2 4.02kΩ
+V
AD8226
R2
J1-4 4.02kΩ
INPUT
VCM = 5V
–V
J1-1
PCB GROUND
Figure 2. Sensor Voltage Drive Configuration for RBRIDGE = 5 Ω, VDRIVE = 10 V
Rev. 0 | Page 2 of 7
10947-002
Component
ADR02
ADA4091-2
AD8226
R5, R6 @ 10 V
R12 @ 5 V
TOTAL
Circuit Note
CN-0289
The voltage drive circuit is normally configured for a bridge
drive voltage of 10 V. In this mode, the minimum allowable
bridge resistance is:
R BRIDGE ≥
2 V REF
=
2 mA
10 V
where
V DRIVE
= 5 kΩ
Note that the loop voltage, VLOOP, should be at least 0.2 V
greater than the bridge drive voltage to allow sufficient
headroom for U2A.
2 mA
For lower than 5 kΩ bridge resistance, the drive voltage can be
decreased down to 5 V using a buffer configuration by
removing R6.
V LOOP ≥ V DRIVE + 0.2 V
Excitation: Current Drive Configuration
Other values of drive voltage can be obtained by properly
selecting R6 using the equation:
The circuit can be switched to the current drive configuration
shown in Figure 3 by moving S1 to the position labeled
CURRENT DRIVE on the PCB.

R5 

V DRIVE = 5 V  1 +

R6 

5 V × R5
V DRIVE – 5 V
+VLOOP
5V
1/2
ADA4091-2
U2A
VDRIVE = 5V + IDRIVE × RBRIDGE
= 11V, FOR RBRIDGE = 3kΩ
VDRIVE
J1-3
PCB
GROUND
RBRIDGE
RBRIDGE
RBRIDGE
RBRIDGE
R1
J1-2 4.02kΩ
+V
AD8226
R2
J1-4 4.02kΩ
INPUT
VCM = 8V
–V
J1-1
IDRIVE × R4 = 5V
IDRIVE = 2mA
R4
2.49kΩ
PCB
GROUND
Figure 3. Sensor Current Drive Configuration for RBRIDGE = 3 kΩ
Rev. 0 | Page 3 of 7
10947-003
R6 =
≥ 2 mA
R BRIDGE
CN-0289
Circuit Note
In the current drive mode, the 2 mA maximum allowable bridge
drive current must be observed. The circuit is configured for R4
= 2.49 kΩ, and IDRIVE = 2 mA. Lower values of IDRIVE can be obtained
by using the following equation to select the value of R4:
5V
R4 =
I DRIVE
The current through R12 is given by:
The resulting drive voltage VDRIVE is calculated from:
The current through R9 is given by:
I 10 =
I LH
=
101
I 12 =
V REF
20 mA
= 198.02 μA
101
=
5V
= 39.60 μA
126.25 kΩ
R12
I9 = I10 − I12 = 198.02 μA − 39.60 μA = 158.42 μA
V DRIVE = 5 V + I DRIVE × R BRIDGE
A headroom of 0.2 V is required for the U2A supply, therefore:
The value of R9 can now be calculated from:
V LOOP ≥ V DRIVE + 0.2 V
R9 =
For the values shown in Figure 3, RBRIDGE = 3 kΩ, IDRIVE = 2 mA,
VDRIVE = 11 V, and VLOOP ≥ 11.2 V.
V OUT
I9
=
5V
158.42 μA
= 31.56 kΩ
The ADA4091-2 op amp is chosen for the circuit because of its
low current consumption (250 µA/amplifier), low offset voltage
(250 µV), and rail-to-rail inputs and outputs.
In practice, the calculated values for R3, R12, and R9 will not be
available as standard values, so there will be fixed errors
introduced by the actual values used in the circuit. These errors
can be calculated as follows.
Bridge Output Instrumentation Amplifier and Selection
of Gain and Offset Resistors
Gain, offset, and total error, measured as a %FSR (where FSR =
16 mA) due to resistors R3, R9, and R12:
Gain Error (%FSR) =
The output of the bridge is filtered by a common-mode filter
(4.02 kΩ, 1 nF) with a bandwidth of 39.6 kHz and a differentialmode filter (8.04 kΩ, 10 nF) with a bandwidth of 2 kHz.
  1008 Ω – R3   31.56 kΩ – R9
+

