CN-0023: Precision, Unipolar, Noninverting Configuration for the...

Circuit Note
CN-0023
Devices Connected/Referenced
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AD5546/
AD5556
Current Output, Parallel Input,
16-/14-Bit DAC
ADR03
2.5 V Precision Voltage Reference
AD8628
Precision Amplifier
Precision, Unipolar, Noninverting Configuration for the AD5546/AD5556 DAC
matching and temperature tracking that minimize the number
of components needed for multiquadrant applications.
CIRCUIT FUNCTION AND BENEFITS
This circuit provides precision, unipolar noninverting data
conversion using the AD5546/AD5556 current output DAC with
the ADR03 precision reference and AD8628 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.
This circuit uses the ADR03, which is a highly accuracy, high
stability, 2.5 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.
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 AD8628 auto-zero op
amp, which has ultralow offset voltage (1 µV typical) and bias
current (30 pA typical). The 2.2 pF capacitor (C7) at the DAC
output is used to compensate for the external output
capacitance of the DAC.
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
+ V+
U4
+5V
AD8628
2
C1
1µF
C2
0.1µF
VIN
– V–
U3
C8
0.1µF
ADR03
TRIM
VOUT
GND
4
5
+2.5V
6
C4
0.1µF
–5V
R1A
C9
1µF
RCOMA
R1
–2.5V
REFA
R2
ROFSA
RFBA
ROFS
RFB
+5V
C7
2.2pF
C5
1µF
VDD
C3
0.1µF
IOUT
16-BIT/
14-BIT
U1
AD5546/AD5556
–
C6
0.1µF
V+
U2
AD8628
GND
+
V–
VOUT
0V TO +2.5V
16-BIT/14-BIT DATA
WR LDAC RS
MSB
08245-001
WR
LDAC
RS
MSB
Figure 1. Unipolar 2-Quadrant Multiplying Mode, VOUT = 0 V to +VREF (Simplified Schematic)
Rev. B
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CN-0023
Circuit Note
Note that the AD8628 has rail-to-rail input and output stages,
but the output can only come within a few millivolts of either
rail depending on load current. For the circuit shown, the
output can swing from approximately 1 mV to +2.5 V.
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 ADR03 and the
AD5546, the LSB size is
2. 5 V
216
= 38 µV
(1)
The input offset voltage of the AD8628 auto-zero op amp is
typically 1 µV, which is negligible compared to the LSB size.
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 30 pA typical, which flowing through the
RFB resistor (10 kΩ typical), produces an error of only 0.3 µV.
The AD5546/AD5556 DAC architecture uses a current-steering
R-2R ladder design that requires an external reference and
op amp to convert the unipolar to an output voltage. VOUT
can be calculated for the AD5546 using the equation
VOUT =
+VREF × D
216
(2)
where D is the decimal equivalent of the input code. VOUT can
be calculated for the AD5556 using the equation
VOUT =
+VREF × D
214
(3)
where D is the decimal equivalent of the input code.
ADR01 and ADR02 are other low noise references available
from the same reference family as the ADR03. Other 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.
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.
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.
AD8628 Data Sheet.
ADR03 Data Sheet.
REVISION HISTORY
4/14—Rev. A to Rev. B
Changes to Figure 1 ...........................................................................1
5/09—Rev. 0 to Rev. A
COMMON VARIATIONS
The AD8629 is a dual version of the AD8628. 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
Updated Format .................................................................. Universal
10/08—Revision 0: Initial Version
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CN08245-0-4/14(B)
Rev. B | Page 2 of 2
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