CN-0194: Galvanically Isolated, 2-Channel, 16-Bit, Simultaneous Sampling,...

Circuit Note
CN-0194
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/CN0194.
AD7685
16-Bit, 250 kSPS PulSAR® ADC
ADuM1402
Quad-Channel Digital Isolator
ADR391
2.5 V, Micropower, Low Noise Precision
Voltage Reference with Shutdown
AD8615
Precision, 20 MHz, CMOS Rail-to-Rail
Input/Output Op Amp
Galvanically Isolated, 2-Channel, 16-Bit, Simultaneous Sampling,
Daisy-Chained Data Acquisition System
EVALUATION AND DESIGN SUPPORT
CIRCUIT FUNCTION AND BENEFITS
Circuit Evaluation Boards
CN-0194 Circuit Evaluation Board (EVAL-CN0194-SDPZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
This circuit, shown in Figure 1, provides galvanic isolation for
high speed, high accuracy, simultaneous sampling analog-todigital conversion applications. The 16-bit AD7685 PulSAR
ADC is versatile and allows monitoring of multiple channels
4.5V REF
±10V INPUT
AIN1
5V
10µF
R1
1kΩ
R2
4.42kΩ
5V
100nF
REF VDD VIO
33Ω
U4
1.84V REF
AD7685
IN+
2.7nF
IN– GND
5V
100nF
SDO
SCK
CNV
SDI
VDD1 , VE1
VDD2 , VE2
GND1
GND2
VIA
VOA
VIB
VOB
VOC
VIC
VOD
VID
3.3V
100nF
SDP BOARD
DATA
RSCLK
AD8615
4.5V REF
±10V INPUT
AIN2
5V
33Ω
1.84V REF
AD7685
IN+
2.7nF
IN– GND
5V
100nF
SDO
SCK
CNV
SDI
AD8615
R5
1.24kΩ
ADuM1402C
100nF
REF VDD VIO
U14
TFS
5V
10µF
R19
1kΩ
R20
4.42kΩ
TSCLK
R4
1kΩ
VDD1 , VE1
VDD2 , VE2
GND1
GND2
VIA
VOA
VIB
VOB
VOC
VIC
VOD
VID
3.3V
100nF
RFS
5V
U13
4.5V REF
ADR391
5V
IN OUT
GND
1kΩ
R3
10µF
ADuM1402C
1.84V REF
2.80kΩ
R6
09612-001
AD8615
100nF
Figure 1. Galvanically Isolated, 2-Channel, Simultaneous Sampling, 16-Bit Analog-to-Digital Conversion System with Daisy Chain
(Simplified Schematic: All Connections and Decoupling Not Shown)
Rev.0
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CN-0194
Circuit Note
through daisy chaining. An input circuit based on the AD8615
op amp level shifts, attenuates, and buffers a ±10 V industrial
signal to match the input requirements of the ADC. The flexible
circuit includes a precision ADR391 reference and two quadchannel ADuM1402 digital isolators to provide a compact and
cost effective solution to a popular industrial data acquisition
application.
CIRCUIT DESCRIPTION
The AD7685 is a 16-bit, charge redistribution successive
approximation (SAR) analog-to-digital converter (ADC) that
operates from a single VDD power supply from 2.3 V to 5.5 V. It
contains a low power, high speed, 16-bit sampling ADC with no
missing codes, no pipeline delay, an internal conversion clock,
and a versatile serial interface port. The ADC also contains a
low noise, wide bandwidth, short aperture delay, track-and-hold
circuit. On the CNV rising edge, it samples an analog input
(IN+) between 0 V to VREF with respect to the ground sense
(IN−). The reference voltage, VREF, is applied externally and is
set from 0.5 V to VDD. The circuit of Figure 1 uses a 4.5 V
reference voltage.
