CN-0179: Less Than 200 μA, Low Power, 4 mA-to-20 mA, Process Control Current...

Circuit Note
CN-0179
Devices Connected/Referenced
Circuits from the Lab® reference designs 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/CN0179.
AD8657
18 V, Precision, Micropower CMOS Rail-toRail I/O Dual Operational Amplifier
ADR02
Ultracompact Precision 5 V Voltage
Reference
AD5641
2.7 V to 5.5 V, <100 µA, 14-Bit nanoDAC, SPI
Interface
4-20 mA Low Power, 14-Bit, Process Control Current Loop Transmitter
EVALUATION AND DESIGN SUPPORT
budget for higher power devices, such as microcontrollers and
digital isolators. The circuit output is 0 mA to 20 mA of current,
and it operates on a single supply from 8 V to 18 V. The 4 mA to
20 mA range is usually mapped to represent the input control
range from the DAC or micro-controller, while the output
current range of 0 mA to 4 mA is often used to diagnose fault
conditions.
Circuit Evaluation Boards
CN0179 Circuit Evaluation Board (EVAL-CN0179-PMDZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
SDP Interposer Board (SDP-PMD-1B1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
CIRCUIT FUNCTION AND BENEFITS
The 14-bit, 5 V AD5641 requires 75 µA typical supply current.
The AD8657 is a rail-to-rail input/output dual op amp and is
one of the lowest power amplifiers currently available in the
industry (22 µA per amplifier over the full supply voltage and
input common-mode range) with high operating voltage of up
to 18 V. The ADR02 ultracompact precision 5 V voltage reference
requires only 650 µA. Together, these three devices consume a
typical supply current of 747 µA.
The circuit in Figure 1 is a 4 mA-to-20 mA current loop
transmitter for communication between a process control
system and its actuator. Besides being cost effective, this circuit
offers the industry’s low power solution. The 4 mA-to-20 mA
current loop has been used extensively in programmable logic
controllers (PLCs) and distributed control systems (DCS’s),
with digital or analog inputs and outputs. Current loop
interfaces are usually preferred because they offer the most cost
effective approach to long distance noise immune data
transmission. The combination of the low power AD8657 dual
op amp, AD5641DAC, and ADR02 reference allows more power
The circuit has a 12-pin Pmod™ digital interface (Digilent
specification).
18V
VSY
18V
VOUT
10µF
0.1µF
VREF
5V
0.1µF
GND
10µF
1/2
18V ZENER
BZX84C18
AD8657
A2
PMOD
SCLK
SDIN
SI2319DS-T1-E3
100Ω
BA S21LT1
IOUT
VDD
SYNC
VDAC
AD5641
GND
GND
R2
100Ω
0.1%
25ppm/°C
VOUT
1/2
AD8657
2N700T
A1
J1
RLOAD
250Ω
RSENSE
2.49kΩ
0.1%
25ppm/°C
GND
09371-001
ADR02
VIN
R1
1kΩ
0.1%
25ppm/°C
P1
Figure 1. Low Power 4 mA-to-20 mA Process Control Current Loop (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. A
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CN-0179
Circuit Note
(1)
20
18
where:
VREF is the output of ADR02 and the power supply to the AD5641.
D is the decimal equivalent of the binary code that is loaded to
the AD5641.
OUTPUT CURRENT (mA)
16
The DAC output voltage sets the current flowing through the
sense resistor, RSENSE, where
(2)
6
4
2
DAC INPUT CODE
16384
09371-002
15360
14336
13312
11264
12288
9216
10240
8192
7168
6144
5120
4096
3072
2048
0
Figure 2. 0 mA to 20 mA Output Current
Figure 3 shows the output current error plot in percent full-scale
range. The overall worst-case error is approximately 0.35%
measured over the output range between Code 256 and
Code 16,128.
Rev. A | Page 2 of 5
0.25
0.20
0.15
0.10
0.05
0
16384
15360
14336
09371-003
DAC INPUT CODE
Figure 3. Output Current Error Plot
13312
12288
11264
10240
9216
8192
–0.05
7168
The ADR02 is an ultracompact, precision 5 V voltage reference.
