Circuit Note CN-0261 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/CN0261. AD7691 18-Bit 1.5 LSB INL, 250 kSPS PulSAR ADC AD8597 Ultralow Distortion, Ultralow Noise Amplifier ADR435 Ultralow Noise XFET 5 V Reference 18-bit, 250 kSPS, Data Acquisition System Optimized for AC Performance EVALUATION AND DESIGN SUPPORT CIRCUIT FUNCTION AND BENEFITS Circuit Evaluation Boards CN-0261 Circuit Evaluation Board (EVAL-CN0261-SDPZ) System Demonstration Platform (EVAL-SDP-CB1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials Choosing complementary products for high performance ADCs can be a challenge. The circuit in Figure 1 shows a complete front end solution for the 18-bit, 250 kSPS PulSAR® ADC, which is optimized for ac performance. ADR435 5V VREF VDD = 8V 2 0.1µF VIN VOUT 6 1µF GND 4 VDD = 8V 0.1µF VIN+ VCM = VREF ÷ 2 = 2.5V 49.9Ω VIO = 1.8V TO 5V VDD = 5V AD8597 2 7 6 180Ω 22µF 0.1µF 3 1 590Ω VCM 0.1µF VREF = 5V VIN+ VCM = 2.5V 590Ω 49.9Ω 1 2.7nF VDD REF IN+ 3 0.1µF 2 10 VIO SDI AD7691 SCK VIN– GND VOLTAGE DIVIDER FOR BIASING PURPOSES 4 VCM = VREF ÷ 2 = 2.5V 49.9Ω AD8597 2 3 0.1µF 1 CNV 7 3-WIRE INTERFACE 6 180Ω 18-BIT, 1.5LSB INL 250kSPS DIFFERENTIAL PulSAR ADC 2.7nF VCM 590Ω VOLTAGE DIVIDER FOR BIASING PURPOSES 49.9Ω 0.1µF 10467-001 590Ω 7 GND 8 5 VDD = 8V 0.1µF VIN– SDO IN– VSS = –2V 0.1µF 9 VSS = –2V Figure 1. High Performance, 18-Bit ADC Front End (Simplified Schematic: All Connections and Decoupling Not Shown) Rev. A 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 ©2012–2013 Analog Devices, Inc. All rights reserved. CN-0261 Circuit Note The circuit centers on the AD7691, which is a low power ADC (1.35 mW @ 2.5 V and 100 kSPS) from the PulSAR family. The ADC is driven directly from the AD8597 ultralow distortion, ultralow noise amplifier, and the ADC’s reference is the ultralow noise 5 V ADR435. The circuit achieves 101 dB SNR and 118 dB THD with a 1 kHz input tone. The ADR435 is available in either an 8-lead MSOP or an 8-lead narrow SOIC package. The dynamic performance of the above configuration is shown in Figure 2 and Figure 3 and is summarized below: SNR = 101.02dB THD = 118.44 dB CIRCUIT DESCRIPTION SINAD = 100.94 dB The heart of this circuit is the AD7691, an 18-bit, 250 kSPS charge redistribution, successive approximation, analog-todigital converter (ADC) that operates from a single power supply. Dynamic Range = 101.5 dB It contains a low power, high speed, 18-bit sampling ADC with no missing codes, an internal conversion clock, and a versatile serial interface port. On the CNV rising edge, it samples the voltage difference between the IN+ and IN− pins. The voltages on these pins swing in opposite phases between 0 V and REF. The reference voltage, REF, is applied externally and can be set up to the supply voltage. The AD7691 power scales linearly with throughput. 10467-002 For the experiments carried out for this circuit note, the AD7691was interfaced to the SDP (System Demonstration Platform board, EVAL-SDP-CB1Z), and the ADC SPIcompatible serial interface was connected to the DSP SPORT interface VIO supply. Figure 2. Evaluation Board Software Output Screen Capture The AD7691 is housed in a 10-lead MSOP or a 10-lead QFN (LFCSP). The AD8597 is available in 8-lead SOIC and LFCSP packages. The 180 Ω resistors and the 2.7 nF capacitors form a single-pole 327 kHz low-pass filter to further reduce noise. FUNDAMENTAL = 1kHz, –0.3dB FROM FS SAMPLING FREQUENCY = 250kSPS SNR = 101.02dB THD = –118.44dB SINAD = 100.94dB DYNAMIC RANGE = 101.5dB –50 –100 –150 –200 –250 The voltage reference used in this application is the ADR435, which is one of a family of XFET® voltage references featuring low noise, high accuracy, and low temperature drift performance. Using patented temperature drift curvature correction and XFET (eXtra implanted junction FET) technology, voltage change vs. temperature is minimized. The ADR43x family can source up to 30 mA of output current and sink up to 20 mA. It also has a trim terminal to adjust the output voltage over a 0.5% range without compromising performance. Rev. A | Page 2 of 4 0 20 80 40 60 FREQUENCY (kHz) 100 Figure 3. FFT Details for 1 kHz Tone Sampling at 250 kSPS 120 10467-003 The AD8597 can be operated on supply voltages up to ±15 V. In the circuit, supply voltages of +8 V and −2 V were chosen in order to minimize power dissipation. 