AN-1234 APPLICATION NOTE One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com Interfacing the ADL5534 Dual IF Gain Block to the AD9640 High Speed ADC Figure 1 shows an application circuit where the ADL5534 drives the AD9640, a 150 MSPS, 14-bit ADC. The two 49.9 Ω resistors connected to the AD9640 CML output establish the 0.9 V dc bias for the AD9640 inputs and set the AD9640 differential input impedance to 100 Ω. CIRCUIT FUNCTION AND BENEFITS This circuit uses the ADL5534 IF amplifier to provide a dual IF gain block for the AD9640 14-bit, 150 MSPS dual ADC. The ADL5534 is a high linearity, dual fixed, 20 dB gain amplifier that can be adapted for use as a driver for a high performance IF sampling ADC. The ADL5534 provides a simple approach to interfacing the RFIN signal level of 200 mV p-p to the 2 V p-p full scale of the high speed ADC. The low noise (2.5 dB NF at 70 MHz) and low distortion (IP3 of 40 dBm at 70 MHz) of the ADL5534 ensure that the ADC performance is not compromised. When operated as a quasi-differential amplifier, the ADL5534’s two amplifiers present 100 Ω differential input and output impedances. The input signal from a 50 Ω RF source is converted to a differential signal with a 1:2 impedance ratio flux-coupled transformer. This matches the 50 Ω source to the 100 Ω differential load of the ADL5534. Between the ADL5534 output and the AD9640 input, a third-order low-pass filter presenting a 100 Ω differential impedance to the source and load is implemented. CIRCUIT DESCRIPTION Table 1. Devices Connected/Referenced Product ADL5534 AD9640 Description 20 MHz to 500 MHz dual IF amplifier 14-bit, 80/105/125/150 MSPS, dual ADC Due to the different common mode voltage levels required by the ADL5534 and the AD9640, the ADL5534 must be ac-coupled to the AD9640. Capacitors of 100 pF were chosen to reduce any low frequency noise coming from the ADL5534 and to provide dc blocking. 5V 10nF 13 Z1 ADL5534 1nF CLIN1 NC 15 14 NC RFOUT1 NC CLIN2 NC GND CML = 0.9V NC RFOUT2 NC 5 1:2 Z 4 1µF NC 12 1nF 120nH 11 100pF 10 120nH 33Ω 1.8V VIN+ 49.9Ω 16pF 100pF AVDD CML DVDD AD9640 4.7pF 49.9Ω 9 120nH 1.8V 120nH 33Ω VIN– AGND DRGND 8 3 10nF NC NC RFIN 7 2 NC 6 1 RFIN2 MINI-CIRCUITS ADT2-1T-1P+ TRANSFORMER RFIN1 16 470nH 470nH 1µF GND Figure 1. Interface Between the ADL5534 and AD9640 with 49.9 Ω Resistors to CML to Establish the 0.9 V DC Bias for the AD9640 Inputs and Set the AD9640 Differential Impedance to 100 Ω (Simplified Schematic: Decoupling and All Connections Not Shown) Rev. B | Page 1 of 3 08537-001 5V 10nF AN-1234 Application Note The measured results for this filter show 0.5 dB insertion loss for a 20 MHz bandwidth centered around 92 MHz. Figure 2 shows the measured wideband response for the filter. 0 SFDR = 78.267dBc NOISE FLOOR = –110.131dB –15 FUND 1 = –7.181dBFs FUND 2 = –7.191dBFs –30 IMD (2F1 – F2) = –80.538dBc IMD (2F2 – F1) = –82.086dBc –45 5 (dBFS) 0 –10 –60 –75 –90 –15 –105 –20 –120 –25 –135 –30 0 6 –35 18 24 30 36 42 48 54 60 FREQUENCY (MHz) Figure 4. Measured Two-Tone Performance for Input Tones at 93 MHz and 92 MHz, Sample Rate of 122.8 MSPS –45 50 100 150 200 250 300 350 400 FREQUENCY (MHz) 08537-002 –40 COMMON VARIATIONS Figure 2. Measured Frequency Response of the LC Filter The single-tone FFT results shown in Figure 3 for an input signal of approximately 93 MHz show an SNR of 69.3 dB and an SFDR of 82 dBc. Note that because of aliasing, the fundamental frequency in the FFT is at 122.8 MHz – 93 MHz = 31.8 MHz. 0 SNR = 69.334dBc SFDR = 82.267dBc –15 NOISE FLOOR = –109.519dB FUND = –1.05dBFs –30 SECOND = –82.262dBc THIRD = –88.688dBc –45 (dBFS) 12 08537-004 NORMALIZED LOSS (dB) –5 This application circuit can be modified for any IF frequency within the operating range of the ADL5534 and AD9640. As an alternative to the ADL5534, the AD8375, a digitally programmable differential variable gain amplifier, can be used. Alternatively, the AD8352, a resistor programmable differential amplifier, can be used to convert from single ended to differential without the requirement of an external balun. The AD8352, AD8375, and AD8376 (dual version of the AD8375) are all true differential amplifiers that provide rejection of common-mode signals at their input. 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, MT-101 Tutorial, the ADL5534 evaluation board layout, and the AD9640 evaluation board layout). Both the ADL5534 and the AD9640 have exposed thermal pads that should be soldered directly to the low impedance ground plane. –60 –75 –90 –105 –120 0 6 12 18 24 30 36 42 48 54 60 FREQUENCY (MHz) 08537-003 –135 Figure 3. Measured Single-Tone Performance for An Input Frequency of 93 MHz, Sample Rate of 122.8 MSPS The two-tone results for tones at 91 MHz and 93 MHz are shown in Figure 4 and yield an IMD3 of −80.5 dBc and SFDR of 78 dBc. The AD9640 was clocked at a sample rate of 122.8 MSPS for both single and two-tone tests. LEARN MORE AN-742 Application Note, Frequency Domain Response of Switched Capacitor ADCs. Analog Devices. AN-827 Application Note, A Resonant Approach to Interfacing Amplifiers to Switched-Capacitor ADCs. Analog Devices. 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-101 Tutorial, Decoupling Techniques. Analog Devices. Rev. B | Page 2 of 3 Application Note AN-1234 Data Sheets and Evaluation Boards REVISION HISTORY ADL5534 Data Sheet 4/13—Rev. A to Rev. B Changed Document Title from CN-0049 to AN-1234 .............................................................................. Universal ADL5534 Evaluation Board AD9640 Data Sheet AD9640 Evaluation Board AD8352 Data Sheet AD8375 Data Sheet 2/10—Rev. 0 to Rev. A Updated Format ................................................................. Universal Changes to Circuit Function and Benefits..................................... 1 Changes to Circuit Description....................................................... 1 Changes to Common Variations ..................................................... 2 8/08—Revision 0: Initial Version ©2008–2013 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. AN08537-0-4/13(B) Rev. B | Page 3 of 3