CN-0233: 16-Bit Isolated Industrial Voltage and Current Output DAC with...

Circuit Note
CN-0233
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/CN0233.
ADuM3471
Quad Isolator with Integrated
Transformer Driver and PWM Controller.
AD5422
16-Bit Current Source and Voltage
Output DAC
ADR445
Precision 5.0 V Reference
16-Bit Isolated Industrial Voltage and Current Output DAC
with Isolated DC-to-DC Supplies
The circuit uses digital isolation, as well as PWM-controlled
power regulation circuitry along with associated feedback
isolation. External transformers are used to transfer power
across the isolation barrier, and the entire circuit operates on a
single +5 V supply located on the primary side. This solution is
superior to isolated power modules, which are often bulky and
may provide poor output regulation.
EVALUATION AND DESIGN SUPPORT
Circuit Evaluation Boards
CN0233 Circuit Evaluation Board (EVAL-CN0233-SDPZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
CIRCUIT FUNCTION AND BENEFITS
Digital isolators are superior to opto-isolators especially when
multichannel isolation is needed. The integrated design isolates
the circuit from the local system controller to protect against
ground loops and also to ensure robustness against external
events often encountered in harsh industrial environments.
The circuit in Figure 1 provides 16-bit fully isolated ±10 V and
4 mA-to-20 mA outputs suitable for programmable logic
controllers (PLCs) and distributed control systems (DCSs).
47µF
D1
D2
GND_ISO
VDD1
3
X2 4
19
ADuM3471
I/OA 5
I/OB 6
I/OC 7
I/OD 8
VDDA
0.1µF
GND1
18
17
16
15
14
13
9
12
10
11
GND
T1: COILTRONICS KA4976-AL
1:5 TURNS RATIO
+10µF
0.1µF
–15V (AVSS)
GND_ISO
GND_ISO
GND_ISO
AVDD AVSS
R1 24.9kΩ
20
2
VOUT
47µH
D4
X1 1
GND1
VIN
10µF + 0.1µF
47µF
47µF
L2
D3
GND
ADR445
+15V (AVDD)
L1
47µH
VREG
GND2
VDD2
FB
VFB
1.25V
R2 90.9kΩ 10µF + 0.1µF
R3
10.5kΩ
GND_ISO
0.1µF
+5V
GND_ISO +5V
GND_ISO
DVCC
I/OA
LATCH
I/OB
SCLK
I/OC
10µF
GND_ISO
AVDD
AVSS
GND_ISO
REFIN
AD5422
ROC 100kΩ
GND_ISO
VOUT
FAULT
CLR
CURRENT
OUTPUT
+VSENSE
SDO
OC
GND2
0.1µF
IOUT
SDIN
I/OD
0.1µF
+
–VSENSE
RL
GND_ISO
VOLTAGE
OUTPUT
GND CLR SEL RSET CCOMP
15kΩ
0.1%
5ppm/°C
GND_ISO
D1 TO D4: MBR0540
GND_ISO
ISO
4nF
GND_ISO
P4
GND_ISO
10126-001
1:5
T1
+5V
IN
Figure 1. Isolated 16-Bit Current and Voltage Output DAC with Isolated Power Supplies
Rev. A
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CN-0233
Circuit Note
0.6
CIRCUIT DESCRIPTION
The AD5422 is a fully integrated, fully programmable 16-bit
voltage and current output DAC, capable of programming
ranges from 4 mA to 20 mA, 0 mA to 24 mA, 0 V to 5 V, 0 V to
10 V, ±5 V, ±10 V. The voltage output headroom is typically
1 V, and the current output needs about 2.5 V headroom.
This means that the 20 mA current output can drive a load up
to approximately 600 Ω with a 15 V supply.
DNL (LSB)
0
8192
16384 24576
32768 40960
CODE
10126-002
–0.6
49152 57344
Figure 2. Measured DNL of Circuit for ±10 V Output Range
The average output noise was also tested and measured over
time, as shown in Figure 3. The total drift is approximately
75 µV, corresponding to only 0.25 LSB.
–4.99855
–4.99860
–4.99865
–4.99870
–4.99875
–4.99880
0
500
1000
1500
2000
SAMPLES
10126-003
The negative supply is loosely regulated, and for light loads can
be as high as −23 V. This is within the maximum operating
value of −26.4 V specification for the AD5422. With nominal
loads greater than 1 kΩ, the additional power dissipation due to
the larger unregulated negative supply voltage is not a problem.
