Circuit Note CN-0268 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/CN0268. ADL5565 6 GHz Ultrahigh Dynamic Range Differential Amplifier AD9467 16-Bit, 200 MSPS/250 MSPS ADC Resonant Approach to Designing a Band-Pass Filter for Narrow-Band, High IF, 16-Bit, 250 MSPS Receiver Front End EVALUATION AND DESIGN SUPPORT CIRCUIT DESCRIPTION Design and Integration Files Schematics, Layout Files, Bill of Materials The advantages of using a differential amplifier to drive a high speed ADC include signal gain, isolation, and source impedance matching to the ADC. The ADL5565 allows pin-strappable gain adjustments of 6 dB, 12 dB, or 15.5 dB. Alternatively, by applying two external resistors to the inputs, finer gain steps can be achieved within the 0 dB to 15.5 dB range. Additionally, the ADL5565 offers high output linearity, low distortion, low noise, and wide input bandwidth. The 3 dB bandwidth is 6 GHz, and the 0.1 dB flatness is 1 GHz. The ADL5565 is capable of achieving an output third-order intercept (OIP3) of greater than 50 dB. CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 is a 16-bit, 250 MSPS, narrow-band, high IF receiver front end with an optimum interface between the ADL5565 differential amplifier and the AD9467 ADC. The AD9467 is a buffered input 16-bit, 200 MSPS or 250 MSPS ADC with SNR performance of approximately 75.5 dBFS and SFDR performance between 95 dBFS and 98 dBFS. The ADL5565 differential amplifier is suitable for driving IF sampling ADCs because of its high input bandwidth, low distortion, and high output linearity. This circuit note describes a systematic procedure for designing the interface circuit and the antialiasing filter that maintains high performance and ensures minimal signal loss. A resonant approach is used to design a maximally flat Butterworth fourthorder band-pass filter with a center frequency of 200 MHz. +3.3V +3.3V 5.6Ω 0.1µF VIP2 INPUT Z = 50Ω 33Ω 15Ω 150nH AD9467 5Ω ADL5565 VIN1 33Ω 39nH VIP1 ZI = 200Ω 0.1µF 1nF 8.2pF 2pF 39nH 150nH 310Ω 530Ω 180nH +1.8V 3.5pF 16-BIT 250MSPS ADC G = 6dB 5Ω VIN2 5.6Ω 1nF INTERNAL INPUT Z 15Ω FS = 2V p-p DIFF 10560-001 XFMR 1:1 Z ECT1-1-13M Figure 1. Resonant Filter Design for Narrow Band High IF Applications Using the ADL5565 Differential Amplifier and the AD9467 ADC 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-0268 Circuit Note Figure 2 shows a single-ended-to-differential approach to driving the ADL5565 using a balun or transformer. This configuration may not be a viable or desirable option in certain applications. The ADL5565 offers flexibility in its driver interface and can be driven single ended, as shown, or differentially with a differential mixer, for example. Refer to the ADL5565 data sheet for details on the different input interfaces. To achieve the optimal level of performance that the ADL5565 and AD9467 have to offer, it is important to properly follow the design guidelines as specified on the respective data sheets. Some of the important design criteria include properly matching the input and output impedance of the ADL5565 for minimum signal loss and optimum linearity performance, systematic design of an antialiasing filter for improved dynamic range, and source impedance matching to the ADC inputs. ADL5565 Output Load Matching ADL5565 Input Impedance Matching The ADL5565 linearity performance has been optimized for a 200 Ω output load. This is a common output impedance used to interface to ADCs and for filter design. With an optimized output load of 200 Ω, the output IP3 of the ADL5565 at 200 MHz is 46 dBm. R3 R1 VIP2 0.1µF R4 ETC1-1-13 50Ω VIP1 VOP ADL5565 In situations where a 200 Ω output load may not fit the application, tradeoffs can be made between the output load of the ADL5565 and its linearity performance. Figure 3 shows a plot of thirdorder intermodulation (IMD3) vs. frequency for commonly used output loads. R5 VIN1 VON R6 VIN2 10560-002 R2 0.1µF Figure 2. ADL5565 Input Impedance Match 0 Figure 2 shows the recommended input matching network for the ADL5565. The input impedance of the ADL5565 is gain dependent, and the differential input impedance is 200 Ω for 6 dB gain, 100 Ω for 12 dB gain, and 67 Ω for 15.5 dB gain. To match the 50 Ω source impedance of the signal generator to the input impedance of the ADL5565, R1 and R2 must be chosen so