Circuit Note CN-0023 Devices Connected/Referenced Circuits from the Lab® reference designs are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0023. 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 Circuits from the Lab® reference designs 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 toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage) 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 ©2008–2014 Analog Devices, Inc. All rights reserved. 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 (Continued from first page) Circuits from the Lab reference designs 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 reference designs 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 reference designs. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab reference designs 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 reference designs at any time without notice but is under no obligation to do so. ©2008–2014 Analog Devices, Inc. All rights reserved. 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