Circuit Note CN-0096 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/CN0096. Devices Connected/Referenced AD8331 Ultralow Noise VGA with Preamplifier and Programmable RIN AD9215 10-Bit, 65 MSPS/80 MSPS/105 MSPS, 3 V Analog-to-Digital Converter High Frequency Variable Gain Amplifier Extends the Dynamic Range of a 10-Bit, 65 MSPS ADC to Greater Than 100 dB EVALUATION AND DESIGN SUPPORT This represents a dynamic range of approximately 60 dB. A wideband VGA can be used ahead of the ADC to amplify input signals with amplitudes less than the minimum resolution and attenuate large signals that would otherwise saturate the ADC. The circuit shown in Figure 1 uses the AD8331 low noise wideband VGA as a driver to extend the dynamic range of the AD9215 10-bit ADC. The gain range of the AD8331 is 48 dB and is linear-in-dB with respect to the control voltage. The overall dynamic range of the combined VGA and ADC is greater than 100 dB. Examples of such applications are ultrasound receivers where the signal strength ranges from microvolts to several volts, and the intermediate frequency (IF) amplifier used in virtually all receivers. For dc or low frequency analog signals, Σ-Δ ADCs with resolutions up to 24 bits are economical and plentiful, but typically limited in sampling frequency to a few hundred kilohertz. State-of-the-art, available ADC resolution decreases as sampling frequency increases. This makes accurate digitization of high frequency, low amplitude signals extremely difficult using standard ADCs. Variable gain amplifiers (VGAs) serve a critical function when an analog signal with wide dynamic range is converted to digital format, and the ADC resolution is insufficient to capture all useful information. For example, a 10-bit converter with a 2 V p-p input range has an LSB weight of 2 ÷ 1024, or just under 2 mV. Variable gain amplifiers are a convenient solution to this problem. The AD8331/AD8332/AD8334 are single-, dual-, and quadchannel, ultralow noise, linear-in-dB, variable gain amplifiers (VGAs). Optimized for ultrasound systems, they are usable as a low noise variable gain element at frequencies up to 120 MHz. CIRCUIT FUNCTION AND BENEFITS 18nF EIN OPTIONAL OUTPUT DECOUPLING 100Ω 0.1µF VOH LON FB1 1nF TO 0.1µF +5V 274Ω FB1 +3V 1kΩ 22pF FB1 AD8331ARU LOP LMD 0.1µF 1kΩ 100nF 33Ω INH 0pF TO 22pF +3V VCM 0.1µF VOL 100Ω CLMP 100nF 33Ω 1kΩ 22pF 1kΩ 10pF VIN+ ADC 10 AD9215 VIN– AGND DGND 0.1µF 1FB = FERRITE BEAD, MURATA BLM21BB750SN1, 75Ω @ 100MHz. Figure 1. Interconnecting an AD8331 VGA and an AD9215 ADC (Simplified Schematic, All Connections/Decoupling Not Shown) 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/circuits Fax: 781.461.3113 ©2009–2010 Analog Devices, Inc. All rights reserved. 08352-001 Circuit Evaluation Boards AD8331ARU-VGA-ADC Evaluation Board HSC-ADC-EVALB-DCZ Data Capture Board Design and Integration Files Schematics, Layout Files, Bill of Materials CN-0096 Circuit Note The 48 dB gain range of the VGA makes these devices suitable for a variety of applications. Excellent bandwidth uniformity is maintained across the entire range. The gain control interface provides precise linear-in-dB scaling of 50 dB/V for control voltages between 40 mV and 1 V. Factory trim ensures excellent part-to-part and channel-to-channel gain matching. CIRCUIT DESCRIPTION VGAs and modern ADCs have much greater functionality than the conventional op amps used with early ADC designs. In the VGA used in this example, gain is controlled externally. Pins are provided for selecting between gain values mapped for 10-bit or 12-bit converters, and the impedance of the low noise stage is adjustable over a wide range of impedance values with a series R-C network. The high speed converter options are available with simple pin strapping. which can be a serious problem with converters. A simple resistor controls the clamp amplitude. If no connection is made to the CLMP pin, the