CN-0034: Unipolar, Precision DC Digital-to-Analog Conversion Using the...

Circuit Note
CN-0034
Devices Connected/Referenced
Circuit Designs Using Analog Devices Products
Apply these product pairings quickly and with confidence.
For more information and/or support call 1-800-AnalogD
(1-800-262-5643) or visit www.analog.com/circuit.
AD5426/AD5432/
AD5443
8-Bit/10-Bit/12-Bit Multiplying
DACs
AD8065
High Performance FastFETTM
Amplifier
ADR01
Low Noise Precision Reference
Unipolar, Precision DC Digital-to-Analog Conversion Using the
AD5426/AD5432/AD5443 8-Bit to12-Bit DACs
This circuit is a high performance, unipolar, precision dc DAC
configuration that employs the AD5426/AD5432/AD5443
family of precision multiplying DACs, the AD8065 low noise
operational amplifier (op amp), and the ADR01 precision
reference. Since the op amp dictates the overall circuit
performance in terms of precision or speed, the AD8065, a high
precision, low noise op amp is well matched for performancedriven applications. This circuit also uses the ADR01, which is a
high accuracy, high stability, 10 V precision voltage reference.
Because voltage reference temperature coefficient and longterm drift are primary considerations for applications requiring
high precision conversion, this device is an ideal candidate.
The input offset voltage of an op amp is multiplied by the
variable noise gain (due to the code-dependent output
resistance of the DAC) of the circuit. A change in this noise gain
between two adjacent digital codes produces a step change in
the output voltage due to the amplifier’s input offset voltage.
This output voltage change is superimposed on the desired
change in output between the two codes and gives rise to a
differential linearity error that, if large enough, could cause the
DAC to be nonmonotonic. In general, the input offset voltage
should be a fraction of an LSB to ensure monotonic behavior
when stepping through codes. For the 12-bit AD5443, the LSB
size is 10 V/212 = 2.44 mV, while the input offset voltage of the
AD8065 is only 0.4 mV.
CIRCUIT DESCRIPTION
This circuit utilizes the AD5426/AD5432/AD5443 CMOS,
current output DACs that provide 8-bit, 10-bit, and 12-bit
operation, respectively. Because this is a current output DAC, an
op amp is required for current-to-voltage (I-V) conversion at
the output of the DAC. Since an op amp’s bias current and
offset voltage are both important selection criteria for precision
current output DACs, this circuit employs the AD8065 op amp,
which has ultralow offset voltage (0.4 mV typical) and bias
current (2 pA typical). The AD8065 and the AD5426/AD5432/
AD5443 can be easily configured to provide a 2-quadrant
multiplying operation or a unipolar output voltage swing, as
shown in Figure 1.
When an output amplifier is connected in unipolar mode, the
output voltage is given by
VOUT = −VREF × (D/2N)
where D is the digital word loaded to the DAC and N is the
number of bits: D = 0 to 255 (8-bit AD5426); D = 0 to 1023
(10-bit AD5432); and D = 0 to 4095 (12-bit AD5443).
VDD1 = +5.0V
VDD
VDD = +12V
C1
VDD
VIN
VOUT
VREF
ADR01
0.1µF
AD5426/
AD5432/
AD5443
SYNC SCLK SDIN
RFB
IOUT1
A1
A1
AD8065
VOUT = 0V TO –VREF
IOUT2
VDD
GND
VSS = –12V
10µF
0.1µF
MICROCONTROLLER
AGND
VSS
10µF
0.1µF
VDD1
10µF
0.1µF
NOTES
1. C1 PHASE COMPENSATION (1pF TO 5pF) MAY BE REQUIRED IF A1 IS A HIGH SPEED AMPLIFIER.
08269-001
CIRCUIT FUNCTION AND BENEFITS
Figure 1. Unipolar Output Precision DC Configuration
(Simplified Schematic)
COMMON VARIATIONS
The OP1177 is another excellent op amp candidate for the I-V
conversion circuit. It also provides a low offset voltage (15 µV
typical) and ultralow bias current (0.5 nA typical).
Rev. A
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CN-0034
Circuit Note
The 10.0 V ADR01 reference can be replaced by either the
ADR02 or ADR03, which are low noise references available
from the same reference family as the ADR01 and provide 5.0 V
and 2.5 V, respectively. The ADR445 and ADR441 ultralow
noise references are suitable substitutes that also provide 5.0 V
and 2.5 V, respectively. Note that the size of the reference input
voltage is restricted by the rail-to-rail voltage of the operational
amplifier selected.
Data Sheets
LEARN MORE
ADR03 Data Sheet.
ADIsimPower Design Tool. Analog Devices.
ADR441 Data Sheet.
Kester, Walt. 2005.The Data Conversion Handbook. Analog
Devices. Chapters 3 and 7.
ADR445 Data Sheet.
MT-015 Tutorial, Basic DAC Architectures II: Binary DACs.
Analog Devices.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of AGND and DGND. Analog Devices.
MT-033 Tutorial, Voltage Feedback Op Amp Gain and
Bandwidth. Analog Devices.
AD5426 Data Sheet.
AD5432 Data Sheet.
AD5443 Data Sheet.
AD8065 Data Sheet.
ADR01 Data Sheet.
ADR02 Data Sheet.
OP1177 Data Sheet.
REVISION HISTORY
7/09—Rev. 0 to Rev. A
Updated Format .................................................................. Universal
MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and
Rail-to-Rail Issues. Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
Voltage Reference Wizard Design Tool. Analog Devices.
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CN08269-0-7/09(A)
Rev. A | Page 2 of 2
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