that their sum in parallel with the input impedance of the ADL5565, ZI, is equal to 50 Ω. To maintain balance in the differential circuit, R1 must equal R2. The following formula can be used to calculate the necessary matching resistors. –20 IMD3 (dBc) –40 50Ω LOAD 100Ω LOAD 200Ω LOAD 400Ω LOAD –60 –80 –100 –120 –140 2R1 || Zl = 50 Ω R1 = R2 = 0 50 100 150 200 250 300 350 400 FREQUENCY (MHz) 25 1 − (50 / Z l ) 450 500 10560-003 R1 = R2 Figure 3. ADL5565 IMD3 vs. Frequency for 50 Ω, 100 Ω, 200 Ω, and 400 Ω Output Loads, 3.3 V Supply, Gain = 6 dB Table 1 shows the calculated termination resistors and pin configuration for the different gain settings of the ADL5565. An alternative configuration to the one shown in Figure 2 is to replace the 1:1 balun, ETC1-1-13, with an impedance transformation RF transformer. This can eliminate the need for R1 and R2. A 1:4 transformer can be used for the 6 dB gain configuration or a 1:2 transformer for the 12 dB gain configuration. The advantages of this alternative configuration are lower component count and minimum signal loss. However, pay attention to the bandwidth of the transformer. Impedance transformation transformers have narrower bandwidths and higher insertion loss as compared to a 1:1 balun. Table 1. Gain, Input Impedance, and R1, R2, R3, R4, R5, and R6 Values for ADL5565 Gain (dB) 6 12 15.5 ADL5565 Input Impedance, Zl, (Ω) 200 100 67 R1 (Ω) 33 50 Open Rev. 0 | Page 2 of 6 R2 (Ω) 33 50 Open R3 (Ω) Open 0 0 R4 (Ω) 0 Open 0 R5 (Ω) 0 Open 0 R6 (Ω) Open 0 0 Circuit Note CN-0268 39nH When interfacing to the ADC, it is recommended that the real input impedance be reduced from 530 Ω to a lower value within the 200 Ω to 400 Ω range. By lowering the input impedance of the ADC, the kickback due to the sample-and-hold structure settles out faster, yielding improved linearity performance. The tradeoff is increased input power because more power is required to drive the full scale of the ADC. In this circuit example, the input impedance of the AD9467 was reduced to 200 Ω to match the output impedance of the ADL5565 and also to balance the linearity vs. input power of the ADC. The input impedance of the AD9467 was reduced to 200 Ω by placing a 310 Ω resistor in parallel with the ADC differential input. 8.2pF 39nH In the circuit in Figure 1, the ADS program was used to design a fourth-order maximally flat (Butterworth) low-pass filter. Figure 4 shows the low-pass filter design with a source and load impedance of 200 Ω and a 3 dB cutoff frequency of 300 MHz. The 200 Ω impedance was chosen because it is the common source and load impedance of the driver amplifier and ADC. The first elements are series inductors to ease driver requirements. In the final optimized circuit of Figure 1, the filter source impedance is equal to approximately 21.6 Ω; however, 200 Ω was chosen to design the low-pass portion of the filter because the overall filter is ultimately a resonant band-pass filter, and it is more critical that the amplifier and ADC see the correct load and source impedance for optimized linearity performance. The effect of doing this is amplitude loss due to the impedance mismatch. 150nH Figure 4. Low-Pass Filter Design The low-pass filter design was further tuned by creating resonance to cause peaking at the band of interest. This resulted in a narrow-band, band-pass filter at a high IF. Placing an inductor across the ADC differential inputs nulls the input capacitance of the ADC and creates peaking. Figure 5 shows the calculation used to determine the resonant inductor value. In the case of the 3.5 pF source impedance of the AD9467, a parallel inductor of 181 nH is necessary to null the capacitive susceptance; leaving only the high impedance resistive portion of the RC parallel equivalent. The resonant frequency chosen for the calculation was 200 MHz. ZL ZR ZC Figure 5. Resonant Match 1 j ωC Z L = j ωL ZC = Antialiasing Filter Design An antialising filter ahead of the ADC helps reduce signal content and noise from unwanted Nyquist zones that would otherwise alias in band and degrade the dynamic performance. Antialiasing filters are often designed using LC networks and must have well defined source and load impedances to achieve the desired stop-band and pass-band characteristics. The filter design is accomplished using software available from Nuhertz Technologies or Agilent Technologies Advanced Design Systems (ADS), for example. 