CN-0257: 20-Bit, Linear, Low Noise, Precision, Unipolar +10 V DC Voltage Source

Circuit Note
CN-0257
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/CN0257.
AD5790
AD8675
AD8676
True 20-Bit, Voltage Output DAC
Ultraprecision, 36 V, 2.8 nV√Hz,
Rail-to-Rail Output Op Amp
Ultraprecision, 36 V, 2.8 nV√Hz, Dual
Rail-to-Rail Output Op Amp
20-Bit, Linear, Low Noise, Precision, Unipolar +10 V DC Voltage Source
supply of up to 33 V. The AD5790 accepts a positive reference
input range of 5 V to VDD − 2.5 V, and a negative reference input
range of VSS + 2.5 V to 0 V. Both reference inputs are buffered
on the chip, and external buffers are not required. The AD5790
offers a relative accuracy specification of ±2 LSB maximum, and
operation is guaranteed monotonic with a −1 to +2 LSB
maximum DNL specification.
EVALUATION AND DESIGN SUPPORT
Circuit Evaluation Boards
AD5790 Circuit Evaluation Board (EVAL-AD5790SDZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
CIRCUIT FUNCTION AND BENEFITS
The AD8675 precision op amp has low offset voltage (75 µV
maximum) and low noise (2.8 nV/√Hz typical) and is an
optimum output buffer for the AD5790. The AD5790 has two
internal matched 6.8 kΩ feedforward and feedback resistors,
which can either be connected to the AD8675 op amp to
The circuit, shown in Figure 1, is a 20-bit, linear, low noise,
precision, unipolar (+10 V) voltage source with a minimum
amount of external components. The AD5790 DAC is a 20-bit,
unbuffered voltage output DAC that operates from a bipolar
TO HIGH PRECISION
+5V REFERENCE
R1
1.5kΩ
+ C1
10µF
+15V
AD8675
+3.3V +15V
R2
R3
1kΩ
+10V
A1
1kΩ
–15V
10µF
+
10µF
+
0.1µF
0.1µF
9
6 CLR
8
IOVCC VCC
3
2
VDD
VREFP
7 LDAC
SPI INTERFACE
AND DIGITAL
COTNROL
20
14 SYNC
12 SDIN
4 RESET
A2
VOUT 1
AD5790
13 SCLK
AD8675
6.8kΩ
24
INV
6.8kΩ
11 SDO
+15V
RFB
0V TO +10V
OUTPUT
VOLTAGE
–15V
DGND
VSS
VREFN
AGND
15
17
16
19
0V TO +10V
0.1µF
+
10394-001
10µF
–15V
Figure 1. 20-Bit Accurate, 0 V to +10 V Voltage Source (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev.0
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CN-0257
Circuit Note
1.5
1.0
0
–0.5
–1.0
–2.0
The digital-to-analog converter (DAC) shown in Figure 1 is the
AD5790, a high voltage, 20-bit converter with SPI interface,
offering ±2 LSB INL, −1 to +2 LSB DNL, and 8 nV/√Hz noise
spectral density. The AD5790 also exhibits an extremely long
term linearity error stability of 0.1 LSB.
10394-002
–1.5
CIRCUIT DESCRIPTION
0
200000
400000
600000
800000
1000000
DAC CODE
Figure 2. Integral Nonlinearity vs. DAC Code
1.5
1.0
0.5
DNL (LSB)
Figure 1 shows the AD5790 in a unipolar buffered configuration.
The output buffer is the AD8675, used for its low noise and
low drift. This amplifier is also used (A1) to amplify the +5 V
reference voltage from the low noise precision reference, in this
case a Krohn Hite Model 523 precision reference. The resistors
R2 and R3 in this gain circuit are precision metal foil resistors
with 0.01% tolerance and a temperature coefficient of 0.6 ppm/°C.
For optimum performance over temperature, R2 and R3 should
be in a single package, such as the Vishay 300144 or VSR144
series. R2 and R3 are selected to be 1 kΩ to keep noise in the
system low. R1 and C1 form a low-pass filter with a cutoff
frequency of approximately 10 Hz. The purpose of this filter
is to attenuate voltage reference noise.
0.5
0
–0.5
–1.0
10394-003
The digital input to the circuit is serial and is compatible with
standard SPI, QSPI, MICROWIRE®, and DSP interface standards.
For high accuracy applications, the compact circuit offers
high precision, as well as low noise—this is ensured by the
combination of the AD5790 and AD8675 precision components.