Power dissipation of the AD7685 scales linearly with sampling
rate. Power dissipation is 15 mW maximum for VDD = 5 V and
a sampling rate of 250 kSPS. The AD7685 is housed in a 10-lead
MSOP or a 10-lead QFN (LFCSP) with operation specified
from −40°C to +85°C. The SPI-compatible serial interface also
features the ability, using the SDI input, to either daisy chain
several ADCs on a single 3-wire bus or provide an optional
BUSY indicator. It is compatible with 1.8 V, 2.5 V, 3 V, or 5 V
logic using the separate VIO supply pin.
The complete analog signal chain runs on a single 5 V supply.
The ADR391 low dropout 2.5 V reference and the U13 rail-torail CMOS AD8615 op amp develop the 4.5 V reference voltage
for the ADCs. A 4.5 V reference gives 0.5 V headroom at the
output of U13 so that the op amp remains in the linear region
for nominal variations in the 5 V supply. The ADR391 2.5 V
output is amplified by the noise gain of U13, which is 1 + R4/R5.
For the R4 and R5 values chosen, the noise gain is 1.8, and the
reference voltage is 1.8 × 2.5 V = 4.5 V.
The U4 and U14 AD8615 op amps provide a signal gain of
0.225 (set by the ratio of R1 to R2 and R19 to R20), which
reduces the amplitude of the 20 V p-p input signals to 4.5 V p-p
at the inputs of the ADCs. A 2.25 V offset is required at the
outputs of U4 and U14 for a 0 V input. This requires a commonmode voltage of 1.84 V at the noninverting input of U4 and
U14. This voltage is developed by the resistor divider R3-R6.
The R-C network on the output of U4 and U14 (33 Ω, 2.7 nF)
forms a single-pole noise filter with a bandwidth of 1.8 MHz.
The AD8615 is a CMOS rail-to-rail input and output, singlesupply amplifier featuring very low offset voltage, wide signal
bandwidth, and low input voltage and current noise. The part
uses a patented DigiTrim® trimming technique that achieves
superior precision without laser trimming. The AD8615 is fully
specified to operate from 2.7 V to 5 V single supplies.
The combination of greater than 20 MHz bandwidth, low
offset, low noise, and low input bias current makes this
amplifier useful in a wide variety of applications. Filters,
integrators, photodiode amplifiers, and high impedance sensors
all benefit from the combination of performance features.
AC applications benefit from the wide bandwidth and low
distortion. The AD8615 offers the highest output drive
capability of the DigiTrim family, which is excellent for audio
line drivers and other low impedance applications.
Applications for the parts include portable and low powered
instrumentation, audio amplification for portable devices,
portable phone headsets, barcode scanners, and multipole
filters. The ability to swing rail-to-rail at both the input and
output enables designers to buffer CMOS ADCs, DACs, ASICs,
and other wide output swing devices in single-supply systems.
The ADR391 micropower, low dropout voltage reference
provides a stable output voltage from a minimum supply of
300 mV above the output. Its advanced design eliminates the
need for external capacitors, which further reduces board space
and system cost. The combination of low power operation,
small size, and ease of use makes the ADR391 precision voltage
reference ideally suited for battery-operated applications. Using
patented temperature drift curvature correction techniques
from Analog Devices, the ADR391 reference achieves a low
9 ppm/°C of temperature drift in a TSOT package.
The ADuM1402 is a quad-channel digital isolator based on
Analog Devices’ iCoupler® technology. Combining high speed
CMOS and monolithic air core transformer technology, this
isolation component provides outstanding performance characteristics superior to alternatives such as optocoupler devices.
By avoiding the use of LEDs and photodiodes, iCoupler devices
remove the design difficulties commonly associated with optocouplers. The typical optocoupler concerns regarding uncertain
current transfer ratios, nonlinear transfer functions, and
temperature and lifetime effects are eliminated with the simple
iCoupler digital interfaces and stable performance characteristics.
The need for external drivers and other discrete components is
eliminated with these iCoupler products. Furthermore,
iCoupler devices consume one-tenth to one-sixth of the power
of optocouplers at comparable signal data rates.