2pF 10560-005 The AD9467 is an ideal choice for an ADC in this circuit because it is an IF sampling ADC optimized for high performance over wide bandwidths and ease of use. The AD9467 has an integrated buffer that presents a fixed input impedance to the driver amplifier. This input structure is an advantage over ADCs that use an unbuffered front end directly coupled to the sampling switches. Unbuffered ADCs present time varying input sample-and-hold impedances to the drive amplifier. The addition of the input buffer eases the drive requirements at the expense of slightly higher power consumption. The buffered source impedance of the AD9467 is modeled as a fixed impedance of a 530 Ω resistance in parallel with a 3.5 pF capacitance. 150nH 10560-004 AD9467 Source Impedance YC = 1 ZC YL = 1 ZL YC + YL = 0 L= 1 ω 2C Measured Performance Figure 1 shows the final circuit configuration. The outputs of the ADL5565 were padded with 5.6 Ω on each output to improve the stability of the driver amplifier. The recommended series resistance is generally between a few ohms to several tens of ohms. A larger resistor value improves on stability; however, the tradeoff is a power loss because the series resistor forms a voltage divider with the impedance at the ADC inputs, resulting in signal attenuation. Following the series resistors at the output of the ADL5565 are 1 nF dc blocking capacitors. Following that is the antialiasing filter and then the parallel resistor of 310 Ω to reduce the input impedance of the ADC. Finally, the 15 Ω resistors in series with the ADC inputs isolate the internal switching transients from the filter and the amplifier. Rev. 0 | Page 3 of 6 CN-0268 Circuit Note Figure 6 and Figure 7 shows the resulting antialiasing filter response with a 1 dB bandwidth of 41 MHz and a 3 dB bandwidth of 89 MHz, centered at an IF of 203 MHz. Figure 8 shows the FFT spectrum for the final receiver circuit of Figure 1, where the SNR is 72.5 dBFS, and the SFDR performance approaches 90 dBc. 20.000 Using ADS as a simulation tool, the filter components can be further tuned to shift the resonant peak to the desired IF. For example, by changing the parallel 8.2 pF capacitor of the antialiasing filter to 10 pF shifts the resonance peak lower to 180 MHz. Figure 9 through Figure 11 show the filter profile and single-tone FFT performance for this condition. 20.000 14.286 14.286 8.571 AMPLITUDE (dB) AMPLITUDE (dB) 8.571 2.857 –2.857 –8.571 2.857 –2.857 –8.571 –14.286 100 200 FREQUENCY (MHz) 300 400 –20.000 0 100 200 FREQUENCY (MHz) Figure 6. Antialiasing Filter Response, fC = 203 MHz 300 400 Figure 9. Antialiasing Filter Response, fC = 183 MHz 0 0 –0.5 –0.5 1dB BW = 41MHz –1.0 AMPLITUDE (dB) AMPLITUDE (dB) –1.0 –1.5 –2.0 –2.5 1dB BW = 40MHz –1.5 –2.0 –2.5 –3.0 3dB BW = 89MHz 160 180 –3.0 200 220 240 FREQUENCY (MHz) 3dB BW = 75MHz –3.5 100 120 Figure 7. Antialiasing Filter Response, fC = 203 MHz, 1 dB and 3 dB Bandwidth 220 240 fIN = 183MHz fS = 245.76MHz –10 –20 –50 –30 –60 –40 AMPLITUDE (dBFS) –40 –70 –80 –90 –100 –110 SNR = 73dB SFDR = –91dBc H2/H3 = –94dBc/–91dBc –50 –60 –70 –80 –90 –100 –120 0 20 40 60 80 100 FREQUENCY (MHz) 120 –110 –120 –130 Figure 8. Single Tone FFT Plot, Input = 203 MHz, Sampling Rate = 245.76 MSPS 0 20 40 60 80 100 FREQUENCY (MHz) Figure 11. Single Tone FFT Plot, Input = 183 MHz, Sampling Rate = 245.76 MSPS Rev. 0 | Page 4 of 6 120 10560-011 –130 10560-008 AMPLITUDE (dBFS) 200 0 SNR = 72.5dB SFDR = –88.9dBc H2/H3 = –89.1dBc/–88.9dBc –30 180 Figure 10. Antialiasing Filter Response, fC = 183 MHz, 1 dB and 3 dB Bandwidth fIN = 203MHz fS = 245.76MHz –20 160 FREQUENCY (MHz) 0 –10 140 10560-010 140 10560-007 –3.5 120 10560-009 0 10560-006 –14.286 –20.000 Circuit Note CN-0268 COMMON VARIATIONS CIRCUIT EVALUATION AND TEST Quite a few combinations of drivers and high speed ADCs are available; however, for optimum performance, it is important to pay attention to the input and output impedance of the ADC driver and the input reactance of the ADC. Each device has its own unique impedance characteristic. A common variation to the Figure 1 circuit is the ADL5562 (3.3 GHz bandwidth) driving the AD9467 with a low-pass, antialiasing filter design for wideband receiver