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Circuit Note
CN-0183
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/CN0183.
AD5668
16-Bit Voltage Output denseDAC with
5 ppm/°C On-Chip Reference and SPI
Interface
AD8638
16 V Auto-Zero, Rail-to-Rail Output Op Amp
ADP2300
1.2 A, 20 V, 700 kHz, Nonsynchronous StepDown Switching Regulator
REF192
Precision Micropower, 2.5 V Low Dropout
Voltage Reference
Precision 16-Bit, Bipolar Output Voltage Source with +12 V to ±5 V Supply
EVALUATION AND DESIGN SUPPORT
of the AD8638 is only 0.06 ppm/°C. The external REF192
reference ensures a maximum drift of 5 ppm/°C (E grade) and
provides a low impedance pseudo ground for the AD8638 level
gain and shifting circuit.
Circuit Evaluation Boards
CN-0183 Circuit Evaluation Board (EVAL-CN0183-SDZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
The circuit offers an efficient solution to a problem often
encountered in systems with a single +12 V supply rail. Proper
printed circuit board (PCB) layout and grounding techniques
ensure that the ADP2300 switching regulator does not degrade
the overall performance of the circuit.
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 provides a precision 16-bit, low
drift bipolar voltage output of ±2.5 V and operates on a single
+10 V to +15 V supply. The unipolar voltage outputs of the
AD5668 octal denseDAC are amplified and level shifted by the
AD8638 auto-zero op amps. The maximum drift contribution
+5V
VIN = +12V
REF192
+5V
L1
100nF
+
1µF
+
+
ADP2300
BST
VS
+5V
FB
EN
0.1µF
10µF
1µF
10µF
10µF
VIN
+2.5V
R1
10kΩ
1.5kΩ
SW
GND
OUTPUT
GND
+5V
0.1µF
52.3kΩ
10kΩ
+
VDD
+
4.7µF
10kΩ
R2
10kΩ
10µF
SYNC
1.5kΩ
SCLK
DIN
VREFIN/VREFOUT
TP1
DAC_OUT
AD8638
DAC A
VOUTA
TP2
BIPOLAR_OUT
−2.5V TO +2.5V
0V TO +2.5V
–5V
AD5668
–5V
LDAC
CLR
DAC H
GND
VOUTH
09471-001
L1: COILCRAFT, LPD4012-472MLB,
COUPLED INDUCTOR, 4.7µH
Figure 1. Bipolar Output DAC Circuit with ±5 V Power Supplies
Rev. A
Circuits from the Lab™ circuits from Analog Devices have been designed and built by Analog Devices
engineers. Standard engineering practices have been employed in the design and construction of
each circuit, and their function and performance have been tested and verified in a lab environment at
room temperature. However, you are solely responsible for testing the circuit and determining its
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whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page)
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CN-0183
Circuit Note
CIRCUIT DESCRIPTION
0.6
0.4
0.2
DNL (LSB)
–0.2
–0.4
The output voltage of the AD5668 is 0 V to 2.5 V at TP1, and
this signal drives the noninverting input of the AD8638 op amp.
The signal gain of the op amp is 1 + R2/R1, which is 2 for R1 = R2.
A negative 2.5 V offset is injected into the op amp output by
driving R1 with the 2.5 V reference. The result is a bipolar
output voltage at TP2 that swings from −2.5 V to +2.5 V.
–0.8
0
10000
20000
30000
40000
DAC CODE
50000
60000
09471-003
–0.6
Figure 3. DNL Performance of Bipolar Output (TP2)
8
6
4
INL (LSB)
The circuit operates on a single supply voltage of nominally 12 V,
which can vary between 10 V and 15 V. The regulated −5 V
supply rail is developed from an ADP2300 switching regulator
connected in the inverting buck-boost configuration. The circuit
can be designed using the ADIsimPower program available at
www.analog.com/ADIsimPower. The L1 coupled inductor is
used to develop an unregulated 5 V supply for the circuit using
a Zeta configuration. This circuit yields high efficiency for small
output currents.
0
2
0
–2
The integral nonlinearity (INL) and differential nonlinearity
(DNL) measured at TP2 (bipolar output) are shown in Figure 2
and Figure 3, respectively.
