Circuit Note CN-0307 Devices Connected/Referenced Circuits from the Lab™ reference circuits 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/CN0307. AD7625 16-Bit, 6 MSPS, PulSAR, Differential ADC ADA4897-1/ ADA4897-2 Low Power, Low Noise, Single/Dual Amplifier Ultralow Noise XFET Voltage Reference with Current Sink and Source Capability 2.7 V, 800 µA, 80 MHz Single/Dual Rail-toRail I/O Amplifier ADR434 AD8031/ AD8032 A 16-Bit, 6 MSPS SAR ADC System with Low Power Input Drivers and Reference Optimized for Multiplexed Applications EVALUATION AND DESIGN SUPPORT distortion [THD] = −110 dBc) at low power. The circuit is ideal for high performance multiplexed data acquisition systems, such as portable digital x-ray systems and security scanners, because the SAR architecture can sample without the latency or pipeline delay typically incurred with pipeline ADCs. The 6 MSPS sampling rate allows fast sampling of multiple channels, and the ADC has true 16-bit dc linearity performance and a serial low voltage differential signaling (LVDS) interface for low pin count and low digital noise. Design and Integration Files Schematics, Layout Files, Bill of Materials CIRCUIT FUNCTION AND BENEFITS The circuit in Figure 1 is a 16-bit, 6 MSPS, successive approximation (SAR) analog-to-digital converter (ADC) and differential-todifferential driver combination optimized for low noise (signal-tonoise ratio [SNR] = 88.6 dB) and low distortion (total harmonic +5V +4.096V +7V +4.096V AD8031 ADR434 +5V VDD +7V/+5V 0.1µF VIN+ 590Ω 20Ω VCM 2.7nF ADA4897-1 0.1µF VREF = 4.096V +2.5V VCM = VREF ÷ 2 = 2.048V 49.9Ω REFIN REF VDD1 VDD2 VIO 0.1µF CNV+/ CNV– 100Ω VIN+ VCM = 2.048V IN+ –2V/0V VSS VIN– D+/D– 100Ω DCO+/DCO– 100Ω GND AD7625 VDD +7V/+5V 0.1µF VIN– VCM = VREF ÷ 2 = 2.048V 49.9Ω 590Ω 0.1µF VCM IN– 20Ω GND 2.7nF ADA4897-1 0.1µF VCM CLK+/CLK– 100Ω +2.048V +5V AD8031 11130-001 –2V/0V VSS Figure 1. The ADA4897-1 Driving the AD7625 (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 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 ©2012 Analog Devices, Inc. All rights reserved. CN-0307 Circuit Note The driver uses two low noise (1 nV/√Hz) ADA4897-1 op amps that maintain the dynamic performance of the AD7625 ADC at low power levels (3 mA per amplifier). The fast settling time (45 ns to 0.1%) of ADA4897-1 makes them ideal for multiplexed applications. This combination offers industry-leading dynamic performance at low power in a small board area with the AD7625 in a 5 mm × 5 mm, 32-lead LFCSP; the ADA4897-1 in an 8-lead SOIC; and the AD8031 in a 5-lead SOT-23 package. CIRCUIT DESCRIPTION The ADA4897-1 has low distortion (−93 dB spurious-free dynamic range [SFDR] at 1 MHz), a fast settling time (36 ns to 0.1%), and high bandwidth (230 MHz, −3 dB, G = 1). Both ADA4897-1 drivers are configured with a gain of 1. The single-pole 2.95 MHz low-pass RC filter, using a 20 Ω resistor and 2.7 nF capacitor, is placed between each of the drivers and the ADC. This filter limits the output noise of the op amp at the inputs of the AD7625 and provides some attenuation of the out-of-band harmonics. The ADR434 is available in either an 8-lead MSOP or an 8-lead, narrow SOIC package. An AD8031 op amp isolates the ADR434 output from the reference input of the AD7625 and provides low impedance and fast settling to the transient current on the REF input. The dual driver requires only 54 mW, and when added to the ADC power of 135 mW, and the reference and reference buffer power of 12 mW, yields a total power of only 201 mW for the entire circuit. The circuit uses supplies of +7 V and −2 V for the input of the ADA4897-1 drivers to minimize power dissipation and to achieve the optimum system distortion performance. The ADA4897-1 output stage is rail-to-rail and swings between 150 mV and 4.85 V when operating on a single 5 V supply. However, the additional 2 V headroom at each end of the range provides lower distortion. Figure 2 shows the ac performance of the circuit using +7 V and −2 V supplies for the input stage. The SNR = 88.6 dB, THD = −110.7 dB, with a 20 kHz input signal 0.6 dB below full scale (93% full scale). 