  1008 Ω   31.56 kΩ

The AD8226 is an ideal choice for the in-amp because of its low
gain error (0.1%, B-grade), low offset (58 µV @ G = 50, B-grade;
112 µV @ G = 50, A-grade), excellent gain nonlinearity (75 ppm
= 0.0075%), and rail-to-rail output.
The AD8226 instrumentation amplifier amplifies the 100 mV
FS signal by a factor of 50 V to 5 V using a gain setting resistor
R3 = 1.008 kΩ. The relationship between the gain, G, and R3 is
given by
R3 =
49.4 kΩ
For the output zero-value loop current, ILO = 4 mA:
I LO = I 8 + I 10
Because of the 100:1 ratio of R10 to R8:
I 8 = 100 × I 10
However, the total error at full-scale output (20 mA) is equal to:
• R9 = 30.9 kΩ + 655 Ω = 31.555k Ω (calculated = 31.56 kΩ)
ILO = 101 × I10
• R12 = 124 kΩ + 2.26 kΩ = 126.26 Ω (calculated = 126.25 Ω).
The AD8226 output is 0 V for ILO = 4 mA, and the offset resistor
R12 can now be calculated from:
=
The total error at zero output (4 mA) is not affected by the gain
error.
• R3 = 1 kΩ + 8.06 Ω = 1008.06 Ω (calculated = 1008 Ω)
Combining the last two equations:
I 10
 126.25 kΩ – R12 
 × 100%
0.25 


126.25 kΩ


In the practical circuit, the nearest EIA standard 0.1% resistor
values must be chosen, thereby resulting in fixed gain and offset
errors as described in the previous equations. It is possible to
use combinations of two 0.1% values to come closer to the
calculated values. For instance, the following series combinations of
0.1% resistors come very close to the calculated values:
where G = 50, and R3 = 1008 Ω.
R12 =
Offset Error (%FSR) =
Full Scale Error = Gain Error + Offset Error
G –1
V REF

