circuit note cn- 0046 circuit designs using analog devices products apply these product pairings quickly and with confidence. for more information and/or support call 1 -800- analogd (1 -800-262-5643) or visit www.analog.com/circuit . devices connected /referenced ad8352 ultral ow distortion differential rf/if amplifier ad9445 14- bit, 105 msps/ 125 msps analog -to - digital converter using the ad8352 as a n ultral ow distortion differenti al rf/if front end for high speed adcs rev. a circuits from the lab from analog devices have been designed and buil t by analog devices engineers. standard engineering practices ha ve 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 suit ability 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 circuit fr om the lab. (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 ? 2008 C 2009 analog devices, inc. all rights reserved. circuit function and benefits these circuits provide both a single - ended and a differential configuration for driving high sp eed adcs using the ad8352 ultra low distortion differential rf/if amplifier. the ad8352 provides the gain, isolation, and distortion performance necessary for efficiently driving high linearity converters, such as the ad9445. this device also provides balanced outputs whether driven differentially or single - ended, thereby maintaining excellent second - order distortion levels. circuit de scription figure 1 and figure 2 illustrate two front - end circuit s for driving th e ad9445 14 - bit adc at 105 msps . figure 1 provide s a differential input configuration, while figure 2 provides a s ingle - ended input configuration. in the differential configuration shown in figure 1 , the input 49.9 ? resistor provides a differential input impedance to the 50 ? rf/if source. when the driver is located less than approximately one eighth of the wavelength of the maximum input rf/if frequency from the ad8352, impedance matching is not required, thereb y eliminating the need for this termination resistor. the output 24 ? series resistors provide isolation from the input capacitance of the adc, and the optimum value is determined empirically. the 100 mhz fft plots shown in figure 3 and figure 4 display the performance results for the differential configuration. in the single - ended input configuration shown in figure 2 , the net input impedance at v ip is r n (200 ?) plus the external 24.9 ? balancing resistor, or ~225 ? . this requires a 64.9 ? parallel resistor to provide the input impedance match for a 50 ? source. if input reflections are minimal, this impedance match is not required. the 200 ? resistor (r n ) is req uired to balance the output voltages to minimize second - order distortion. the single - ended configuration provides ?3 db bandwidths similar to input differential drive and shows little or no degradation in overall thi rd - order harmonic performance. the singl e - ended, third - order distortion levels are similar to the 08446-001 ad8352 49.9 ? c d r d r g 0.1 f 0.1 f 0.1 f 0.1 f 16 1 2 3 4 5 24? 0.1 f 10 24? 0.1 f 11 8, 13 14 6, 7, 9, 12 ad9445 50 ? source if/rf input minicircuits adt1-1wt 5v figure 1 . differential input to the ad8352 driving the ad9445 14 - b it, 105 msps/125 msps adc (simplified schematic, all connections not shown)
cn- 0046 circuit note rev. a | page 2 of 3 ad8352 r d r g 24.9 ? 64.9 ? 50 ? c d 0.1 f 0.1 f ac vip 5v vin r n 200 ? 33? 33? 0.1 f 0.1 f 0.1 f vop von ad9445 vin+ vin? 08446-002 figure 2 . single- ended input to the ad8352 driving the ad9445 adc (simplified schematic, all connections not shown) differential fft plots in figure 3 and figure 4 . the single - ended circuit avoids the use of a transformer or balun in front of the amplifier while still maintaining excellent distortio n up to approximately 100 mhz. however, at frequencies above approximately 100 mhz, second - order distortion increases when the ad8352 is driven single - ended due to phase - related errors. 