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 “Circuits from the Lab” from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any“Circuit from the Lab”. (Continued on last page) One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2008–2009 Analog Devices, Inc. All rights reserved. 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 (Continued from first page) "Circuits from the Lab" are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the "Circuits from the Lab" in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the "Circuits from the Lab". Information furnished by Analog Devices is believed to be accurate and reliable. However, "Circuits from the Lab" are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any "Circuits from the Lab" at any time without notice, but is under no obligation to do so. Trademarks and registered trademarks are the property of their respective owners. ©2008–2009 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN08249-0-5/09(A) Rev. A | Page 2 of 2

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