Circuit Note CN-0185
Circuit Note
CN-0185
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/CN0185.
AD7400A
Isolated Sigma-Delta Modulator
ADuM5000
Isolated DC-to-DC Converter
AD8646
Dual, 24 MHz, Rail-to-Rail, I/O Op Amp
ADP121
150 mA, Low Quiescent Current, CMOS
Linear Regulator
ADP3301
High Accuracy, 100 mA, Low Dropout
Linear Regulator
ADG849
3 V/5 V CMOS 0.5 Ω SPDT/2:1 Mux in SC70
ADR443
Ultralow Noise, LDO XFET® 3.0 V Voltage
Reference
A Novel Analog-to-Analog Isolator Using an Isolated Sigma-Delta Modulator,
Isolated DC-to-DC Converter, and Active Filter
EVALUATION AND DESIGN SUPPORT
The circuit is based on the AD7400A, a second-order, sigma-delta
(Σ-Δ) modulator with a digitally isolated 1-bit data stream output.
The isolated analog signal is recovered with a fourth-order active
filter based on the dual, low noise, rail-to-rail AD8646 op amp.
With the ADuM5000 as the power supply for the isolated side,
the two sides are completely isolated and use only one power
supply for the system. The circuit has 0.05% linearity and benefits
from the noise shaping provided by the modulator of the AD7400A
and the analog filter. The applications of the circuit include
motor control and shunt current monitoring, and the circuit is
also a good alternative to isolation systems based on optoisolators.
Circuit Evaluation Boards
CN-0185 Circuit Evaluation Board (EVAL-CN0185-EB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 is a complete low cost
implementation of an analog-to-analog isolator. The circuit
provides isolation of 2500 V rms (1 minute per UL 1577).
+5V
+5V_ISO
VISO
GNDISO
VSEL
VISO
GNDISO
VDD1
GND1
RCIN
RCOUT
RCSEL
VDD1
GND1
OUT
IN
GND
SD
POWER IN
+5.5V TO +12V
ADP3301-5
+3VREF
OUT
ADuM5000
IN
VOUT VIN
GND_ISO
GND
GND
EN
ADR443
ADP121-3.3
VIN+
22Ω
1%
0.1µF
VIN–
22Ω
1%
VDD1
VDD2
VIN+
MDAT
VIN– MCLKOUT
GND1
+3VREF
ADG849
GND2
AD7400A
GND_ISO
24kΩ
1%
24kΩ
1%
100pF
5%
1/2
+5V
AD8646
51kΩ
1%
51kΩ
1%
100pF
5%
1/2
AD8646
VOUT
8.2pF
5%
22Ω
1%
22Ω
1%
09499-001
68pF
5%
Figure 1. Analog Isolator Using AD7400A (Simplified Schematic: All Connections and Decoupling Not Shown
Rev. B
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CN-0185
Circuit Note
CIRCUIT DESCRIPTION
Analog Active Filter Design
A block diagram of the circuit is shown in Figure 1. The analog
input is sampled at 10 MSPS by the AD7400A Σ-Δ modulator.
The 22 Ω resistors and 0.1 µF capacitor form a differential input
noise reduction filter with a cutoff frequency of 145 kHz. The
output of the AD7400A is an isolated 1-bit data stream. The
quantization noise is shaped by a second-order Σ-Δ modulator,
which shifts the noise to the higher frequencies (see the MT-022
Tutorial).
The cutoff frequency of the low-pass filter mostly depends on
the desired bandwidth of the circuit. There is a trade-off between
the cutoff frequency and noise performance, and there is more
noise if the cutoff frequency of the filter increases. This is especially
true in this design because the Σ-Δ modulator shapes the noise
and moves a large portion into the higher frequencies. The
cutoff frequency in this design is 100 kHz.
The signal is then filtered by an active filter whose order is higher
than the order of the modulator. A fourth-order Chebyshev filter is
used for better noise attenuation. Compared to other filter
responses (Butterworth or Bessel), the response of the
Chebyshev provides the steepest rolloff for a given filter order.
The filter is implemented using the dual AD8646, a rail-to-rail,
input and output, low noise, single-supply op amp.
The ADuM5000 is an isolated dc-to-dc converter based on
Analog Devices, Inc., iCoupler® technology. It is used for the
power supply to the isolated side of the circuit containing the
AD7400A. The isoPower® technology of the ADuM5000 uses
high frequency switching elements to transfer power through a
chip-scale transformer.