The ADuM1402 isolators provides four independent isolation
channels with 2/2 directionality and multiple data rates up to
90 Mbps for the C grade (see the data sheet Ordering Guide).
All models operate with the supply voltage on either side
ranging from 2.7 V to 5.5 V, providing compatibility with lower
Rev. 0 | Page 2 of 5
Circuit Note
CN-0194
AIN1
60,000
50,000
40,000
30,000
20,000
10,000
AIN2
0
0
2,000
4,000
6,000
8,000 10,000 12,000 14,000
SAMPLE NUMBER
Figure 2. 15,000 Samples of Two Separate ±10V Input Signals (AIN1,AIN2),
90° Out of Phase, Applied to AIN1 and AIN2 Sampled at 250 kSPS on the
EVAL-CN0194-SDPZ Board in Daisy Chain Mode
09612-003
Two ±10 V signals 90° out of phase were applied to the two
channels (AIN1 and AIN2) of the EVAL-CN0194-SDPZ board.
The results of their conversion are seen in Figure 2, which was
obtained using the supplied evaluation software. The supplied
evaluation software can also be used to view FFT data (Figure 3)
as well as a histogram for codes at a fixed dc level (Figure 4).
A complete design support package for this circuit note can be
found at www.analog.com/CN0194-DesignSupport.
09612-002
Figure 1 shows how the AD7685’s are daisy-chained to reduce
the number of signals requiring isolation. Note that the RSCLK
and RFS are readbacks of the AD7685’s serial clock (SCK) and
serial frame sync (CNV), respectively. These readback signals
need to have a very short skew with respect to the DATA signal.
This skew is the channel-to-channel matching propagation
delay of the digital isolator (tPSKCD), which is less than 2 ns for
the ADuM1402C. This allows running the serial interface at the
maximum speed of the digital isolator (90 Mbps for the
ADuM1402C), which corresponds to a maximum serial clock
frequency of 90 MHz. This might have been otherwise limited
by the cascade of the propagation delays of the digital isolators
if the delays were too long. In the circuit, the TSCLK frequency
is 30 MHz for a sampling frequency of 250 kSPS.
The performance of this or any high speed circuit is highly
dependent on proper PCB layout. This includes, but is not
limited to, power supply bypassing, controlled impedance lines
(where required), component placement, signal routing, and
power and ground planes. (See MT-031 Tutorial, MT-101 Tutorial,
and the article, A Practical Guide to High-Speed Printed-CircuitBoard Layout, for more detailed information regarding PCB layout.)
OUTPUT CODE
voltage systems, as well as enabling a voltage translation
functionality across the isolation barrier. In addition, the
ADuM1402 provides low pulse width distortion (<2 ns) and
tight channel-to-channel matching (<2 ns). Unlike other
optocoupler alternatives, the ADuM1402 isolators have a
patented refresh feature that ensures dc correctness in the
absence of input logic transitions and when power is not applied
to one of the supplies.
Figure 3. FFT with a Kaiser Window (Parameter = 20), 20 kHz Input, 250 kSPS Sampling Rate
Rev. 0 | Page 3 of 5
CN-0194
Circuit Note
•
200,000
• Power supply: +6 V, or +6 V “wall wart”
180,000
• Low distortion signal source, such as Audio Precision System
Two 2322
NUMBER OF OCCURENCES
160,000
140,000
Getting Started
120,000
40,000
Load the evaluation software by placing the CN0194 Evaluation
Software disc in the CD drive of the PC. Using "My Computer,"
locate the drive that contains the evaluation software disc and
open the Readme file. Follow the instructions contained in the
Readme file for installing and using the evaluation software.
20,000
Functional Block Diagram
100,000
80,000
60,000
33,025
33,026
33,027 33,028 33,029
ADC OUTPUT CODE
33,030
09612-004
0
33,031
Figure 4. Histogram of a DC Input at the Code Center for 390,000 Samples
Setup
COMMON VARIATIONS
Other pin-compatible 16-bit ADCs in the PulSAR family are
available at higher sampling rates: AD7686 (500 kSPS), AD7980
(1 MSPS), and AD7983 (1.33 MSPS).
