CN-0064: 16-Bit Fully Isolated 4 mA to 20 mA Output Module Using the AD5662 DAC, ADuM1401 Digital Isolator, and External Amplifiers PDF

Circuit Note
CN-0064
Devices Connected/Referenced
Circuit Designs Using Analog Devices Products
Apply these product pairings quickly and with confidence.
For more information and/or support call 1-800-AnalogD
(1-800-262-5643) or visit www.analog.com/circuit.
AD5662
16-Bit VOUT/IOUT DAC
ADR02
Precision 5 V Reference
AD822
Dual, Precision 36 V FET Amplifier
ADuM1401
Quad-Channel Digital Isolator
16-Bit Fully Isolated 4 mA to 20 mA Output Module Using the AD5662 DAC,
ADuM1401 Digital Isolator, and External Amplifiers
CIRCUIT FUNCTION AND BENEFITS
AD822 op amp. The AD822 amplifier was chosen for its high
performance, high voltage operation. The first stage of the
current mirror needs to operate at 0 V input to ensure there is
no dead band when the DAC operates at zero-scale output. This
requires that the negative supply to the AD822 be at least −1 V
so that there is sufficient headroom on the output of the first
stage AD822. The two diodes in the second stage, which are in
series with the positive power supply, ensure that the output
voltage of the second stage does not go to the positive supply
rail of the AD822, which can be as high as 35 V. The output of
the FET is also protected by a series diode. Note that this series
diode will increase the power dissipation of the circuit, which
may be an issue in some applications.
This circuit provides a complete solution for an industrial
control output module. This design is suitable for process
control programmable logic controllers (PLCs) and distributed
control system (DCS) modules and transmitters that require a
4 mA to 20 mA current output range. The AD5662 nanoDAC®
is a 5 V16-bit DAC in a SOT-23 package. The ADuM1401 fourchannel digital isolator provides all the necessary signal
isolation between the microcontroller and the DAC.
CIRCUIT DESCRIPTION
For industrial control modules, analog output current ranges
are typically 4 mA to 20 mA or 0 mA to 20 mA. The AD5662
provides a 0 V to 5 V output, which sets the current through the
sense resistor RS and, therefore, the current through R1.This
current is mirrored (10×) to R2, using the second half of the
15VISO
ADR02
VOUT
0.1µF
ISO
+15VISO
+
10µF
EXTERNAL INPUT
SYNC
SCLK
DIN
3.3V
0V
VDD1
VDD2
GND1
GND2
R1
1kΩ
10ppm/°C
ISO
0.1µF
VDD
VIA
VOA
SYNC
VIB
VOB
SCLK
VIC
VOC
DIN
VOD
VID
VE1
VE2
GND1
+15VISO
0.1µF
22µH
3.3V
0.1µF
VREF
+
VFB
10µF
0.1µF
AD5662
VOUT
GND
ISO
(1)
G
BZX84C18LT1
100Ω
(2)
2kΩ
+
0.1µF
ISO
S
G
10µF
IRFU9014
D
AD822
S
BAS21LT1
IRFU024N
D
AD822
ISO
+
10µF
GND2
DIGITAL ISOLATOR
R2
100Ω
10ppm/°C
ISO
ISO
ADuM1401
1N4002 (2)
0.1µF
ISO
VSS
<-1VISO
0.1µF
RS
2.5kΩ, 0.1%
10ppm/°C
ISO
RL
CURRENT
OUTPUT
ISO
08346-001
VIN
Figure 1. 16-Bit Isolated 4 mA to 20 mA Industrial Control Output Module (Simplified Schematic)
Rev. B
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Circuit Note
CN-0064
The ADR02 has drift specs of 9 ppm/°C maximum. It is also
often used in industrial applications due to its high input
voltage range to 36 V.
Learn More
The ADuM1401 is a quad-channel digital isolator based on
Analog Devices iCoupler® technology. It is used to provide
isolation between the AD5662 and the system microcontroller,
with an isolation rating of 2.5 kV rms. Three wires are used
(SYNC, SCLK, and DIN), which connect the standard SPI
connections to the AD5662.
Figure 2 shows an output error plot (integral nonlinearity) of
the current output of the circuit into RL when the AD5662 is
used with the ADR02 external reference. Results are shown in
% FSR (full scale range) as a function of input code. The ADR02
was chosen as the reference for this circuit.
See Colm Slattery, Derrick Hartmann, Li Ke, “PLC Evaluation
Board Simplifies Design of Industrial Process Control Systems,”
Analog Dialogue, April 2009, for more discussion of external
protection techniques.
Slattery, Colm, Derrick Hartmann, and Li Ke. “PLC Evaluation
Board Simplifies Design of Industrial Process Control
Systems.” Analog Dialogue (April 2009).
MT-014 Tutorial, Basic DAC Architectures I: String DACs and
Thermometer (Fully Decoded) DACs, Analog Devices.
MT-015 Tutorial, Basic DAC Architectures II: Binary DACs,
Analog Devices.
MT-016 Tutorial, Basic DAC Architectures III: Segmented DACs,
Analog Devices.
Wayne, Scott. “iCoupler® Digital Isolators Protect RS-232, RS485, and CAN Buses in Industrial, Instrumentation, and
Computer Applications.” Analog Dialogue (October 2005).
Data Sheets and Evaluation Boards
0.005
AD5662 Data Sheet.
0
AD822 Data Sheet.
ADR02 Data Sheet.
–0.005
ADuM1401 Data Sheet.
–0.010
ADuM1401 Evaluation Board.
–0.015
PLC Demo System.
–0.020
REVISION HISTORY
–0.025
08346-002
OUTPUT ERROR (%FSR)
Chen, Baoxing. 2006. iCoupler® Products with isoPower™
Technology: Signal and Power Transfer Across Isolation
Barrier Using Microtransformers. Analog Devices.
CODE
Figure 2. INL Accuracy Plot for 24 mA Full-Scale Output Range
3/11—Rev. A to Rev. B
Change to Circuit Function and Benefits.......................................1
Changes to Circuit Description .......................................................2
1/10—Rev. 0 to Rev. A
Change to Figure 1 ............................................................................1
7/09—Revision 0: Initial Version
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CN08346-0-3/11(B)
Rev. B | Page 2 of 2
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