The circuit must be constructed on a multilayer printed circuit
board (PCB) with a large area ground plane. Proper layout,
grounding, and decoupling techniques must be used to achieve
optimum performance (see the MT-031 Tutorial, Grounding Data
Converters and Solving the Mystery of "AGND" and "DGND," the
MT-101 Tutorial, Decoupling Techniques, and the ADuC7060/
ADuC7061 evaluation board layout). Ensure that the PCB layout
meets the emissions standards and isolation requirements between
the two isolated sides (see the AN-0971 Application Note).
In order not to overdrive the AD8646, the input signal must be
lower than the power supply (5 V). The output of the AD7400A
is a stream of 1s and 0s with an amplitude equal to the
AD7400A VDD2 supply voltage. Therefore, the VDD2 digital
supply is 3.3 V supplied by the ADP121 linear regulator.
Alternatively, if a 5 V supply is used for VDD2, attenuate the
digital output signal before connecting to the active filter. In
either case, well regulate the supply because the final analog
output is directly proportional to VDD2.
The fourth-order filter is made up of two second-order filters,
with a Sallen-Key structure. The Analog Filter Wizard and the
NI Multisim were used to design the filter. The parameters used
include the following:
• Filter type = low-pass, Chebyshev, 0.01 dB ripple
• Order = 4
• fC = 100 kHz, Sallen-Key (updated format for clarity)
The recommended values generated by the program were
used with the exception of the feedback resistors, which were
reduced to 22 Ω.
Measurements
The circuit has a gain of 4.6875 and an output offset voltage of
1.5 V. A differential signal of 0 V results in a digital bit stream of
1s and 0s, where each occurs 50% of the time. The ADR443 output
is 3.0 V; therefore, after filtering, there is a 1.5 V dc offset. A
differential input of 320 mV ideally results in a stream of all 1s,
which after filtering, yields a 3.0 V DC output. Therefore, the
effective gain of the circuit is
GAIN = (3.0 − 1.5)/0.32 = 4.6875
From the measurements, the actual measured offset is 1.504V , and
the gain is 4.69. The dc transfer function of the system is shown
in Figure 2. Linearity was measured as 0.0465%.
The 5 V supply for the circuit in Figure 1 is supplied from an
ADP3301 5 V linear regulator, which accepts an input voltage
of 5.5 V to 12 V.
2.9
2.7
OUTPUT VOLTAGE (V)
To reconstruct the analog input signal, follow the data stream by
an ADG849 switch connected to a 3 V ADR443 reference to
stabilize the peak-to-peak output of the MDAT.
For a given cutoff frequency, the smaller the transition band of
the filter is, the smaller the noise is that passes through the filter.
Of all the filter responses (Butterworth, Chebyshev, Bessel, and
so on), the Chebyshev filter was chosen for this design because it
has a smaller transition band for a given filter order. However,
this smaller transition band comes at the expense of a slightly
worse transient response.
2.5
2.3
2.1
1.9
1.5
0
50
100
150
200
INPUT VOLTAGE (mV)
Figure 2. System DC Transfer Function
Rev. B | Page 2 of 4
250
09499-002
1.7
Circuit Note
CN-0185
CIRCUIT EVALUATION AND TEST
Figure 3 shows the output voltage with no dc offset voltage
vs. the input frequency. The input signal voltage is 40 mV p-p,
which causes an output signal of 40 × 4.6875= 190 mV p-p.
Note that there is approximately 10 mV of peaking in the
frequency response function, corresponding to about 0.42 dB.
The circuit can be easily evaluated using a signal generator and an
oscilloscope after powering on the circuit with a 6 V power supply.
Equipment Needed (Equivalents Can Be Substituted)
The system has good noise performance, with a noise density of
2.50 µV/√Hz at 1 kHz and 1.52 µV/√Hz at 10 kHz.
A complete design support package for this circuit note can be
found at www.analog.com/CN0185-DesignSupport.
100
120
0
80
–100
40
–200
0
100
1k
10k
FREQUENCY (Hz)
100k
1M
Setup and Test
The block diagram of the linearity measurement setup is shown in
Figure 4. Connect the 6 V power supply to the EVAL-CN0185-EB1Z
power terminal.
The dc input voltage is generated with the Fluke 5700A, and the
Agilent 3458A DVM is used to measure the output. The dc output
from the Fluke 5700A is stepped, and the data recorded with a
1 mV increase from 1 mV to 250 mV.
DC POWER
SUPPLY
+6V
Figure 3. Frequency and Phase Response of the Circuit for a
40 mV Peak-to-Peak Input Signal
DC SOURCE
FLUKE 5700A
COMMON VARIATIONS
The circuit can be used for isolated voltage monitoring and for
current sensing applications where the voltage across a shunt
resistor is monitored. The requirements of the input signal for
the system are detailed in the AD7400A data sheet.