–4
–6
0
The INL and DNL measured at TP1 (unipolar DAC output) are
shown in Figure 4 and Figure 5, respectively.
10000
20000
40000
30000
DAC CODE
50000
60000
09471-004
The AD5668 is a 16-bit, octal, voltage output denseDAC
controlled by an SPI interface. It contains an on-chip reference
with a 10 ppm/°C maximum drift specification. The on-chip
reference is off at power-up, allowing the use of an external
reference. The internal reference is enabled via a software write.
In the circuit shown in Figure 1, an external REF192 is used
because a low output impedance is required to drive the 2.5 V
pseudo ground reference for the AD8638 op amps.
Figure 4. INL Performance of Unipolar DAC Output (TP1)
0.8
6
0.6
5
0.4
3
0.2
DNL (LSB)
4
1
0
0
–0.2
–0.4
–1
–2
–0.6
–3
–0.8
–1.0
–5
0
10000
20000
30000
40000
DAC CODE
50000
60000
0
10000
20000
40000
30000
DAC CODE
50000
60000
Figure 5. DNL Performance of Unipolar DAC Output (TP1)
Figure 2. INL Performance of Bipolar Output (TP2)
Rev. A | Page 2 of 5
09471-005
–4
09471-002
INL (LSB)
2
Circuit Note
CN-0183
COMMON VARIATIONS
Getting Started
The AD5628 and AD5648 are 12-bit and 14-bit versions of the
AD5668. All have an on-chip reference with an internal gain of 2.
The AD5628-1/AD5648-1/AD5668-1 have a 1.25 V, 5 ppm/°C
reference, giving a full-scale output range of 2.5 V; and the
AD5628-2/AD5648-2/AD5668-2 and AD5668-3 have a 2.5 V,
5 ppm/°C reference, giving a full-scale output range of 5 V. The
on-board reference is off at power-up, allowing the use of an
external reference. The internal reference is enabled via a software
write. The part incorporates a power-on-reset circuit that ensures
that the DAC output powers up to 0 V (AD5628-1/AD5648-1/
AD5668-1, AD5628-2/AD5648-2/AD5668-2) or midscale
(AD5668-3) and remains powered up at this level until a valid
write takes place.
Load the evaluation software by placing the CN-0183 evaluation
software CD in the CD drive of the PC. Using My Computer,
locate the drive that contains the evaluation software CD and
open the Readme file. Follow the instructions contained in the
Readme file for installing and using the evaluation software.
The evaluation software main window is shown in Figure 6.
09471-006
The AD8639 is a dual version of the AD8638 and can be used, if
desired. The circuit in Figure 1 uses the single AD8638 to
minimize crosstalk between the eight channels.
Other 2.5 V references can be used, such as the ADR4525, which
has an accuracy of ±0.02% and a temperature coefficient of
2 ppm/°C maximum (B grade).
Figure 6. Evaluation Software Main Window
Functional Diagram of Test Setup
A functional diagram of the test setup is shown in Figure 7. This
setup allows the DAC output (TP1) and the bipolar output (TP2) to
be observed with an oscilloscope.
CIRCUIT EVALUATION AND TEST
Equipment Needed (Equivalents Can Be Substituted)
The following equipment is needed:
•
The System Demonstration Platform (EVAL-SDP-CB1Z)
•
The CN-0183 circuit evaluation board (EVAL-CN0183-SDZ)
•
The CN-0183 evaluation software
•
The Tektronix TDS2024, 4-channel oscilloscope
•
The HP E3630A 0 V to 6 V/2.55 A; ±20 V/0.5 A triple
output dc power supply
•
A PC (Windows 32-bit or 64-bit)
Linearity measurements require a precision, digital voltmeter
(DVM) that can be read by the PC via a USB port.