applications, as described in Circuit Note CN-0227. The circuit shown in Figure 1 is implemented using the AD9467 evaluation board (AD9467-250EBZ). The bottom side of the AD9467 evaluation board includes the ADL5562 and a prototype area for a fourth-order filter. The ADL5562 was replaced with the ADL5565 because both ADC drivers are pin compatible. See User Guide UG-200 for the complete schematics, BOM, and layout for the AD9467-250EBZ board. Table 2 shows the modifications to the AD9467 evaluation board required to duplicate the circuit shown in Figure 1. Complete documentation for this circuit note can be found in the CN-0268 Design Support package located at: http://www.analog.com/CN0268DesignSupport. Similarly, Circuit Note CN-0110 describes using the ADL5562 differential driver amplifier to drive wide bandwidth ADCs, such as the AD9445, for high IF ac-coupled applications. Another alternative where variable gain is desired, the ADL5565 can be replaced with the AD8375 variable gain amplifier. The AD8375 is a digitally controlled, variable gain, wide bandwidth amplifier that provides precise gain control across a broad 24 dB gain range with 1 dB resolution. The AD8376 is a dual version of the AD8375. Circuit Note CN-0002 describes how to use the AD8376 VGA to drive wide bandwidth ADCs for high IF, ac-coupled applications. This circuit uses the modified AD9467-250EBZ circuit board and the HSC-ADC-EVALCZ FPGA-based data capture board to run the tests. The two boards have mating high speed connectors, allowing for the quick setup and evaluation of the circuit’s performance. The modified AD9467-250EBZ board contains the circuit evaluated as described in this note, and the HSCADC-EVALCZ data capture board is used in conjunction with VisualAnalog evaluation software, as well as the SPI controller software to properly control the ADC and capture the data. Application Note AN-835 contains complete details on how to set up the hardware and software to run the tests described in this circuit note. Table 2. AD9467 Evaluation Board Modification for the ADL5565 Driver Option Reference Designator R121, R122, C109, C110, C117, R103, C116, R130, C118 R125, R110, R107, R113, R114, R119, R120 T103 R105, R106 C101, C105, C106, C107 U100 R117, R118 C113, C114 L101, L102 C119 L103, L104 C120 L100 R111, R112 R127, R128 Description DNI 0Ω Balun, 1:1 impedance ratio 33 Ω 0.1 µF ADL5565 5.6 Ω 1 nF 39 nH 8.2 pF 150 nH 2 pF 180 nH 155 Ω 15 Ω Rev. 0 | Page 5 of 6 Manufacturer Part Number M/A-Com MABA-007159-000000 Analog Devices Coilcraft Murata Coilcraft Murata Coilcraft 0805CS GRM15 0805CS GRM15 0805CS CN-0268 Circuit Note LEARN MORE CN-0268 Design Support Package: http://www.analog.com/CN0268-DesignSupport UG-200 User Guide: Evaluating the AD9467 16-Bit, 200 MSPS/250 MSPS ADC, Analog Devices. CN-0002 Circuit Note, Using the AD8376 VGA to Drive Wide Bandwidth ADCs for High IF AC-Coupled Applications. Analog Devices CN-0110 Circuit Note, Using the ADL5562 Differential Amplifier to Drive Wide Bandwidth ADCs for High IF ACCoupled Applications, Analog Devices Newman, Eric and Rob Reeder. AN-827 Application Note, A Resonant Approach to Interfacing Amplifiers to SwitchedCapacitor ADCs. Analog Devices. Reeder, Rob. AN-742 Application Note, Frequency Domain Response of Switched Capacitor ADCs. Analog Devices. MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND." Analog Devices. MT-073 Tutorial, High Speed Variable Gain Amplifiers (VGAs). Analog Devices. MT-075 Tutorial, Differential Drivers for High Speed ADCs Overview. Analog Devices. CN-0227 Circuit Note, High Performance, 16-Bit, 250 MSPS Wideband Receiver with Antialiasing Fitler, Analog Devices. Arrants, Alex, Brad Brannon and Rob Reeder, AN-835 Application Note, Understanding High Speed ADC Testing and Evaluation, Analog Devices. MT-101 Tutorial, Decoupling Techniques, Analog Devices. Data Sheets and Evaluation Boards AD9467 Data Sheet ADL5565 Data Sheet Ardizzoni, John. A Practical Guide to High-Speed PrintedCircuit-Board Layout, Analog Dialogue 39-09, September 2005. Circuit Evaluation Board (AD9467-250EBZ) Standard Data Capture Platform (HSC-ADC-EVALCZ) REVISION HISTORY 4/10—Rev. 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. 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. CN10560-0-4/12(0) Rev. 0 | Page 6 of 6