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 “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 ©2008–2010 Analog Devices, Inc. All rights reserved. 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 (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. ©2008–2010 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN08247-0-4/10(B) Rev. B | Page 2 of 2

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