  × 100%


101 × V REF
I LO
=
101 × 5 V
• Offset Error = −0.008% FSR
• Gain Error = +0.010% FSR
= 126.25 kΩ
4 mA
For VOUT = 5.00V, the output loop current is ILH = 20 mA, and
hence:
I LH = I 8 + I 10 = 100 × I 10 + I 10 = 101 × I 10
The error calculated with these combinations is as follows:
• Full Scale Error = +0.002% FSR
In some cases, however, even standard 0.1% resistor values may
not be obtainable from resistor suppliers, and substitutions are
required.
Rev. 0 | Page 4 of 7
Circuit Note
CN-0289
For example, the resistors supplied with the EVAL-CN0289EB1Z board are as follows:
In order for the circuit to operate properly, the supply voltage,
VLOOP, must be greater than 7 V in order to provide sufficient
headroom for the ADR02 voltage reference.
• R3 = 1000 Ω (calculated = 1008 Ω)
Therefore,
• R9 = 31.6 kΩ (calculated = 31.56 kΩ)
VLOOP_SUPPLY > 7 V + R7 × ILOOP
• R12 = 124 kΩ (calculated = 126.25 kΩ)
With the values supplied with the board, the errors due to the
resistor values are:
VLOOP_SUPPLY > 7 V + 250 Ω × 20 mA = 12 V
The minimum loop supply voltage is also dependent on the
configuration of the drive circuit for the bridge. In the voltage
drive mode with VDRIVE = 10 V, the supply voltage VLOOP must be
greater than 10.2 V in order to maintain sufficient headroom
for U2A (see Figure 2).
• Offset Error = +0.45% FSR
• Gain Error = +0.66% FSR
• Full Scale Error = +1.11% FSR
Voltage Reference
The ADR02 5 V reference is used to set the drive voltage or
current to the bridge and to set the 4 mA zero offset. It has an
initial accuracy of 0.1% (A-grade), 0.06% (B-grade), and
10 µV p-p voltage noise. In addition it will operate on supply
voltages up to 36 V and consumes less 1 mA maximum, making
it an ideal choice for loop-powered applications.
Voltage-to-Current Conversion
The 4 mA-to-20 mA output is produced by forcing a current
through R10 that is the sum of the signal component (I9) and
the offset component (I12). The current I10 = I9 + I12 develops a
voltage across R10 that is applied to the sense resistor R10
through U2B and Q1. The current through R8 is 100× the
current in R10. The loop current, ILOOP, is therefore :
I LOOP = I 8 + I 10 = 100 × I 10 + I 10 = 101 × I 10
The values of R8 (10 Ω) and R10 (1 kΩ) were chosen to be
values that are easily obtainable in 0.1% tolerances.
In order for the circuit to operate properly, the circuit current,
ICIRCUIT, must always be less than the minimum loop current of
4 mA. In addition, the ground of the PCB must not be connected
to the loop ground in any manner, and the PCB ground and the
sensor must be free to float with respect to the loop ground.
The bipolar NPN transistor that is controlled by the output of
U2B and generates the loop current should have a gain of at
least 300 to minimize the linearity error. It should also have a
breakdown voltage of at least 50 V.
The output transistor Q1 is a 50 V NPN power transistor
capable of dissipating 1.1 W @ 25°C. The worst case power
dissipation in the circuit is for an output current of 20 mA into
a loop load resistance of 0 Ω with a VCC supply of 36 V. Under
these conditions the power dissipation of Q1 is 0.68 W.
The supply voltage, VLOOP, driving the circuit board is
dependent on the loop supply VLOOP_SUPPLY, the loop load, R7,
and the loop current, ILOOP. These values are related by the
following equation:
For a maximum loop current of 20 mA, and R7 = 250 Ω,
In the current drive mode, the supply voltage VLOOP must be
greater than 11.2 V in order to maintain sufficient headroom
for U2A (see Figure 3).
The loop supply voltage is limited to 36 V maximum.
Error Analysis for Active Components
The maximum and rss errors due to the active components in
the system for A- and B-grade levels of the AD8226 and ADR02
are shown in Table 2 and Table 3. Note that the ADA4091-2 op
amp is only available in one grade level.
Table 2. Errors Due to Active Components (A-Grade)
Error Component
AD8226-A
ADR02-A
ADA4091-2
AD8226-A
RSS Offset
RSS Gain
RSS FS Error
Max Offset
Max Gain
Max FS Error
Error
Offset
Offset
Offset
Gain
Error Value
112µV
0.10%
250µV
0.15%
Error %FSR
0.11%
0.02%
0.16%
0.15%
0.20%
0.15%
0.35%
0.29%
0.15%
0.44%
Table 3. Errors Due to Active Components (B-Grade)
Error Component
AD8226-B
ADR02-B
ADA4091-2
AD8226-B
RSS Gain
RSS Offset
RSS FS Error
Max Offset
Max Gain
Max FS Error
VLOOP = VLOOP_SUPPLY – R7 × ILOOP
Rev. 0 | Page 5 of 7
Error
Offset
Offset
Offset
Gain
Error Value
58µV
0.06%
250µV
0.10%
Error %FSR
0.06%
0.01%
0.16%
0.10%
0.10%
0.17%
0.27%
0.23%
0.10%
0.33%
CN-0289
Circuit Note
Total Circuit Accuracy
0.9
A good approximation to the total error contributed by the
resistor tolerances is to assume that each of the critical resistors
contribute equally to the total error. The five critical resistors
are R3, R8, R9, R10, and R12. Worst-case tolerance build up of
0.1% resistors yields a total resistor error of 0.5% maximum. If