2.0
INL (LSB)
provide a 10 V offset voltage for a ±10 V output swing, or
connected in parallel to provide bias current cancellation. In
this example, a unipolar +10 V output is demonstrated, and the
resistors are used for bias current cancellation. The internal
resistor connection is controlled by setting a bit in the AD5790
control register (see AD5790 data sheet).
0
200000
400000
600000
800000
1000000
DAC CODE
Figure 3. Differential Nonlinearity vs. DAC Code
Linearity Measurements
The precision performance of the circuit shown in Figure 1 is
demonstrated on the EVAL-AD5790SDZ evaluation board
using an Agilent 3458A multimeter. Figure 2 shows that the
integral nonlinearity as a function of DAC code is well within
the specification of ± 2 LSB from 0°C to 105°C.
Figure 3 shows that the differential nonlinearity as a function of
DAC code is within the −1 LSB to +2 LSB specification.
Noise Drift Measurements
To be able to realize high precision, the peak-to-peak noise at
the circuit output must be maintained below 1 LSB, which is
9.5 µV for 20-bit resolution and a +10 V unipolar voltage range.
A real-time noise application will not have a high-pass cutoff at
0.1 Hz to attenuate 1/f noise but will include frequencies down
to dc in its pass band. With this in mind, the measured peak-topeak noise is shown in Figure 4. In this case, the noise at the
output of the circuit was measured over a period of 100 seconds,
effectively including frequencies as low as 0.01 Hz in the
measurement.
A temperature controlled ultralow noise reference was required
for this measurement so as not to dominate the noise
performance.
Rev. 0 | Page 2 of 6
Circuit Note
CN-0257
The zero-scale output voltage exhibits the lowest noise because
it represents the noise from the DAC core only. The noise
contribution from each voltage reference path is attenuated by
the DAC when the zero-scale code is selected.
CIRCUIT EVALUATION AND TEST
Equipment Required
•
•
•
•
•
•
•
4
3
FULL SCALE
HALF SCALE
NOISE (µV)
2
1
Software Installation
The AD5790 evaluation kit includes self-installing software on
a CD. The software is compatible with Windows XP (SP2) and
Vista (32-bit and 64-bit). If the setup file does not run
automatically, you can run the setup.exe file from the CD.
0
–1
ZERO SCALE
10394-004
–2
–3
System Demonstration Platform (EVAL-SDP-CB1Z)
EVAL-AD5790SDZ evaluation board and software
Krohn-Hite Model 523 precision reference
Agilent 3458A multimeter
PC (Windows 32-bit or 64-bit OS)
National Instruments GPIB to USB-B interface cable
SMB cable (2)
0
20
40
60
80
100
TIME (Seconds)
Install the evaluation software before connecting the evaluation
board and SDP board to the USB port of the PC to ensure that
the evaluation system is correctly recognized when connected to
the PC.
Figure 4. DAC Output Voltage Noise Measured Over 100 Second Period for
Full Scale (Green), Half Scale (Red), and Zero Scale (Blue) with Precision
Reference Source
1.
As the time period over which the measurement is taken is
increased, lower frequencies will be included, and the peak-topeak value will increase. At low frequencies, temperature drift
and thermocouple effects become contributors to noise. These
effects can be minimized by choosing components with low
thermal coefficients.
Complete schematics and layout of the printed circuit board
can be found in the CN-0257 Design Support package:
www.analog.com/CN0257-DesignSupport.
2.
After installation from the CD is complete, power up
the AD5790 evaluation board as described in the Power
Supplies section. Connect the SDP board (via either
Connector A or Connector B) to the AD5790
evaluation board and then to the USB port of your PC
using the supplied cable.
When the evaluation system is detected, proceed through
any dialog boxes that appear. This completes the
installation.
Functional Diagram
A functional diagram of the test setup is shown in Figure 6.
Power Supplies
COMMON VARIATIONS
The AD5790 will support a wide variety of output ranges from
0 V to +5 V up to ±10 V, and values in between. If a bipolar
configuration is required, then the VREFN pin needs to be
supplied with an inverted high precision reference voltage.
Again, precise amplifier technology and precision temperature
stable resistors are required.
The following external supplies must be provided:
The AD8676 is a dual version of the AD8675 op amp and can be
used in the circuit if desired.
