CN0295:Flexible, 4 mA-to-20 mA Pressure Sensor Transmitter with Voltage or Current Drive PDF

Circuit Note
CN-0295
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/CN0295.
AD8226
Wide Supply Range, Rail-to-Rail
Output Instrumentation Amplifier
ADR02
Ultracompact, Precision 5.0 V Voltage
Reference
ADA4091-4
Precision Micropower, OVP, RRIO
Operational Amplifier
Flexible, 4 mA-to-20 mA Pressure Sensor Transmitter with Voltage or Current Drive
The circuit is optimized for a wide variety of bridge-based voltage
or current driven pressure sensors, utilizes only five active
devices, and has a total unadjusted error of less than 1%. The
power supply voltage can range from 7 V to 36 V depending on
the component and sensor driver configuration.
EVALUATION AND DESIGN SUPPORT
Circuit Evaluation Boards
CN0295 Evaluation Board (EVAL-CN0295-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 1is a flexible current transmitter
that converts the differential voltage output from a pressure
sensor to a 4 mA-to-20 mA current output.
VCC
+VCC
1/4
ADA4091-4
U2A
+VREF
VDRIVE
R6
10kΩ
I15
4mA,
20mA
+VCC
+5V
J3-3
R5
2kΩ
ADR02
+VCC
RBRIDGE
RBRIDGE
R1
J3-2 4.02kΩ
VIN
RBRIDGE
RBRIDGE
+VREF
R7
11.5kΩ
R8
1kΩ
+VSS
G = 16.06
VCC
R12
1kΩ
+VCC
10nF
R2
4.02kΩ
J3-4
VIN = 0mV; 100mV
J3-1
1nF
R3
3.28kΩ
U1
AD8226
1nF
REF
VOUT
Q2
1/4
ADA4091-4
100Ω
Q1
U2D
P1-1
1/4
ADA4091-4
Q1: BC847C, 215
Q2: SI2319DS-T1-E3
U2C
+VCC
J1-1
J1-2
0.4V, 2.0V
0.4V
1/4
ADA4091-4
R15
100Ω
I13
0.4mA,
2.0mA
R13
1kΩ
R14
499Ω
P1-2
PCB GROUND
11610-001
+5V
U2B
Figure 1. 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
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CN-0295
Circuit Note
CIRCUIT DESCRIPTION
The design provides a complete solution for the 4 mA-to-20 mA
transmitter for pressure sensor measurements. The circuit has
three critical stages: the sensor excitation drive, the sensor output
amplifier, and the voltage to current converter.
The total current required by the circuit (neglecting the bridge
drive current and the output current) is 5.23 mA (maximum) as
shown in Table 1.
Table 1. Maximum circuit currents at 25°C