rss errors are assumed, then the total rss error is 0.1√5 = 0.224%.
0.8
TOTAL ERROR (%FS)
0.7
Adding the worst case resistor tolerance error of 0.5% to the
previous worst-case errors due to the active components (Agrade) yields:
0.6
AD8226 OUTPUT ZERO ERROR = +0.35%
0.5
0.4
0.3
0.2
GAIN ERROR = –0.47%
FULL SCALE
ERROR = –0.02%
0.1
–0.1
• Gain Error = 0.15% + 0.5% = 0.65%
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
BRIDGE OUTPUT (mV)
• Full Scale Error = 0.44% + 0.5% = 0.94%
10947-004
0
• Offset Error = 0.29% +0.5% = 0.79%
Figure 4. Total Error in Output Current (%FSR) vs. Bridge Output for 3 kΩ
Bridge, 24 V Loop Supply
These errors assume ideal resistors are selected and that the
only errors are due to their tolerances.
Although the circuit is capable of 1% or less total error, if better
accuracy is required offset and gain adjustment capability
should be added to the circuit. Offset can be calibrated by
adjusting R12 for 4 mA output with zero input, and full-scale
can then be adjusted by varying R9 for a full-scale 100 mV
input. The two adjustments are independent provided the offset
is calibrated first.
Actual error data from the circuit is shown in Figure 4. The total
output error (%FSR) is calculated by taking the difference between
the ideal output current and the measured output current,
dividing by the FSR (16 mA), and multiplying the result by 100.
Note that the error between 0 mV and 1 mV input is due to the
saturation voltage of the AD8226 output stage that can range from
20 mV to 100 mV under the loading conditions in the circuit.
All rail-to-rail output stages are limited in their ability to approach
the supply rails by either the saturation voltage (bipolar outputs)
or on-resistance (CMOS outputs).
If the error caused by the output saturation voltage is troublesome,
the input signal from the bridge can be offset by connecting an
appropriate resistor from the +5 V reference to one side of the
bridge output.
COMMON VARIATIONS
The circuit is proven to work with good stability and accuracy
with component values shown. Other voltage references, precision
op-amps and in-amp can be used in this configuration to
develop 4 mA-to-20 mA analog current output and for other
various applications for this circuit.
The ADA4091-4 is a quad version and can be used as a
substitute for the ADA4091-2, dual channel, if additional
precision op amps are needed.
The AD8426, a dual-channel, low cost, and wide supply range
instrumentation amplifier can also be used for multiple input
channel applications.
The ADR4550, high precision, low power, low noise voltage
references can be used to replace ADR02 for a low voltage
supply applications.
CIRCUIT EVALUATION AND TEST
Equipment Required
• EVAL-CN0289-EB1Z evaluation board
• Agilent E36311A dual dc power supply or equivalent
• Agilent 3458A multimeter or equivalent
Current Output Measurement
The current output of the evaluation board was measured with
a setup seen in Figure 5. The test conditions were as follows:
• Loop supply: 24 V
• Loop load: 250 Ω
• RBRIDGE = 3 kΩ
• VDRIVE = 5 V
• VCM = 2.5 V
The bridge resistors are connected to both terminal inputs of
the in-amp to simulate sensor output.
Rev. 0 | Page 6 of 7
Circuit Note
CN-0289
Test Setup Configuration and Tests
LEARN MORE
The circuit was tested using the test setup in Figure 5.
CN-0289 Design Support Package:
http://www.analog.com/CN0289-DesignSupport.
The Agilent E36311A dual power supply was used to generate a
common-mode voltage of 2.5 V and a 0 mV to 100 mV differential
input voltage.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of AGND and DGND. Analog Devices.
The Agilent 3458A was used to measure the actual loop current
output of the evaluation board.
MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and
Rail-to-Rail Issues. Analog Devices.
EVAL-CN0289-EB1Z
DUAL
POWER SUPPLY
J1-2
RBRIDGE ÷ 2
24V
J1-4
COM
VDIFF
J1-3
VOUT1
CHANNEL 1
COM1
MT-087 Tutorial, Voltage References. Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
CURRENT METER
RBRIDGE
VCM
MT-066 Tutorial, In-Amp Bridge Circuit Error Budget Analysis.
Analog Devices.
VOUT
J2-1
COM
J1-1
PCB GND
J2-2
Voltage Reference Wizard Design Tool.
I
FOR TESTS, VCM = 2.5V, RBRIDGE = 3kΩ, VDRIVE = 5V
Figure 5. Functional Block Diagram of Test Setup
10947-005
VOUT2
CHANNEL 2
COM2
MT-065 Tutorial, In-Amp Noise. Analog Devices.
POWER SUPPLY
RBRIDGE ÷ 2
Data Sheets and Evaluation Boards
CN-0289 Circuit Evaluation Board (EVAL-CN0289-EB1Z)
AD8226 Data Sheet
ADA4091-2 Data Sheet
ADR02 Data Sheet
REVISION HISTORY
5/13—Revision 0: Initial Version
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©2013 Analog Devices, Inc. All rights reserved. Trademarks and
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CN10947-0-5/13(0)
Rev. 0 | Page 7 of 7
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