If 18-bit resolution is needed, the following are also
pin-compatible members of the PulSAR family: AD7691
(250 kSPS), AD7690 (500 kSPS), AD7982 (1 MSPS), and
AD7984 (1.33 MSPS).
The AD8615 op amp is also available in dual (AD8616) and
quad (AD8618) package versions. Other possible driver op
amps are the ADA4841-1 (single), ADA4841-2 (dual), and
ADA4941-1 (dual differential).
This circuit uses the EVAL-CN0194-SDPZ circuit board and the
EVAL-SDP-CB1Z System Demonstration Platform (SDP)
evaluation board. The two boards have 120-pin mating
connectors, allowing for the quick setup and evaluation of the
circuit’s performance. The EVAL-CN0194-SDPZ board contains
the circuit to be evaluated, as described in this note, and the
SDP evaluation board is used with the CN0194 evaluation
software to capture the data from the EVAL-CN0194-SDPZ
circuit board.
• PC with a USB port and Windows® XP, Windows Vista
(32-bit), or Windows 7 (32-bit)
• EVAL-CN0194-SDPZ circuit evaluation board
• EVAL-SDP-CB1Z SDP evaluation board
Connect the 120-pin connector on the EVAL-CN0194-SDPZ
circuit board to the connector marked “CON A” on the
EVAL-SDP-CB1Z evaluation (SDP) board. Nylon hardware
should be used to firmly secure the two boards, using the holes
provided at the ends of the 120-pin connectors. With power to
the supply off, connect a +6 V power supply to the pins marked
“+6V CFTL” and “+6 V SDP” on the board. If available, a +6 V
"wall wart" can be connected to the barrel jack connector on the
board and used in place of the +6 V power supply. Connect the
USB cable supplied with the SDP board to the USB port on the PC.
Note: Do not connect the USB cable to the mini USB connector
on the SDP board at this time.
Test
CIRCUIT EVALUATION AND TEST
Equipment Needed
See Figure 1 of this circuit note for the circuit block diagram,
and the PDF file “EVAL-CN0194-SDPZ-SCH" for the circuit
schematics. This file is contained in the CN0194 Design
Support Package.
Apply power to the +6 V supply (or “wall wart”) connected to
the EVAL-CN0194-SDPZ circuit board. Launch the evaluation
software, and connect the USB cable from the PC to the USB
mini connector on the SDP board. The software will be able to
communicate to the SDP board if the Analog Devices System
Development Platform driver is listed in the Device Manager.
Once USB communications are established, the SDP board
can be used to send, receive, and capture serial data from the
EVAL-CN0194-SDPZ board.
The data in this circuit note were generated using an Audio
Precision System Two 2322 signal source and a GlobTek power
supply. The signal source was set to the frequencies indicated in
the graphs.
Information regarding the SDP board can be found at
www.analog.com/SDP.
• CN-0194 evaluation software
Rev. 0 | Page 4 of 5
Circuit Note
CN-0194
LEARN MORE
CN-0194 Design Support Package:
www.analog.com/CN0194-DesignSupport
Data Sheets and Evaluation Boards
SDP User Guide
System Demonstration Platform (EVAL-SDP-CB1Z)
Ardizzoni, John. “A Practical Guide to High-Speed PrintedCircuit-Board Layout,” Analog Dialogue. 39-09, September
2005.
AD7685 Data Sheet
CN-0194 Circuit Evaluation Board (EVAL-CN0194-SDPZ)
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of “AGND” and “DGND”, Analog Devices.
AD7685 Evaluation Board
ADuM1402 Data Sheet
AD8615 Data Sheet
MT-073 Tutorial, High Speed Variable Gain Amplifiers (VGAs),
Analog Devices.
ADR391 Data Sheet
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
REVISION HISTORY
9/11—Revision 0: Initial Version
(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
may use the Circuits from the Lab circuits in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by
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©2011 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN09612-0-9/11(0)
Rev. 0 | Page 5 of 5
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