•
•
•
•
Rev. 0 | Page 3 of 6
3.3 V between the VCC and DGND on Connector J1
for the digital supply of the AD5790. Alternatively,
place Link 1 in Position A to power the digital
circuitry from the USB port via the SDP board
(default).
+12 V to +16.5 V between the VDD and AGND inputs
of J2 for the positive analog supply of the AD5790.
−12 V to −16.5 V between the VSS and AGND inputs
of J2 for the negative analog supply of the AD5790.
+5 V precision reference connected to SMB connector
labeled VREF.
Circuit Note
10394-005
CN-0257
Figure 5. Evaluation Software Main Window
KROHN HITE 523
PRECISION
REFERENCE
GPIB
DUAL POWER SUPPLY
+15V
COM
AGILENT
3458A MULTIMETER
USB
PC
USB
−15V
+VDD
AGND
−VSS
J2-3
J2-2
J2-1
USB
SMB
SMB
VREF
VOUT_BUF
SDP
CON A
OR
CON B
10394-006
J4
120-PIN SDP
EVAL-AD5790SDZ
Figure 6. Functional Block Diagram of Test Setup
Rev. 0 | Page 4 of 6
Circuit Note
CN-0257
Default Link Option Setup
The default link options are listed in Table 1. By default, the
board is configured with VREFP = +10 V and VREFN = −10 V
for a ±10 V output range.
Table 1. Default Link Options
Link No.
LK1
LK2
LK3
LK4
LK5
LK6
LK7
LK8
LK9
LK11
Option
A
B
A
Removed
Removed
Removed
Removed
C
Inserted
Inserted
CN0257 Design Support Package:
www.analog.com/CN0257-DesignSupport
Egan, Maurice. "The 20-Bit DAC Is the Easiest Part of a 1-ppmAccurate Precision Voltage Source," Analog Dialogue, Vol.
44, April 2010.
Kester, Walt. 2005. The Data Conversion Handbook. Analog
Devices. Chapters 3, 5, and 7.
MT-015 Tutorial, Basic DAC Architectures II: Binary DACs.
Analog Devices.
MT-016 Tutorial, Basic DAC Architectures III: Segmented DACs.
Analog Devices.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of AGND and DGND. Analog Devices.
MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and
Rail-to-Rail Issues. Analog Devices.
In order to configure the board for the circuit shown in Figure 1,
the following changes must be made to the default link configuration in Table 1:
1.
2.
3.
4.
LEARN MORE
Place LK3 in position A.
Remove LK4.
Place LK8 in position B.
Remove LK9.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
Voltage Reference Wizard Design Tool.
CN-0177 Circuit Note, 18-Bit, Linear, Low Noise, Precision
Bipolar ±10 V DC Voltage Source.
CN-0191 Circuit Note, 20-Bit, Linear, Low Noise, Precision,
Bipolar ±10V DC Voltage Source.
These changes configure the output buffer amplifier for a gain
of 1 and connect the VREFN pin of the AD5790 to ground.
In addition, the board is now configured to accept a precision
+5 V reference on the SMB connector labeled VREF.
Please refer to User Guide UG-342 for more information on the
EVAL-AD5790SDZ test setup.
CN-0200 Circuit Note, 18-Bit, Linear, Low Noise, Precision
Bipolar ±10 V DC Voltage Source.
User Guide UG-342, Evaluation Board for a 20-Bit Serial Input,
Voltage Output DAC with Integrated Precision Reference
Buffer Amplifiers.
Data Sheets and Evaluation Boards
Test
AD5790 Data Sheet and Evaluation Board
The VOUT_BUF SMB connector is connected to the Agilent
3458A multimeter. The linearity measurements are run using
the Measure DAC Output Tab on the AD5790 GUI.
AD8675 Data Sheet
AD8676 Data Sheet
The noise drift measurement is measured on the VOUT_BUF
SMB connector also. The output voltage is set using the
Program Voltage tab in the AD5790 GUI. The peak-to-peak
noise drift is measured over 100 seconds.
For more details on the definitions and how to calculate the
INL, DNL, and noise from the measured data, see the
"TERMINOLOGY" section of the AD5790 data sheet and also
the following reference: Data Conversion Handbook, "Testing
Data Converters," Chapter 5, Analog Devices.
Rev. 0 | Page 5 of 6
CN-0257
Circuit Note
REVISION HISTORY
12/11—Revision 0: Initial Version
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CN10394-0-12/11(0)
Rev. 0 | Page 6 of 6
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