R5 

V DRIVE = 5 V  1 +

R6 

Note that the power supply voltage VCC should be at least 0.2 V
greater than the bridge drive voltage to allow sufficient headroom
for U2A, the ADA4091-4:
Current (mA)
0.80
1.00
0.43
0.60
0.40
2.00
5.23
V CC ≥ V DRIVE + 0.2 V
For the values shown in Figure 2, R5 = 2 kΩ, R6 = 10 kΩ, IDRIVE
= 2 mA, VDRIVE = 6 V, and VCC ≥ 6.2 V.
The ADA4091-4 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.
Excitation: Voltage Drive Configuration
The ADR02 is chosen for the 5 V reference because of its
accuracy (A-Grade: 0.1%, B-Grade: 0.06%) and low quiescent
current (0.8 mA).
Depending on the pressure sensor selected, either a voltage or
current drive is required. The circuit uses one-fourth of the
ADA4091-4 (U2A) with different configurations chosen by
switching S1 to support either option. Figure 2 shows the
+VCC
+5V
1/4
ADA4091-4
U2A
VDRIVE
IDRIVE
IDRIVE = 2mA FOR
VDRIVE = 6V, RBRIDGE = 3kΩ
J3-3
R5
2kΩ
RBRIDGE
RBRIDGE
J3-2
R1
4.02kΩ
+V
R6
10kΩ
RBRIDGE
AD8226 INPUT
RBRIDGE
VCM = 3V
–V
J3-1
J3-4
R2
4.02kΩ
11610-002
Component
ADR02
ADA4091-4
AD8226
R5, R6 at 6 V
R7, R8 at 5 V
R13 at 2 V
Total
configuration for the voltage drive with S1 in the position
closest to the identifying marking (see complete circuit layouts
and schematics in the CN0295 design support package:
http://www.analog.com/CN0295-DesignSupport). The voltage
drive is normally configured for a bridge drive voltage of 6 V by
the gain of the stage, 1 + R5/R6. Other drive voltages can be
obtained by changing the resistor ratio appropriately:
Figure 2. Sensor Voltage Drive Configuration (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. 0 | Page 2 of 6
Circuit Note
CN-0295
Excitation: Current Drive Configuration
The resulting drive voltage VDRIVE is calculated from:
V DRIVE  5 V  I DRIVE  R BRIDGE
The circuit can be switched to the current drive configuration
shown in Figure 3 by moving S1 to the position that is furthest
away from the identifying marking.
A headroom of 0.2 V is required for the VCC supply, therefore:
V CC  V DRIVE  0.2 V
In the current drive mode, the circuit is configured for R4 = 2.5 kΩ,
and IDRIVE = 2 mA. Lower or higher values of IDRIVE can be obtained
by using the following equation to select the value of R4:
5V
I DRIVE
+VCC
+5V
1/4
ADA4091-4
U2A
VDRIVE
IDRIVE
VDRIVE = 5V + IDRIVE × RBRIDGE = 11V FOR
RBRIDGE = 3kΩ
J3-3
RBRIDGE
RBRIDGE
J3-2
R1
4.02kΩ
+V
RBRIDGE
J3-1
IDRIVE = 2mA
AD8226 INPUT
RBRIDGE
J3-4
R4
2.5kΩ
R2
4.02kΩ
VCM = 8V
–V
11610-003
R4 
For the values shown in Figure 3, RBRIDGE = 3 kΩ, IDRIVE = 2 mA,
VDRIVE = 11 V, and VCC ≥ 11.2 V.
Figure 3. Sensor Current Drive Configuration (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. 0 | Page 3 of 6
Circuit Note
CN-0295
Bridge Output Instrumentation Amplifier and Offset Circuit
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 1.98 kHz.
The AD8226 is an ideal choice for the in-amp because of its low
gain error (0.1%, B-grade), low offset (58 μV at G = 16, B-grade;
112 μV at G = 16, A-grade), excellent gain nonlinearity (75 ppm
= 0.0075%), and rail-to-rail inputs and output.
The AD8226 instrumentation amplifier amplifies the 100 mV
FS signal by a factor of 16 to 1.6 V using a gain setting resistor
R3 = 3.28 kΩ. The relationship between the gain, G, and R3 is
given by
R3 
49.4 kΩ
G –1
For G = 16, R3 = 3.2933 kΩ. The nearest standard 0.05% value
of 3.28 kΩ is chosen for R3, yielding a gain of G = 16.06, which
introduces an overall gain error of +0.4%.
For a 0 V bridge output, the output loop current should be 4
mA. This is achieved by simply applying a +0.4 V offset to the
REF input of the AD8226 in amp as shown in Figure 1. The
+0.4 V is derived from the ADR02 5 V reference using divider
resistors R7/R8 and buffering the voltage with U2B.
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 0.06% (B-grade) and 10 μV p-p voltage noise.
In addition, it operates on supply voltages up to 36 V and
consumes less than 1 mA, making it an ideal choice for low
power applications.
Voltage to Current Conversion
The 0 V to 100 mV input to the AD8226 generates an output
swing at VOUT of 0.4 V to 2.0 V. The buffer, U2C, applies this
voltage across R13 that produces a corresponding current I13 of
0.4 mA to 2.0 mA. Transistor Q1 then mirrors the I13 current to
R12, and the resulting voltage is applied to R15, thereby
developing the final loop current of 4 mA to 20 mA. Transistor
Q1 should have a high gain of at least 300 to minimize the
linearity error due to its base current.
The output transistor Q2 is a 40 V P-channel MOSFET power
transistor capable of dissipating 0.75 W at 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 Q2 is 0.68 W.
However, the power in Q2 can be significantly reduced by
properly selecting VCC so that it is at least 3 V greater than the
maximum loop load voltage. This ensures sufficient headroom
due to the voltage dropped across the sense resistor R15.
The minimum VCC supply voltage is also dependent on the
configuration of the drive circuit for the bridge. In the voltage
drive mode with VDRIVE = 6 V, the supply voltage VCC must be
greater than 6.2 V in order to maintain sufficient headroom for
U2A (see Figure 2).
In the current drive mode, the supply voltage VCC must be
greater than 11.2 V in order to maintain sufficient headroom
for U2A (see Figure 3).
The VCC 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 the
ADR02 are shown in Tables 2 and 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
ADA4091-4 (U2B)
ADA4091-4 (U2C)
ADA4091-4 (U2D)
AD8226-A
RSS Offset
RSS Gain
RSS FS Error
Max Offset
Max Gain
Max FS Error
Error
Offset
Offset
Offset
Offset
Offset
Gain
Error Value
112 μV
0.10%
250 μV
250 μV
250 μV
0.15%
Error %FSR
0.11%
0.02%
0.02%
0.02%
0.02%
0.15%
0.12%
0.15%
0.27%
0.19%
0.15%
0.34%
Table 3. Errors Due to Active Components (B-Grade)
Error Component
AD8226-B
ADR02-B
ADA4091-4 (U2B)
ADA4091-4 (U2C)
ADA4091-4 (U2D)
AD8226-B
RSS Offset
RSS Gain
RSS FS Error
Max Offset
Max Gain
Max FS Error
Voltage Supply Requirement
In order for the circuit to operate properly, the supply voltage,
VCC, must be greater than 7 V in order to provide sufficient
headroom for the ADR02 voltage reference.
Rev. 0 | Page 4 of 6
Error
Offset
Offset
Offset
Offset
Offset
Gain
Error Value
58 μV
0.06%
250 μV
250 μV
250μV
0.10%
Error %FSR
0.06%
0.01%
0.02%
0.02%
0.02%
0.10%
0.07%
0.10%
0.17%
0.13%
0.10%
0.23%
Circuit Note
CN-0295
Total Circuit Accuracy
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 six critical resistors are R3, R7,
R8, R12, R13, and R15. Worst-case tolerance build up of 0.1%
resistors yields a total resistor error of 0.6% maximum. If rss
errors are assumed, then the total rss error is 0.1√6 = 0.245%.
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
Adding the worst-case resistor tolerance error of 0.6% to the
previous worst-case errors due to the active components (Agrade) yields:
 Offset Error = 0.19% + 0.6% = 0.79%