clamp voltage is 4.5 V p-p differential centered on a common-mode voltage of 2.5 V. A 1 MHz sinewave was selected for the test waveform, and a recovered waveform from the ADC Analyzer™ software is shown in Figure 2. The sampling frequency was 65 MSPS, corresponding to the 65 MSPS version of the AD9215. The LNA input signal was 70 mV p-p, following external lowpass and high-pass filters to reject signal generator spurs. The VGA gain was 29 dB, amplifying the signal to about one-half the ADC full-scale input voltage. Combined high-pass and lowpass filtering between the VGA and the ADC attenuates low frequency noise below 50 kHz (33 Ω and 100 nF yield a low frequency cutoff frequency of 48 kHz) and frequencies above 100 MHz (42 pF and 33 Ω yield a high frequency cutoff frequency of 114 MHz). Additional high-pass filtering is possible by reducing the value of the series capacitor at the LNA input and the series capacitors between the LNA and VGA inputs. 1024 The circuit shown in Figure 1 demonstrates the interconnection of a typical VGA and ADC. For this example, the AD8331 VGA and AD9215 ADC are compatible in frequency range and differential interface matching. For clarity, power supply decoupling is omitted from the figure. The AD8331 includes a low noise preamplifier, followed by a differential attenuator and gain stages. This VGA requires only a single 5 V power supply. Low noise 3 V for the ADC can be provided by an LDO, such as the ADP3339, which is connected to the 5 V supply. The differential output of the VGA is designed to drive ADCs with differential inputs over a wide range of devices from 1 V p-p to about 4.5 V p-p. The input range of the AD9215 can be set between 1 V p-p differential and 2 V p-p differential. For this circuit, the ADC input range was set to 2 V p-p differential. As with most single-supply devices, the AD8331 requires an internal mid-supply reference for a pair of mirrored amplifiers that provide equal but opposite polarity signals at the output referenced to the common-mode voltage (CMV). Consult the AD8331 data sheet for further information regarding this function. Pin 11 (VCM) of the VGA functions as both an input and an output. As an output, the VCM circuit is available for dc coupling at Pin 11, or the pin can be driven from a voltage source to modify the value of the common-mode voltage to accommodate ADCs with various input ranges. Left floating, the VCM voltage is one-half the supply voltage, which is optimum for ac-coupled applications. Pin 12, CLMP, clamps the output swing within the limits of the differential inputs of the ADC, thereby avoiding overdrive, 896 768 640 512 384 256 128 0 08352-002 Included in each channel are an ultralow noise preamp (LNA), an X-AMP® VGA with 48 dB of gain range, and a selectable gain postamp with adjustable output limiting. The LNA gain is 19 dB with a single-ended input and differential outputs. Using a single resistor, the LNA input impedance can be adjusted to match a signal source without compromising noise performance. 6783 6800 6817 6834 6851 6868 6885 6902 SAMPLES Figure 2. Reconstructed Full-Scale 2 VP-P 1 MHz Sinewave, Sampling Rate = 65 MSPS An output decoupling network, consisting of a fixed 100 Ω resistor in parallel with a ferrite bead inserted in series with each AD3331 output, may be required if there is more than approximately 25 pF stray PCB trace capacitance on VOH and VOL. Otherwise, the network is not required. Most modern ADCs provide pin access to the internal reference. For the AD9215, the internal reference voltage is 1 V, and external resistors bias the common-mode input voltage at one-half the 3 V supply. The data capture board interfaces to a laptop computer. The ADC Analyzer software enables the converter and provides waveform or FFT displays. For the AD9215, consult the AD9215 data sheet for the configuration details. Rev. A | Page 2 of 4 CN-0096 Circuit Note