0 AMPLITUDE (dB) The ADC is driven from the AD8597 (4.8 mA/amplifier), which is a low noise, low distortion operational amplifiers ideal for use as an input buffer. The low noise of 1.1 nV/√Hz and low harmonic distortion of less than −120 dB at audio frequencies give the AD8597 the wide dynamic range necessary for preamplifiers in audio, medical, and instrumentation applications. The excellent slew rate of 14 V/μs and 10 MHz gain bandwidth product make it highly suitable for medical applications. Circuit Note CN-0261 COMMON VARIATIONS CIRCUIT EVALUATION AND TEST Other pin-compatible 18-bit ADCs in the PulSAR family are available with higher sampling rates: AD7690 (400 kSPS), AD7982 (1 MSPS), AD7984 (1.33 MSPS). Equipment Needed (Equivalents Can Be Substituted) The AD7986 (2 MSPS) is available in 20-lead 4 mm × 4 mm LFCSP (QFN). • • • • The AD8599 op amp is a dual version of the AD8597 and can be used in the circuit, if desired. • The ADA4841-1 (single) and ADA4841-2 (dual) are lower power op amps (1.1 mA/amp), but have slightly higher noise (2.1 nV/√Hz). • The ADA4941 is optimized for driving differential input 18-bit ADCs and has 2.2 mA/amplifier and 10.2 nV/√Hz noise. A summary of PulSAR ADCs and recommended drivers can be found at www.analog.com/PulSAR. Other reference voltages within the ADR43x family or from other reference families are available from the Analog Devices portfolio. EVAL-CN0261-SDPZ circuit evaluation board System Demonstration Board (EVAL-SDP-CB1Z) Function generator, Audio Precision SYS-2522 External 10 nF ceramic filter capacitor as shown in Figure 4 Power supplies: +8 V @ 50 mA, −2 V @ 50 mA, +5 V @ 500 mA. PC with a USB port and Windows® XP or Windows Vista® (32-bit), or Windows® 7 (32-bit) Setup and Test The block diagram of ac performance measurement setup is shown in Figure 4. The EVAL-CN0261-SDPZ board is driven with bench supplies as shown. Complete documentation for the board can be found at www.analog.com/CN0261-DesignSupport. To measure the frequency response, the equipment was connected as shown in Figure 5. The Audio Precision SYS-2522 was set to output a 1 kHz tone at an input signal level of 0.5 dB below full-scale. The external 10 nF capacitor acts as a low-pass noise reduction filter for the output of the signal generator. Using the evaluation board software, the FFT data was then captured and analyzed. The inclusion of a buffer to create the VCM signal to bias the input would be normal, however, specified performance was achieved without the need of a buffer in this circuit. The software analysis is part of the evaluation board software, which allows the user to capture and analyze ac or dc performance. In addition to ac performance, the evaluation board software also allows users to analyze the waveform data and create a histogram for the measured input signal. DC POWER SUPPLIES +5V −2V GND VIN+ AUDIO PRECISION SYS-2522 VIN− USB SDP BOARD EVAL-CN0261-SDPZ 120-PIN CONNECTOR 10nF CERAMIC Figure 4. Test Setup for Measuring AC Performance Rev. A | Page 3 of 4 PC WITH FFT ANALYSIS SOFTWARE 10467-004 +8V CN-0261 Circuit Note LEARN MORE Data Sheets and Evaluation Boards CN0261Design Support Package: www.analog.com/CN0261-DesignSupport CN-0261 Circuit Evaluation Board (EVAL-CN0261-SDPZ) Ardizzoni, John. A Practical Guide to High-Speed PrintedCircuit-Board Layout, Analog Dialogue 39-09, September 2005. MT-021 Tutorial, Sucessive Approximation ADCs, Analog Devices. System Demonstration Platform (EVAL-SDP-CB1Z) AD7691 Data Sheet AD8597 Data Sheet ADR435 Data Sheet MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND," Analog Devices. REVISION HISTORY MT-101 Tutorial, Decoupling Techniques, Analog Devices. Changes to Title ................................................................................. 1 Voltage Reference selection and Evaluation Tool, Analog Devices. 12/13—Rev. 0 to Rev. A 1/12—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. ©2012–2013 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN10467-0-12/13(A) Rev. 0 | Page 4 of 4