CN-0024: Precision, Bipolar Configuration for the AD5546/AD5556 DAC

Circuit Note
CN-0024
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.
AD5546
Current Output, Parallel Input, 16-/14-Bit
DAC
AD8512
Low Noise, Low Input Bias, JFET Operational
Amplifier
ADR01
Ultracompact Precision 10.0 V Voltage
Reference
Precision, Bipolar Configuration for the AD5546/AD5556 DAC
This circuit uses the ADR01, which is a highly accuracy, high
stability, 10 V precision voltage reference. As voltage reference
temperature coefficient and long-term drift are primary
considerations for applications requiring high precision
conversion, this device is an ideal candidate.
CIRCUIT FUNCTION AND BENEFITS
This circuit provides precision, bipolar data conversion using
the AD5546/AD5556 current output DAC with the ADR01
10 V precision reference and AD8512 operational amplifier
(op amp). This circuit provides accurate, low noise, high speed
output voltage capability and is well suited for process control,
automatic test equipment, and digital calibration applications.
An op amp is used in the current-to-voltage (I-V) stage of this
circuit. An op-amp’s bias current and offset voltage are both
important selection criteria for use with precision current
output DACs. Therefore, this circuit employs the AD8512
op amp, which has ultralow offset voltage (100 µV typical) and
bias current (21 pA typical). C9 is a compensation capacitor.
The value of C9 for this application is 2.2 pF, which is optimized
to compensate for the external output capacitance of the DAC.
The capacitor C8 acts as an integrator to reduce noise, and a
typical value of 47 pF is recommended.
CIRCUIT DESCRIPTION
The AD5546 and AD5556 are 16-bit and 14-bit, precision,
multiplying, low power, current output, parallel input digital-toanalog converters. They operate from a single 2.7 V to 5.5 V
supply with ±15 V multiplying references for 4-quadrant
outputs. Built-in 4-quadrant resistors facilitate the resistance
matching and temperature tracking that minimize the number
of components needed for multiquadrant applications.
2
C1
1µF
C2
0.1µF
+
VIN
U2A
C4
AD8512
0.1µF
–
–10V
C8
47pF
U3
ADR01
TRIM
VOUT
GND
4
5
6
R1
RCOM
R1
+5V
REF
R2
+10V
ROF
RFB
ROFS
+15V
RFB
C9
2.2pF
VDD
C3
0.1µF
U1
AD5546/AD5556
IOUT
16-BIT/
14-BIT
GND
16-BIT/
14-BIT DATA
WR LDAC RS
C5
1µF
–
C6
0.1µF
V+
U2B
AD8512
+
V–
C7
0.1µF
VOUT
–10V TO +10V
MSB
WR
LDAC
RS
MSB
C10
–15V 1µF
08247-001
+15V
Figure 1. Bipolar 2-Quadrant Multiplying Mode with ±10 V Output (Simplified Schematic)
Rev. B
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CN-0024
Circuit Note
The input offset voltage of the 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, which, 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 ADR01 and the
AD5546, the LSB size is
10 V
16
= 153 µV
2
The input offset voltage of the AD8512 is 100 µV typical,
thereby giving adequate margin.
(1)
The AD5546/AD5556 DAC architecture uses a current-steering
R-2R ladder design that requires an external reference and op
amp to convert the bipolar to an output voltage. VOUT can be
calculated for the AD5546 using the equation
(2)
where D = 0 to 65535 for 16-bit DAC (D is the decimal
equivalent of the input code). VOUT can be calculated for the
AD5556 using the equation
 VREF × D 
VOUT = 
14 − 1
 − VREF
 2

These circuits can also be used as a variable gain element by
utilizing the multiplying bandwidth nature of the R-2R
structure of the AD5546/AD5556 DAC. In this configuration,
remove the external precision reference and apply the signal to
be multiplied to the reference input pins of the DAC.
LEARN MORE
ADIsimPower Design Tool.
Kester, Walt. 2005. The Data Conversion Handbook. Analog
Devices. See chapters 3 and 7.
MT-015 Tutorial, Basic DAC Architectures II: Binary DACs.
Analog Devices.
The input bias current of an op-amp also generates an offset at
the voltage output as a result of the bias current flowing through
the feedback resistor, RFB. In the case of the AD8628, the input
bias current is only 21 pA typical, which flowing through the
RFB resistor (10 kΩ typical) produces an error of only 0.21 µV.
 VREF × D 
VOUT = 
16 − 1
 − VREF
 2

low noise references that would be suitable are the ADR441 and
ADR445 products. The size of the reference input voltage is
restricted by the rail-to-rail voltage of the op amp selected.
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.
MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and
Rail-to-Rail Issues. Analog Devices.
MT-055 Tutorial, Chopper Stabilized (Auto-Zero) Precision Op
Amps. Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
Data Sheets
AD5546 Data Sheet.
AD8512 Data Sheet.
ADR01 Data Sheet.
(3)
REVISION HISTORY
where D = 0 to 16383 for 14-bit DAC (D is the decimal
equivalent of the input code).
4/10—Rev. A to Rev. B
COMMON VARIATIONS
The AD8605 is another excellent op amp candidate for the I-V
conversion circuit. It also has a low offset voltage and low bias
current. The ADR02 and ADR03 are other low noise references
available from the same reference family as the ADR01. Other
Changes to Circuit Description .......................................................1
Changes to Figure 1 ...........................................................................1
5/09—Rev. 0 to Rev. A
Updated Format .................................................................. Universal
10/08—Revision 0: Initial Version
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CN08247-0-4/10(B)
Rev. B | Page 2 of 2
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