If the ADuM6000 is used instead of the ADuM5000, the entire
circuit is rated to 5 kV.
The ADP1720 or ADP7102 linear regulator can be used as a
substitute for the ADP3301, if desired.
GND
1mV TO
250mV
EVAL-CN0185-EB1Z
DIGITAL
MULTIMETER
AGILENT 3458
09499-004
160
A multifunction calibrator (dc source), Fluke 5700A
A digital multimeter, Agilent 3458A, 8.5 digits
A spectrum analyzer, Agilent 4396B
A function generator, Agilent 33250A
A power supply, 6 V
Figure 4. Test Setup for Measuring Linearity
To measure the frequency response, connect the equipment as
shown in Figure 5. Set the 33250A function generator for a
40 mV peak-to-peak sine-wave output with a 0 dc offset. Then,
sweep the frequency of the signal from 100 Hz to 500 kHz and
record the data using the Agilent 4396B spectrum analyzer.
DC POWER
SUPPLY
+6V
SIGNAL
GENERATOR
AGILENT
33250A
GND
100Hz TO
500kHz
EVAL-CN0185-EB1Z
SPECTRUM
ANALYZER
AGILENTY 4396B
Figure 5. Test Setup for Measuring Frequency Response
Rev. B | Page 3 of 4
09499-005
200
PHASE (Degrees)
200
•
•
•
•
•
09499-003
P-P OUTPUT VOLTAGE (mV)
240
The following equipment is needed:
CN-0185
Circuit Note
LEARN MORE
Data Sheets and Evaluation Boards
CN0185 Design Support Package:
http://www.analog.com/CN0185-DesignSupport
AD7400A Data Sheet
ADIsimPower™ Design Tool, Analog Devices.
ADuM5000 Data Sheet
Analog Filter Wizard Design Tool, Analog Devices.
ADuM5000 Evaluation Board
Cantrell, Mark. Application Note AN-0971, Recommendations
for Control of Radiated Emissions with isoPower Devices.
Analog Devices.
AD8646 Data Sheet
AD7400A Evaluation Board
Chen, Baoxing, John Wynne, and Ronn Kliger. High Speed
Digital Isolators Using Microscale On-Chip Transformers,
Analog Devices, 2003.
ADP121 Data Sheet
ADP3301 Data Sheet
ADG849 Data Sheet
ADR443 Data Sheet
Chen, Baoxing. iCoupler® Products with isoPower™ Technology:
Signal and Power Transfer Across Isolation Barrier Using
Microtransformers, Analog Devices, 2006.
Chen, Baoxing. “Microtransformer Isolation Benefits Digital
Control.” Power Electronics Technology. October 2008.
Ghiorse, Rich. Application Note AN-825, Power Supply
Considerations in iCoupler® Isolation Products,
Analog Devices.
Krakauer, David. “Digital Isolation Offers Compact, LowCost Solutions to Challenging Design Problems.”
Analog Dialogue. Volume 40, December 2006.
MT-022 Tutorial, ADC Architectures III: Sigma-Delta ADC Basics,
Analog Devices.
REVISION HISTORY
9/13—Rev. A to Rev. B
Added ADR443.............................................................Throughrout
Changes to Devices Connected/Referenced Section and
Figure 1 ...............................................................................................1
Changes to Circuit Description Section, Measurements Section,
and Figure 2 ........................................................................................2
Changes to Figure 3 ...........................................................................3
Change to Data Sheets and Evaluation Boards Section ...............4
4/13—Rev. 0 to Rev. A
Changes to Figure 1 ...........................................................................1
Changes to Figure 3 and Common Variations Section ................3
4/11—Revision 0: Initial Version
MT-023 Tutorial, ADC Architectures IV: Sigma-Delta ADC
Advanced Concepts and Applications, Analog Devices.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND," Analog Devices.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
USB 2.0 Specifications, USB Implementers Forum, Inc.
Wayne, Scott. “iCoupler® Digital Isolators Protect RS-232, RS-485,
and CAN Buses in Industrial, Instrumentation, and
Computer Applications.” Analog Dialogue. Volume 39,
October 2005.
Zumbahlen, Hank. Application Note AN-649, Using the
Analog Devices Active Filter Design Tool, Analog Devices.
(Continued from first page) Circuits from the Lab circuits are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you
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application or use of the Circuits from the Lab circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab circuits are supplied
"as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular
purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices
reserves the right to change any Circuits from the Lab circuits at any time without notice but is under no obligation to do so.
©2011–2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN09499-0-9/13(B)
Rev. B | Page 4 of 4
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