In applications that require higher compliance voltages or
where very low power dissipation is required, a different power
supply design should be considered.
–0.4
Figure 3. Measured Average DAC Output Noise with DAC Output Set at −5 V
on ±10 V Output Range, Vertical Scale: 50 µV/div
(1 LSB = 305 µV), 2000 Samples
This circuit was tested with the ADR445 5 V, high precision,
low drift (3 ppm/°C maximum for B grade) external reference.
This allows total system errors of less than 0.1% to be achieved
over the industrial temperature range (−40°C to +85°C).
The AD5422 has a high precision integrated internal reference
with a drift of 10 ppm/°C maximum. If this reference is used
rather than the external reference, only 0.065% additional error
is incurred across the industrial temperature range.
0.6
0.5
0.4
4mA TO 20mA
0mA TO 20mA
0mA TO 24mA
0.3
0.2
0.1
0
0
12800
25600
38400
51200
64000
CODE
Test Data and Results
The AD5422 differential nonlinearity (DNL) was tested to
ensure no loss in system accuracy was incurred because of the
switching supplies. Figure 2 shows the DNL for a ±10V range.
The result shows less than 0.5 LSB DNL error.
Rev. A | Page 2 of 5
Figure 4. Measured Error (% FSR) For Current Output Ranges
10126-004
The ADuM3471 regulation is from the positive 15 V supply.
The feedback for regulation is from the divider network (R1,
R2, R3). The resistors are chosen such that the feedback voltage
is 1.25 V when the output voltage is 15 V. The feedback voltage
is compared with the ADuM3471 internal feedback voltage of
1.25 V. Regulation is achieved by varying the duty cycle of the
PWM signals driving the external transformer.
0
–0.2
VOUT (V)
The iCoupler chip-scale transformer technology is used to
isolate the logic signals, and the integrated transformer driver
with isolated secondary side control provides high efficiency
for the isolated dc-to-dc converter. The internal oscillator
frequency is adjustable from 200 kHz to 1 MHz and is
determined by the value of ROC. For ROC = 100 kΩ, the
switching frequency is 500 kHz.
0.2
OUTPUT ERROR (% FSR)
The ADuM347x devices are quad-channel digital isolators with
an integrated PWM controller and low impedance transformer
driver outputs (X1 and X2). The only additional components
required for an isolated dc-to-dc converter are a transformer
and simple full-wave diode rectifier. The devices provide up to
2 W of regulated, isolated power when supplied from a 5.0 V or
3.3 V input. This eliminates the need for a separate isolated dcto-dc converter.
0.4
Circuit Note
CN-0233
The ADuM347x isolators (ADuM3470, ADuM3471, ADuM3472,
ADuM3473, ADuM3474) provide four independent isolation
channels in a variety of input/output channel configurations.
These devices are also available with either a maximum data
rate of 1 Mbps (A grade) or 25 Mbps (C grade).
0.5
OUTPUT ERROR (% FSR)
0.4
0.3
0.2
0.1
CIRCUIT EVALUATION AND TEST
0
This circuit was tested using the EVAL-CN0233-SDPZ circuit
board and the EVAL-SDP-CB1Z connected together as shown
in Figure 6.
–0.1
0V TO 5V
0V TO 10V
–5V TO +5V
–10V TO +10V
–0.2
Equipment Used to Collect Test Data
–0.3
0
12800
25600
38400
51200
64000
CODE
•
10126-005
–0.4
Figure 5. Measured Error (% FSR) for Voltage Output Ranges
Actual error data from the circuit is shown in Figure 4 and
Figure 5. The total error in the output current and voltage
(% FSR) is calculated by taking the difference between the ideal
output and the measured output, dividing by the FSR, and
multiplying the result by 100. An error of less than 0.5% FSR
error is achieved in both the current and voltage output modes
as shown in Figure 4 and Figure 5, respectively.
If the VOUT pin must drive large capacitive loads up to 1 μF, a
3.9 nF capacitor can be connected between the VOUT pin and the
CCOMP pin of the AD5422 by connecting the P4 pins on the
board using a jumper. However, the addition of this capacitor
reduces the bandwidth of the output amplifier, increasing the
settling time.
COMMON VARIATIONS
This circuit is proven to work well with good stability and
accuracy with component values shown. Where the application
needs only the 4 mA to 20 mA current output, a single supply
scheme can be used. In this case, the positive AVCC supply can
be as large as 26.4 V and, therefore, the output compliance is
26.4 V – 2.5 V = 23.9 V. With an output current of 20 mA, a
load resistance as high as 1 kΩ is possible.
