Circuit Note CN-0180 Circuits from the Lab tested circuit designs address common design challenges and are engineered for quick and easy system integration. For more information and/or support, visit www.analog.com/CN0180. Devices Connected/Referenced AD8475 Fully Differential Funnel Amplifier and ADC Driver AD7982 Fully Differential, 18-Bit, 1 MSPS SAR ADC ADR435 Ultralow Noise XFET Voltage Reference with Current Sink and Source Capability Precision, Low Power, Single-Supply, Fully Integrated Differential ADC Driver for Industrial-Level Signals Rev. A 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 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 Circuits from the Lab circuits. (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 2010-2011 Analog Devices, Inc. All rights reserved. EVALUATION AND DESIGN SUPPORT Circuit Evaluation Boards AD8475 Evaluation Board (EVAL-AD8475Z) AD7982 Evaluation Board (EVAL-AD7982CBZ) Converter Evaluation and Development Board (EVAL-CED1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials CIRCUIT FUNCTION AND BENEFITS Standard single-ended industrial signal levels of 5 V, 10 V, and 0 V to +10 V are not directly compatible with the differential input ranges of modern high performance 16-bit or 18-bit single-supply SAR ADCs. A suitable interface drive circuit is required to attenuate, level shift, and convert the industrial signal into a differential one with the correct amplitude and common-mode voltage that match the input requirements of the ADC. AD798209411-001AD847520V10V0V+10V+7V TO +18V+0.5V+2.5V+4.5VNCNC+IN 0.4xIN 0.4x+IN 0.8xIN 0.8x4V+0.5V2.5V+4.5V4VOUTININ+OUTVOCM+VS+5VVSADR4352020270pF270pF1.3nFVIOCNV GNDREFVDDSDOSCKSDI+5V+2.5V+1.8V TO +5V10k10k0.1F+5VFigure 1. Single-Ended-to-Differential ADC Driver (Simplified Schematic: All Connections and Decoupling Not Shown) CN-0180 Circuit Note Rev. A | Page 2 of 4 Whereas suitable interface circuits can be designed using resistor networks and dual op amps, errors in the ratio matching of the resistors, and between the amplifiers, produce errors at the final output. Achieving the required output phase matching and settling time can be a challenge, especially at low power levels. The circuit shown in Figure 1 uses the AD8475 differential funnel amplifier to perform attenuation, level shifting, and conversion to differential without the need for any external components. The ac and dc performances are compatible with those of the 18-bit, 1 MSPS AD7982 PulSAR ADC and other 16- and 18-bit members of the family, which have sampling rates up to 4 MSPS. The AD8475 is a fully differential attenuating amplifier with integrated precision thin film gain setting resistors. It provides precision attenuation (by 0.4 or 0.8), common-mode level shifting, and single-ended-to-differential conversion along with input overvoltage protection. Power dissipation on a single 5 V supply is only 15 mW. The 18-bit, 1 MSPS AD7982 consumes only 7 mW, which is 30 lower than competitive ADCs. The total power dissipated by the combination is only 22 mW. CIRCUIT DESCRIPTION The AD8475 attenuating amplifier and AD7982 18-bit differ- ential ADC can be used in a high precision analog front-end system to process large voltages using a single 5 V supply. The AD8475 is connected to attenuate the input signal by a factor of 0.4, accomplished through integrated precision trimmed resistors. On a single 5 V supply, it can accept voltages as large as 25 V p-p. The differential rail-to-rail outputs require only 50 mV of headroom at low frequencies. The AD8475 can be driven by a true differential input, or it can be driven by a single-ended input and provide the single-ended-to-differential conversion as shown in Figure 1. An RC network between the AD8475 and ADC provides a single-pole filter that reduces undesirable aliasing effects and high frequency noise. The common-mode bandwidth of the filter is 29.5 MHz (20 , 270 pF), and the differential bandwidth is 3.1 MHz (40 , 1.3 nF). The AD7982 is an 18-bit successive approximation (SAR) ADC that operates from a single power supply, VDD. The I/O interface voltage, VIO, can range between 1.8 V and 5 V, depending on the interface logic supply. The AD7892 has true differential inp