Circuit Note CN-0348 Devices Connected/Referenced Serial-Input, Voltage Output, AD5541A Circuits from the Lab reference circuits are engineered and Unbuffered 16-Bit DAC tested for quick and easy system integration to help solve todays Rail-to-Rail Input/Output, Zero Input analog, mixed-signal, and RF design challenges. For more ADA4500-2 Crossover Distortion Amplifier information and/or support, visit www.analog.com/CN0348. Ultralow Noise, High Accuracy, 5 V ADR4550 Voltage Reference 16-Bit Single-Supply Buffered Voltage Output Digital-to Analog Conversion with Less Than 1 LSB Integral and Differential Nonlinearity The circuit eliminates the crossover nonlinearity associated EVALUATION AND DESIGN SUPPORT with most rail-to-rail op amps that can be as high as 4 or 5 LSBs Circuit Evaluation Boards for a 16-bit system. CN-0348 Circuit Evaluation Board (EVAL-CN0348-SDPZ) System Demonstration Platform (EVAL-SDP-CB1Z) This industry-leading solution is ideal for industrial process control Design and Integration Files and instrumentation applications where a compact, single-supply, Schematics, Layout Files, Bill of Materials low cost, and highly linear 16-bit buffered voltage source is required. CIRCUIT FUNCTION AND BENEFITS Total power dissipation for the three active devices is less than The circuit in Figure 1 is a complete single-supply,16-bit 25 mW typical when operating on a single 6 V supply. buffered voltage output DAC that maintains 1 LSB integral and differential nonlinearity by utilizing a CMOS DAC followed by an innovative amplifier that has no crossover distortion. 6V 5V V V IN OUT 1F 0.1F ADR4550 0.1F GND REF V DD 3.3V V LOGIC CS V ADA4500-2 SERIAL AD5541A OUT DIN INTERFACE VOUT SCLK LDAC AGND DGND Figure 1. 1 LSB Linear 16-Bit Buffered Voltage Output DAC (Simplified Schematic, All Connections and Decoupling Not Shown) Rev. 0 Circuits from the Lab circuits 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 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices Tel: 781.329.4700 www.analog.com be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 2014 Analog Devices, Inc. All rights reserved. 11994-001CN-0348 Circuit Note The ADA4500-2 is selected as the output buffer. This device is a CIRCUIT DESCRIPTION high precision amplifier with maximum offset voltage of 120 V, Figure 1 shows the single supply signal chain that consists of a offset drift of less than 5.5 V/C, 0.1 Hz to 10 Hz noise of voltage reference, a digital-to-analog converter (DAC) and a 2 V p-p, and maximum input bias current of 2 pA. Its key DAC buffer. The reference voltage of the DAC is equal to the feature of rail-to-rail input and output swing with zero crossover supply voltage, VDD, to maximize dynamic output range and distortion makes it a suitable candidate as a DAC buffer. signal-to-noise ratio. With this configuration, a rail-to-rail input A typical rail-to-rail input amplifier uses two differential pairs and output buffer amplifier is required. to achieve rail-to-rail input swing (see Tutorial MT-035). One The DAC is the AD5541A 16-bit, serial input, voltage output differential pair is active at the higher range of the input common- segmented R/2R CMOS DAC. The output voltage of the DAC is mode voltage, and the other pair is active at the lower end. This dependent on the reference voltage, as shown in the following classic dual differential pair topology introduces crossover equation: distortion during the handoff of one differential pair to the other. V D The change in offset voltage causes nonlinearity when the amplifier REF V = OUT N is used as a DAC buffer. The ADA4500-2 uses an integrated charge 2 pump in its input structure to achieve rail-to-rail input swing where without the need for a second differential pair. Therefore, it does D is the decimal data word loaded in the DAC register. not exhibit crossover distortion. Using a zero crossover distortion N is the number of bits. amplifier in this single supply system provides wide dynamic For a reference of 5 V, and N = 16, the equation simplifies to the output range while maintaining linearity over the input following: common mode/input digital code range. Details of the operation of the ADA4500-2 can be found on the ADA4500-2 5 D 5 D V = = OUT 16 data sheet. 2 65,536 The output impedance of the DAC is constant (typically 6.25 k) This gives a V of 2.5 V at mid-scale, and 5 V at full-scale. OUT and code-independent. However, the output buffer should have a The LSB size is 5 V/65,536 = 76.3 V. high input impedance (low input bias current) to minimize errors. One LSB at 16 bits is also 0.0015% of full-scale or 15 ppm FS. The ADA4500-2 is a suitable candidate with high input impedance and 2 pA maximum of input bias current at room temperature, and The ADR4550 voltage reference provides a high precision, low noise (2.8 V p-p, 0.1 Hz to 10 Hz) and stable reference to the 190 pA maximum of input bias current over temperature. This results in 1.2 V of worst-case error due to input bias current, DAC. The ADR4550 uses an innovative core topology to achieve which is much less than 1 LSB. high accuracy while offering industry-leading temperature stability and noise performance. The low output voltage temperature The AD5541A is available 10-lead MSOP or 10-lead LFCSP. coefficient (2 ppm/C maximum) and low long-term output The ADR4550 is available in 8-lead SOIC, and the ADA4500-2 voltage drift of the device also improve system accuracy over is available in 8-lead MSOP or 8-lead LFCSP. time and temperature variations. Measured results show that the combination of the AD5541A, The initial room temperature accuracy of the ADR4550B is 0.02% ADR4550 and ADA4500-2 is an excellent solution for high maximum, which is approximately 14 LSBs at 16 bits. This initial accuracy, low noise performance applications. The ADA4500-2 error can be removed with a system calibration. The voltage maintains the linearity of the DAC with no crossover distortion. reference drives the REF pin of the DAC as well as provides power Integral Nonlinearity (INL) and Differential Nonlinearity to the DAC and the output buffer. As a result, it must supply up to (DNL) Measurements 3.9 mA of load current. The ADR4550 can drive up to 10 mA with INL error is the deviation in LSB of the actual DAC transfer 25 ppm/mA load current regulation. function from an idealized transfer function. DNL error is the The ADR4550 reference should be placed as close to the REF difference between an actual step size and the ideal value of 1 LSB. pin of the DAC as possible to minimize the length of the output This system solution provides a 16-bit resolution with 1 LSB traces, and therefore, the error introduced by the voltage drop. DNL and INL. Figure 2 and Figure 3 show the DNL and INL Current flowing through a PCB trace produces an IR voltage performance of the circuit. drop, and with longer traces, this voltage drop can be several millivolts or more, introducing a considerable error. A 1 inch long, 0.005 inch wide trace of 1oz copper has a resistance of approximately 100 m at room temperature. With a load current of 10 mA, this can introduce a 1 mV error. Rev. 0 Page 2 of 5