For applications not requiring 16-bit resolution, the 12-bit
AD5412 is available.
•
•
•
•
•
PC with a USB port and Windows® XP, Windows Vista®,
(32-bit), or Windows 7 (32-bit)
EVAL-CN0233-SDPZ
EVAL-SDP-CB1Z
Power supply: +6 V wall wart, Agilent E3630A, or
equivalent
Agilent 3458A, 8.5 digit multimeter or equivalent
National Instruments GPIB to USB-B interface and cable
(only required for capturing analog data from the DAC and
transferring it to the PC).
Setup and Test
The circuit was tested and verified by connecting both the
EVAL-CN0233-SDPZ evaluation board and the EVAL-SDPCB1Z evaluation board, as shown in Figure 6.
The CN-0233 Evaluation Software is used to capture the data
from the EVAL-CN0233-SDPZ circuit board using the setup
seen in Figure 6. Details regarding the use of the software can
be found in the CN-0233 Software User Guide.
The DNL, noise data, and actual FSR error were obtained by
inputting the DAC data to the EVAL-CN0233-SDPZ evaluation
board using EVAL-SDP-CB1Z board connected to the PC and
reading the voltage or current output results from the 3485
multimeter. The GPIB/USB interface was used to transfer the
data to the PC for analysis. The CN0233 Evaluation Software
was used to generate the data to the DAC.
Rev. 0 | Page 3 of 5
CN-0233
Circuit Note
EVAL-CN0233-SDPZ
6V
SUPPLY
P2
EVAL-SDP-CB1Z
120-PINS
CON A OR
CON B
SDP
IOUT
VOUT
GPIB
MULTIMETER
USB
PC
10126-006
USB
10126-007
Figure 6. Functional Block Diagram of Test Setup Showing Evaluation Board Connections
Figure 7. EVAL-CN0233-SDPZ PC Board Photo
Rev. A | Page 4 of 5
Circuit Note
CN-0233
LEARN MORE
Data Sheets and Evaluation Boards
CN-0233 Design Support Package:
http://www.analog.com/CN0233-DesignSupport
EVAL-CN0233-SDPZ
CN-0065 Circuit Note, 16-Bit Fully Isolated Output Module
Using the AD5422 Single Chip Voltage and Current Output
DAC and the ADuM1401 Digital Isolator, Analog Devices.
AD5422 Data Sheet
EVAL-SDP-CB1Z
AD5422 Evaluation Board (EVAL-AD5422EBZ)
Cantrell, Mark. AN-0971 Application Note, Recommendations
for Control of Radiated Emissions with isoPower Devices.
Analog Devices.
Chen, Baoxing. 2006. iCoupler® Products with isoPower™
Technology: Signal and Power Transfer Across Isolation
Barrier Using Microtransformers. Analog Devices.
ADuM3471 Data Sheet
ADuM3471 Evaluation Board (EVAL-ADuM3471EBZ)
UG-197 User Guide for ADuM3471 Evaluation Board
ADR445 Data Sheet
REVISION HISTORY
3/14—Rev. 0 to Rev. A
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.
Slattery, Colm, Derrick Hartmann, and Li Ke. “PLC Evaluation
Board Simplifies Design of Industrial Process Control
Systems.” Analog Dialogue (April 2009).
Changes to Circuit Function and Benefits Section and Figure 1 ..... 1
Changes to Circuit Description Section and Figure 2; Added Test
Data and Results Section and Figure 4, Renumbered
Sequentially; Replaced Figure 3 ...................................................... 2
Added Figure 5; Changes to Circuit Evaluation and Test
Section, Equipment Used to Collect Test Data Section, and
Setup and Test Section ...................................................................... 3
Changes to Figure 6; Added Figure 7 ............................................. 4
10/11—Revision 0: Initial Version
Wayne, Scott. iCoupler® Digital Isolators Protect RS-232, RS-485,
and CAN Buses in Industrial, Instrumentation, and Computer
Applications. Analog Dialogue (October 2005).
Ardizzoni, John. A Practical Guide to High-Speed Printed-CircuitBoard Layout, Analog Dialogue 39-09, September 2005.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of “AGND” and “DGND”, Analog Devices.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
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CN10126-0-3/14(0)
Rev. 0 | Page 5 of 5
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