11130-002 The common-mode voltage at the output of the ADA4897-1 is set by buffering the VCM output voltage (nominally 2.048 V) of the AD7625 using the AD8031 configured as a unity-gain buffer. The common-mode bias voltage is applied to the inputs through the 590 Ω series resistors. The AD8031 is ideal for driving the common-mode voltage because of its low output impedance and fast settling from transient currents. The AD7625 achieves industry breakthrough dynamic performance of 92 dB SNR at 6 MSPS with a 16-bit (1 LSB) integral nonlinearity (INL) performance using an LVDS interface. The ADR434 voltage reference (4.096 V) is a low noise, high accuracy XFET reference with low temperature drift. It can source up to 30 mA of output current and sink up to 20 mA. Figure 2. AD7625 and ADA4897-1 in Dual Supply Operation (+7 V, −2 V), SNR = 88.6 dB, THD = −110.7 dB, Fundamental Amplitude = −0.6 dB of Full Scale Rev. 0 | Page 2 of 5 CN-0307 11130-003 Circuit Note Figure 3. AD7625 and ADA4897-1 in Single Supply Operation (5 V), SNR = 86.7 dB, THD = −101.1 dB, Fundamental Amplitude = −1.55 dB of Full Scale Figure 3 shows the ac performance of the circuit using a single 5 V supply for the input stage. The SNR = 86.7 dB, THD = −101.1 dB, with a 20 kHz input signal 1.55 dB below full scale (84% full scale). Another attractive 4.096 V reference is the ADR4540 low dropout (>300 mV) high accuracy reference that allows operation on a 5 V supply. The data shows an approximate 1.9 dB degradation in SNR and a 9.6 dB degradation in THD due to reducing the supply voltage from −2 V, +7 V to 0 V, + 5V. The ADA4897-1 and AD8031 single op amps can be replaced with their dual versions (ADA4897-2 and AD8032, respectively) if desired. The single supply configuration is useful for the users who do not have dual supplies in their system but still must achieve high performance. For high input frequencies up to 3 MHz, the ADA4899-1 (15 mA/amp) is the recommended driving amplifier. COMMON VARIATIONS The AD7625 has an integrated internal reference as well as two provisions for external references if system requirements dictate. The reference voltage can be generated by applying the ADR3412 reference (1.2 V) output to the REFIN pin, which is amplified internally by the on-chip reference buffer to the correct ADC reference value of 4.096 V. The ADR3412 can be supplied by the same 5 V analog rail used for the AD7625 and also make use of the on-chip reference buffer. Alternatively, a 4.096 V external reference, such as the ADR434 or ADR444, can be connected to the unbuffered REF input of the ADC using a buffer amplifier such as the AD8031 as shown in Figure 1. This approach is common for multichannel applications where the system reference is shared by several ADCs. The ADR434 and ADR444 configurations also excel for single channel applications where a low reference temperature coefficient (3 ppm/°C maximum for ADR434B and ADR444B) is required. The 7 V rail used to supply the ADA4897-1 op amps can also supply the VIN supply pin of the ADR434 or ADR444. The ADA4938-1 (37 mA/amp) is excellent for signals up to 10 MHz and can also be used as a single-ended-to-differential converter. The performance of this or any high speed circuit is highly dependent on proper printed circuit board (PCB) layout. This includes, but is not limited to, power supply bypassing, controlled