With an 18 V input voltage, quiescent current is only 650 µA,
typical. It has an initial accuracy of 0.06% (B-grade) and 10 μV p-p
voltage noise. Connecting a 0.1 μF ceramic capacitor to the
output is highly recommended to improve stability and filter
out low level voltage noise. An additional 1 µF to 10 µF
electrolytic, tantalum, or ceramic capacitor in parallel can
improve load transient response. A 1 μF to 10 μF electrolytic,
tantalum or ceramic capacitor can also be connected to the input
to improve transient response in applications where the supply
voltage may fluctuate. An additional 0.1 μF ceramic capacitor
should be connected in parallel to reduce supply noise.
0.30
6144
In addition, this circuit solution requires a rail-to-rail input
amplifier. The AD8657 dual op amp is an excellent choice, with
low power and rail-to-rail features. The op amp operates with a
typical supply current of 22 µA/amplifier over the specified supply
voltage and input common-mode voltage. It also offers excellent
noise and bandwidth per unit of current. The AD8657 is one of
the lowest power amplifiers that operate on supplies of up to 18 V.
0.35
5120
The AD5641 is a 14-bit DAC from the nanoDAC family and
operates from the 5 V output voltage of the ADR02 reference. It
has an on-chip precision output buffer that is capable of swinging
from rail-to-rail (within 10 mV), thus allowing a high dynamic
output range. With a supply voltage of 5 V, AD5641 consumes
a typical 75 µA of supply current.
4096
With VDAC ranging from 0 V to 5 V, the circuit generates a
current output from 0 mA to 20 mA.
8
3072
(3)
10
2048
IOUT = IR2 = (VDAC/RSENSE ) × ( R1/R2)
IDEAL CURRENT
MEASURED CURRENT
12
1024
The current through RSENSE varies from 0 mA to 2 mA as a function
of VDAC. This current develops a voltage across R1 and sets the
voltage at the noninverting input of the AD8657 amplifier (A2).
The A2 AD8657 closes the loop and brings the inverting input
voltage to the same voltage as the noninverting input. Therefore,
the current flowing through R1 is mirrored by a factor of 10 to
R2. This is represented by Equation 3.
14
0
ISENSE = VDAC/RSENSE
READING ERROR (%FSR)
VDAC = VREF × (D/214)
Figure 2 shows the linearity of the system, that is the measured
output current from the circuit DAC input code from 0 to
full-scale.
1024
For industrial and process control modules, 4 mA-to-20 mA
current loop transmitters are used as a means of communication
between the control unit and the actuator. Located at the control
unit, the 14-bit AD5641 DAC produces an output voltage, VDAC,
between 0 V and 5 V as a function of the input code. The code
is set via an SPI interface. The ideal relationship between the
input code and output voltage is given by
Bypass capacitors (not shown in Figure 1) are required. In this
case, a 10 µF tantalum capacitor in parallel with a 0.1 µF ceramic
capacitor should be placed on each power pin of each dual
op amp. Details of proper decoupling techniques can be
found in Tutorial MT-101.
0
CIRCUIT DESCRIPTION
Circuit Note
CN-0179
Figure 4 shows the calibrated output current error plot.
Removing the gain and offset error from Figure 3, the accuracy
is better than 0.05% measured over the output range between
Code 256 and Code 16,128.
0.05
READING ERROR (%FSR)
0.03
0.01
–0.01
CIRCUIT EVALUATION AND TEST
This circuit uses the EVAL-CN0179-PMDZ circuit board, the
EVAL-SDP-CB1Z system demonstration platform (SDP)
evaluation board and the SDP-PMD-IB1Z, a Pmod interposer
board for the EVAL-SDP-CB1Z. The SDP and the SDP-PMDIB1Z boards have 120-pin mating connectors, allowing the
quick setup and evaluation of the circuit’s performance. In
order to evaluate the EVAL-CN0179-PMDZ board using the
SDP-PMD-IB1Z and the SDP, the EVAL-CN0179-PMDZ is
connected to the SDP-PMD-IB1Z by a standard 100 milspaced, 25 mil square, right angle 12 pin-Pmod header connector.
Information and details regarding how to use the evaluation
software for data capturing and proper hardware installation
can be found in the CN0179 Software User Guide.
–0.03
Equipment Required
15360
16384
•
09371-004
DAC INPUT CODE
14336
13312
11264
12288
9216
10240
8192
7168
6144
5120
4096
3072
2048
0
1024
–0.05
Figure 4. Calibrated Output Current Error Plot
The data in Figure 3 and Figure 4 shows larger errors at zero and
full-scale because the output buffer of the AD5641 DAC limits
when its output is within 10 mV of either supply rail. The
region between Code 0 and Code 255 as well as the region
between Code 16,129 and Code 16,384 are therefore excluded
from the linearity specifications. This corresponds to
approximately 0 V to 80 mV and 4.92 V to 5.00 V at the DAC
voltage output; and 0 mA to 0.32 mA and 19.68 mA to 20.00
mA referenced to the current output.
