CN-0027: Precision, Unipolar, Noninverting Configuration for the...

Circuit Note
CN-0027
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.
AD5547/
AD5557
Dual, Current Output, Parallel Input,
16-/14-Bit DAC
ADR03
Precision Voltage Reference
AD8628
Rail-to-Rail Input/Output Operational
Amplifier
Precision, Unipolar, Noninverting Configuration for the AD5547/AD5557 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 AD5547/AD5557 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 autozero op amp, which has ultralow offset voltage (1 µV typical)
and bias current (30 pA typical). C7 is a compensation
capacitor. The value of C7 for this application is 2.2 pF, which is
optimized to compensate for the external output capacitance of
the DAC.
CIRCUIT DESCRIPTION
The AD5547/AD5557 are dual-channel precision 16-/14-bit,
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
+5V
C1
1µF
2
U3
C2
1µF
VIN
TRIM
VOUT
U2A
5
6
AD8628
GND
4
C4
0.1µF
ADR03
–2.5V
+5V
+2.5V
VDD
C3
0.1µF
R1
VREFA
RCOMA
R2
AD5547/AD5557
WR
LDAC
RS
MSB
A0, A1
RFBA
ROFS
RFB
16-BIT/
14-BIT
C7
2.2pF
IOUTA
AGNDA
16-BIT/
14-BIT DATA
WR LDAC RS
ROFSA
C5
1µF
C6
0.1µF
U2B
+V
AD8628
VOUTA
–V
0V TO +2.5V
MSB A0, A1
2
08249-001
R1A
Figure 1. Unipolar 2-Quadrant Multiplying Mode, VOUT = 0 V to +VREF (Simplified Schematic)
Rev. A
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CN-0027
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
AD5547, the LSB size is
2. 5 V
16
2
= 38 µV
(1)
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 AD5547/AD5557 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 offset voltage of the AD8628 auto-zero op amp is
typically 1 µV, which is negligible compared to the LSB size.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of AGND and DGND. 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 30 pA typical, which flowing through the
RFB resistor (10 kΩ typical) produces an error of only 0.3 µV.
MT-033 Tutorial, Voltage Feedback Op Amp Gain and
Bandwidth. Analog Devices.
The AD5547/AD5557 DAC architecture uses a current-steering
R-2R ladder design that requires an external reference and opamp to convert to an output voltage. VOUT can be calculated
for the AD5547 using the equation
VOUT =
+VREF × D
(2)
216
where D is the decimal equivalent of the input code. VOUT can
be calculated for the AD5557 using the equation
VOUT =
+VREF × D
214
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
AD5547 Data Sheet.
AD5557 Data Sheet.
AD8628 Data Sheet.
ADR03 Data Sheet.
(3)
REVISION HISTORY
where D is the decimal equivalent of the input code.
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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CN08249-0-5/09(A)
Rev. A | Page 2 of 2
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