impedance lines (where required), component placement, signal routing, and power and ground planes. (See MT-031 Tutorial, MT-101 Tutorial, and the article A Practical Guide to High-Speed Printed-Circuit-Board Layout for more detailed information regarding PCB layout.) CIRCUIT EVALUATION AND TEST The EVAL-AD7625EDZ evaluation board was developed to evaluate and test the AD7625 ADC. To test the circuit shown in Figure 1, the two ADA4899-1 op amps (U13, U14) were replaced with two ADA4897-1 op amps. A detailed schematic and user instructions are available in the EVAL-AD7625EDZ documentation. This documentation describes how to run the ac tests described in this circuit note Note that the +7 V and −2 V supplies for the input amplifiers are connected to the EVAL-AD7625EDZ board from the external dual power supply. Rev. 0 | Page 3 of 5 CN-0307 Circuit Note A functional block diagram of the test setup is shown in Figure 4, and a photograph of the evaluation board is shown in Figure 5. • Equipment Needed • The following equipment is required to test the circuit: POWER SUPPLY –2V • • POWER SUPPLY AGILENT E3630A +7V +7V @ 2A ADA4897-1 IN+ SIGNAL GENERATOR AD7625 CED PC (USB) IN– AUDIO PRECISION SYS-2702 ADA4897-1 EVAL-AD7625/AD7626EDZ EVAL-CED1Z (MODIFIED) Figure 4. Functional Diagram of Test Setup 11130-005 • The EVAL-AD7625EDZ modified evaluation board (includes software and 7 V dc wall wart power supply) The EVAL-CED1Z converter evaluation and demonstration platform board 11130-004 • A low distortion signal generator, such as the Agilent 81150A or Audio Precision SYS2702 A PC with a USB 2.0 port running Windows® XP, Windows Vista, or Windows 7 (32-Bit or 64-bit) A 7 V dc wall wart (included with evaluation board) External +7 V and −2 V dc supplies at 50 mA. Figure 5. Modified EVAL-AD7625EDZ Board Connected to EVAL-CED1Z Board Rev. 0 | Page 4 of 5 Circuit Note CN-0307 LEARN MORE Data Sheets and Evaluation Boards CN-0307 Design Support Package: http://www.analog.com/CN0307-DesignSupport AD7625 Data Sheet Ardizzoni, John, and Jonathan Pearson, High Speed Differential ADC Driver Design Considerations, Application Note AN-1026, Analog Devices, Inc. ADA4897-1 Data Sheet AD7625 Evaluation Board, EVAL-AD7625EDZ Ardizzoni, John. “A Practical Guide to High-Speed PrintedCircuit-Board Layout,” Analog Dialogue 39-09, September 2005. AN-742 Application Note, Frequency Domain Response of Switched Capacitor ADCs. Analog Devices. ADA4897-2 Data Sheet AD8031 Data Sheet AD8032 Data Sheet ADR434 Datasheet REVISION HISTORY AN-827 Application Note, A Resonant Approach to Interfacing Amplifiers to Switched-Capacitor ADCs. Analog Devices. 11/12—Revision 0: Initial Version Kester, Walt. 2006. High Speed System Applications. Analog Devices. Chapter 2, “Optimizing Data Converter Interfaces.” MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of “AGND” and “DGND.” Analog Devices. MT-073 Tutorial, High Speed Variable Gain Amplifiers. Analog Devices. MT-074 Tutorial, Differential Drivers for Precision ADCs, Analog Devices. MT-075 Tutorial, Differential Drivers for High Speed ADCs Overview, Analog Devices. MT-076 Tutorial, Differential Driver Analysis, Analog Devices. MT-101 Tutorial, Decoupling Techniques. Analog Devices. Analog Devices DiffAmpCalculator™ Design Tool (Continued from first page) Circuits from the Lab circuits 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 circuits 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 circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab circuits 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 circuits at any time without notice but is under no obligation to do so. ©2012 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN11130-0-10/12(0) Rev. 0 | Page 5 of 5