EVAL-CN0295-EB1Z Evaluation Board

Agilent 36311A Precision DC Power Supply

Yokogawa 2000 Precision DC Power Supply

Agilent 3458A Precision Multimeter
The linearity error at the current output of the evaluation board
was measured with a setup seen in Figure 5.
 Full Scale Error = 0.34% + 0.6% = 0.94%
These errors assume calculated resistor values are selected and
that the only errors are due to their tolerances.
PRECISION
V
POWER SUPPLY DIFF
COM
Although the circuit is capable of 1% or less total error, if better
accuracy is required, add offset and gain adjustment capability
to the circuit. Offset can be calibrated by adjusting R7 or R8 for
4 mA output with zero input, and full-scale can then be adjusted by
varying R3 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 with VCC
= 25 V. The total error in the output current (%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.
FOR TESTS, VCM = 3V, RBRIDGE = 3kΩ, VCC = 25V
V+
RBRIDGE/2
VOUT2
CH2
COM2
VCM
RBRIDGE/2
J3-4
DUAL
POWER SUPPLY
COM1
CH1
VOUT1
J3-2
EVAL-CN0295-EB1Z
VCC
CURRENT
METER
J2-2
J1-1
I
J2-1
J1-2
COM
11610-005
 Gain Error = 0.15% + 0.6% = 0.75%
Figure 5. Block Diagram for the Test Setup
0.15
Test
TOTAL ERROR (%FSR)
0.10
Agilent E3631A and Yokogawa precision voltage supply were
used to power up the board and simulate the sensor output.
CH2 of Agilent E3631A was set at 25 V to serve as the power
supply for the board and the other channel, CH1, was set at 2.5 V
to generate common mode voltage. This channel was connected
in series with the Yokogawa 2000 as shown in Figure 5. The
Yokogawa generates the 0 to 100 mV differential input voltage
at the in-amp input, which then simulates the sensor output.
0.05
0
–0.05
–0.10
–0.15
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
BRIDGE OUTPUT (mV)
11610-004
The Agilent 3458A was used to measure the actual current output
of the evaluation board, which is connected in series with J1.
Figure 4. Total Error in Output Current (%FSR) vs. Bridge Voltage for VCC = 25 V
COMMON VARIATIONS
The circuit is proven to work with good stability and accuracy
with component values shown. Other Analog Devices, Inc.
voltage references, precision op-amps, and in-amps 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 AD8426, a dual-channel, low cost and a wide supply range
instrumentation amplifier can also be used for multiple input
channel application.
Rev. 0 | Page 5 of 6
CN-0295
Circuit Note
LEARN MORE
CN-0295 Design Support Package:
http://www.analog.com/CN0295-DesignSupport
MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and
Rail-to-Rail Issues. Analog Devices.
MT-051 Tutorial, Current Feedback Op Amp Noise Considerations
MT-065 Tutorial, In-Amp Noise
MT-066 Tutorial, In-Amp Bridge Circuit Error Budget Analysis
MT-087 Tutorial, Voltage References
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of AGND and DGND. Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
Voltage Reference Wizard Design Tool.
Data Sheets and Evaluation Boards
AD8226 Data Sheet
ADA4091-4 Data Sheet
ADR02 Data Sheet
REVISION HISTORY
5/13—Revision 0: Initial Version
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CN11610-0-5/13(0)
Rev. 0 | Page 6 of 6
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