DATA CAPTURE BOARD POWER SUPPLY AD8331ARU-VGA-ADC EVALUATION BOARD AD8331 HSC-ADC-EVALB-DCZ ADC AD9215 VOL USB 08352-003 LNA VOH Figure 3. Block Diagram Showing Test Configuration Figure 3 is a simplified block diagram of the test setup. A 20-pin, dual-row header mounted to the evaluation board mates to half of the connector on the converter interface board. The board is controlled using ADC Analyzer software running on a standard laptop PC. The circuit must be constructed on a multilayer PC board with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of AGND and DGND and MT-101 Tutorial, Decoupling Techniques). A complete design support package for this circuit note can be found at http://www.analog.com/CN0096-DesignSupport COMMON VARIATIONS schematics, AD8331ARU-VGA-ADC-AssemblyDrawingRevA.pdf, and the “AD8331ARU-VGA-ADC-BOM-Rev0.pdf ” for the bill of materials. Functional Block Diagram See user guide UG-173 and Figure 1 of this circuit note for the circuit block diagram. Also see file “AD8331ARU-VGA-ADCSCH-Rev0.pdf ” for the schematics. Setup and Test See Circuit Description section for setup and test details and user guide UG-173 for the HSC-ADC-EVALB-DCZ data capture evaluation board. UG-173 describes the board operation, and installation of the ADC Analyzer software. Also see Figure 1 of this circuit note for the block diagram. LEARN MORE Other single-channel, 10-bit ADCs include the AD9214 for lower input frequencies or the AD9411 for faster sampling applications. CN0096 Design Support Package: http://www.analog.com/CN0096-DesignSupport MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of AGND and DGND. Analog Devices. CIRCUIT EVALUATION AND TEST MT-073 Tutorial, High Speed Variable Gain Amplifiers (VGAs). Analog Devices. This circuit note uses the AD8331ARU-VGA-ADC circuit board and the HSC-ADC-EVALB-DCZ evaluation board. The two boards have mating connectors that plug together, allowing for the quick setup and evaluation of the circuit’s performance. The AD8331ARU-VGA-ADC board contains the circuit described in this note. The HSC-ADC-EVALB-DCZ evaluation board, with the Analog Devices ADC Analyzer software is used to capture the ADC’s output data. MT-076 Tutorial, Differential Driver Analysis. Analog Devices. Equipment Needed MT-101 Tutorial, Decoupling Techniques. Analog Devices. Windows® XP, Windows Vista (32-bit), or Windows 7 (32-bit) PC with USB port, AD8331ARU-VGA-ADC, HSC-ADCEVALB-DCZ evaluation boards, and the AD9215 evaluation software, power supplies, spectrum analyzer, signal source. See user guide UG-173 and the AD8331 and AD9215 data sheets for additional details. Data Sheets and Evaluation Boards Getting Started See user guide UG-173 for the HSC-ADC-EVALB-DCZ data capture evaluation board operation and installation of the ADC Analyzer software. See Figure 1 of this circuit note for the block diagram, "AD8331ARU-VGA-ADC-SCH-Rev0.pdf ” for MT-074 Tutorial, Differential Drivers for Precision ADCs. Analog Devices. MT-075 Tutorial, Differential Drivers for High Speed ADCs Overview. Analog Devices. AD8331 Data Sheet AD8331 Evaluation Board AD9214 Data Sheet AD9215 Data Sheet AD9215 Evaluation Board AD9411 Data Sheet ADP3339 Data Sheet Rev. A | Page 3 of 4 CN-0096 Circuit Note REVISION HISTORY 11/10—Rev. 0 to Rev. A Changes to Circuit Note Title ......................................................... 1 Added Evaluation and Design Support Section ........................... 1 Changes to Circuit Function and Benefits Section ...................... 1 Changes to Circuit Description Section ........................................ 2 Changes to Figure 3 .......................................................................... 3 Added Circuit Evaluation and Test Section .................................. 3 7/09—Revision 0: Initial Version (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. 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