The test data was taken using the board shown in Figure 6.
Complete documentation for the system can be found in the
CN-0179 Design Support package.
Information and details regarding how to use the evaluation
software for data capturing and proper hardware installation
can be found in the CN0179 Software User Guide.
Test Setup and Measurements
COMMON VARIATIONS
For a 16-bit resolution solution, consider the AD5660 or
AD5662, respectively. The 16 V CMOS ADA4665-2 op amp is
another option to replace the AD8657. It lower cost and has
lower voltage noise at the expense of a higher supply current.
When selecting amplifiers for this application, always ensure
that the input common-mode voltage range and the supply
voltage are not exceeded.
•
•
•
•
•
•
•
•
PC with a USB port and Windows® XP, Windows® Vista
(32-bit), or Windows® 7 (32-bit)
EVAL-CN0179-PMDZ circuit evaluation board
EVAL-SDP-CB1Z SDP evaluation board
SDP-PMD-IB1Z
CN0179 evaluation software
Agilent E36311A dual dc power supply or equivalent
Agilent 3458A multimeter or equivalent
+6 V wall wart
A GPIB-to-USB cable adapter (only required for capturing
analog data from the output and transferring it to the PC)
The circuit was tested using the test setup in Figure 5.
A photograph of the board is shown in Figure 6.
A jumper should not be connected to the J1 terminals when
driving an external current loop. The jumper connects the
internal 250 Ω load and should be used when making voltage
measurements.
Rev. A | Page 3 of 5
CN-0179
Circuit Note
GPIB INTERFACE
CURRENT
METER
6V
POWER SUPPLY
OR WALL WART
USB
USB
CONA
120
PINS
J1
J4
J3
PMOD
EVAL-SDP-CB1Z
SDP-PMD-IB1Z
SDP BOARD
INTERPOSER BOARD
18V
POWER SUPPLY
P1
PMOD
IOUT
EVAL-CN0179-PMDZ
09371-005
PC
09371-006
Figure 5. Functional Diagram of Test Setup
Figure 6. Photo of EVAL-CN0179-PMDZ Board
Rev. A | Page 4 of 5
Circuit Note
CN-0179
LEARN MORE
Data Sheets
CN0179 Design Support Package:
http://www.analog.com/CN0179-DesignSupport
AD8657 Data Sheet
AN-345 Application Note, Grounding for Low- and HighFrequency Circuits, Analog Devices.
AD5641 Data Sheet
ADR02 Data Sheet
AD5662 Data Sheet
AN-347 Application Note, Shielding and Guarding: How to
Exclude Interference-Type Noise, Analog Devices.
AD5660 Data Sheet
Colm Slattery, Derrick Hartmann, and Li Ke, “PLC Evaluation
Board Simplifies Design of Industrial Process Control
Systems,” Analog Dialogue (April 2009).
Jung, Walt. Op Amp Applications, Analog Devices. Also
available as Op Amp Applications Handbook, Elsevier.
Kester, Walt. 2005. The Data Conversion Handbook. Chapters 3
and 7. Analog Devices.
MT-015 Tutorial, Basic DAC Architectures II: Binary DACs.
Analog Devices.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of “AGND” and “DGND.” Analog Devices.
MT-035, Op Amp Inputs, Outputs, Single-Supply, and Rail-toRail Issues, Analog Devices.
ADA4665-2 Data Sheet
REVISION HISTORY
3/14—Rev. 0 to Rev. A
Changed AD5621 to AD5641....................................... Throughout
Changed ADR125 to ADR02........................................ Throughout
Changes to Figure 1 .......................................................................... 1
Changes to Circuit Description Section, Figure 2, and
Figure 3 ............................................................................................... 2
Changes to Common Variations Section ....................................... 3
Added Figure 4 and Circuit Evaluation and Test Section ............ 3
Added Figure 5 and Figure 6 ........................................................... 4
Changes to Learn More Section and Data Sheets Section ........... 5
11/10—Revision 0: Initial Version
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
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CN09371-0-3/14(A)
Rev. A | Page 5 of 5
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