POWER SUPPLY
+5V
GND
–5V
+12V
OSCILLOSCOPE
GND
PC
USB
AVDD AGND
J5-2
–5V
+12V AGND
J5-3
J4-1
USB
J4-2
DAC_OUT/TP1
BIPOLAR_OUT/TP2
EVAL-CN0183-SDZ
SDP
CON A
OR
CON B
09471-007
J1
120-PIN SDP
J5-1
Figure 7. Functional Diagram of Test Setup
Rev. A | Page 3 of 5
CN-0183
Circuit Note
Setup
Connect the 120-pin connector on the EVAL-CN0183-SDZ to
the CON A connector or the CON B connector on the EVALSDP-CB1Z. Use nylon hardware to firmly secure the two
boards, using the holes provided at the ends of the 120-pin
connectors. After successfully setting the dc output supply to
+5 V, −5 V, and +12 V, turn the power supply off.
With power to the supply off, connect the −5 V power supply to
the −5V pin on J5-3, connect the +5 V power supply to the
AVDD pin on J5-1, connect GND to the AGND pins on J5-2
and J4-2, and connect the +12 V power supply to the +12V pin
on J4-1. Alternatively, place Link 2 and Link 3 in Position B to
power the circuitry using the ADP2300 to supply +5 V and −5 V.
Note that AVDD and the −5 V are not needed in this case.
Turn on the power supply and then connect the USB cable from
the SDP board to the USB port on the PC. Do not connect the
USB cable to the mini-USB connector on the SDP before turning
on the dc power supply for the EVAL-CN0183-SDZ.
After setting up the test equipment, connect the probes of the
oscilloscope to the TP1 and TP2 test points. The TP3, TP4, and
TP5 test points are connected to the reference, the regulated +5 V,
and the regulated −5 V, respectively. Check these test points for
the correct voltages (use TP6 for the ground).
The software provided on the CD allows users to set the value of
VOUTA by loading a code into the DAC and by choosing the source
of the reference. If users keep the default setting, they will have
to supply the +5 V and −5 V voltages, and the +12 V is not
required. The default setting uses the external REF192 reference,
giving you a full-scale DAC output range of 2.5 V (TP1), and
−2.5 V to +2.5 V on the bipolar output (TP2). Loading 0x0000
sets the DAC output to 0 V and the bipolar output to −2.5 V.
Loading 0x8000 sets the DAC output to 1.25 V and the bipolar
output to 0 V. Loading 0xFFFF sets the DAC output to 2.5 V
and the bipolar output to 2.5 V.
Table 1. Jumper Settings for EVAL-CN0183-SDZ (Default Settings in Bold)
Jumper
LK1
Description
Short AD5668 reference pin to
REF192 output
LK2
AVDD supply source
Setting
Inserted
Opened
Position A
Position B
LK5
−5 V voltage source
Position A
Position B
Function
It shorts AD5668 reference pin to REF192 output allowing the use of
an external DAC reference.
Only the internal reference of the AD5668 can be used.
The circuit is powered by an external 5 V supply applied to the AVDD
pin on J5-1.
The digital power is supplied by the 5 V voltage supplied by the ADP2300
regulator.
The analog circuit is supplied by an external power supply apply to the
−5V pin on J5-3.
The digital power is supplied by the −5 V voltage obtained by inverting the
output of the ADP2300 regulator.
Rev. A | Page 4 of 5
Circuit Note
CN-0183
LEARN MORE
Data Sheets and Evaluation Boards
CN-0183 Design Support Package:
http://www.analog.com/CN0183-DesignSupport
CN-0183 Circuit Evaluation Board (EVAL-CN0183-SDZ)
Ardizzoni, John. A Practical Guide to High-Speed PrintedCircuit-Board Layout, Analog Dialogue 39-09, September
2005.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of “AGND” and “DGND”, Analog Devices.
System Demonstration Platform (EVAL-SDP-CB1Z)
AD5668 Data Sheet and Evaluation Board
AD8638 Data Sheet and Evaluation Board
ADP2300 Data Sheet and Evaluation Board
REF192 Data Sheet and Evaluation Board
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
REVISION HISTORY
ADIsimPower Design Tool
6/13—Rev. 0 to Rev. A
Change to Figure 1 ............................................................................ 1
6/12—Rev. 0: Initial Version
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reserves the right to change any Circuits from the Lab circuits at any time without notice but is under no obligation to do so.
©2012–2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN09471-0-6/13(A)
Rev. A | Page 5 of 5