0 ?150 0 52.50 frequency (mhz) amplitude (dbfs) ?10 ?20 ?30 ?40 ?50 ?60 ?70 ?80 ?90 ?100 ?110 ?120 ?130 ?140 5.25 10.50 15.75 21.00 26.25 31.50 36.75 42.00 47.25 snr = 67.26dbc sfdr = 83.18dbc noise floor = ?110.5db fund = ?1.074dbfs second = ?83.14dbc third = ?85.39dbc 08446-003 figure 3 . single tone distortion, ad8352 driving ad9445, sampling clock = 105 msps, analog input f requency = 100 mhz, a v = 10 db. see figure 1. in both co nfigurations, r g is the gain setting resistor for the ad8352, with the r d and c d components providing distortion cancellation. the ad9445 differential input impedance is approximately 2 k ? in parallel with 5 pf and requires a 2.0 v p - p differential signal (v ref = 1 v) between v in+ and v in ? for a full - scale input signal. the output of the amplifier is ac - coupled to allow for an optimum common - mode voltage at the adc input. the common - mode voltage at the input of the ad9445 is set to 3.5 v by an internal netw ork. input ac - coupling can be required if the source also requires a common - mode voltage that is outside the optimum range of the ad8352. a v cm common - mode pin is provided on the ad8352 that equally shifts both input and output common - mode levels. increasi ng the gain of the ad8352 increases the system noise and, thus, decreases the snr (3.5 db at 100 mhz input for a v = 10 db) of the ad9445 when no filtering is used. however, it should be noted that amplifier gains from 3 db to 18 db, with proper selection of c d and r d , do not appreciably affect distortion levels. these circuits, when con figure d properly, can result in sfdr performance of better than 87 dbc at 70 mhz and 82 dbc at 180 mhz input. single - ended drive, with appropriate c d and r d , give s similar results for sfdr and third - order intermodulation levels as shown in these figures. excellent layout, grounding, and decoupling techniques must be utilized in order to achieve the desired performance from the circuits discussed in this note. as a minimum, a 4 - layer pcb should be used with one ground plane layer, one power plane layer, and two signal layers. all ic power pins must be decoupled to the ground plane with low inductance multi layer ceramic capacitors (mlcc) of 0.01 f to 0.1 f (this is not shown in the diagrams for simplicity). follow the recommendations on the individual data sheets for the ics. the product evaluation boards should be consulted for recommended layout and critical component placement. the y can be accessed through the main produc t pages for the devices or their data sheets. frequency (mhz) amplitude (dbfs) 0 ?150 0 52.50 ?10 ?20 ?30 ?40 ?50 ?60 ?70 ?80 ?90 ?100 ?110 ?120 ?130 ?140 5.25 10.50 15.75 21.00 26.25 31.50 36.75 42.00 47.25 08846-004 snr = 61.98dbc noise floor = ?111.2db fund1 = ?7.072dbfs fund2 = ?7.043dbfs imd (2f2-f1) = ?89dbc imd (2f1-f2) = ?88dbc figure 4. two tone intermodulation distortion , ad8352 driving ad9445, sampling clock = 105 m sps, analog input f requency = 98 mhz and 101 mhz, a v = 10 db . see figure 1.
circuit note cn- 0046 rev. a | page 3 of 3 c ommon variations placing antialiasing filters between the adc and the amplifier is a common approach for im proving overall noise and broad band distortion performance for both band - pass and low - pass applications. for high freq uency filtering, matching to the filter is required. the ad8352 maintains a 100 ? output impedance well beyond most applications and is well - suited to drive most filter configurations with little or no degradation in distortion. the ad8352 low distortion d ifferential amplifier can be replaced by the high ip3, low noise figure ad8375 variable gain amplifier (vga). 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. (see circuit no te cn - 0002, using the ad8376 vga to drive wide bandwidth adcs for high if ac - coupled applications .) learn more cn - 0002 circuit note, using the ad8376 vga to drive wide ba ndwidth adcs for high if ac - coupled applications . 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 tutor ia l , differential drivers for high speed adcs overview . analog devices. mt - 101 tutorial, decoupling techniques . analog devices. data sheets ad8352 data sheet. ad8375 data sheet. ad8376 data sheet. ad9445 data sheet. ad9445 evaluation board. high speed adc evaluation kits and evaluation boards. revision history 8/09 rev. 0 to rev. a updated format ................................................................. univers al 10/08 revision 0: initial version (continued from first page) "circuits from the lab" are intended only for use with analog devices products and are the intellectual property of analog de vices or its licensors. while you may use the "circuits from th e lab" in the design of your product, no other license is granted by implication or otherwise under any patents or other inte llectual property by application or use of the "circuits from the lab". information furnished by analog devices is believed to be a ccurate and reliable. however, "circuits from the lab" 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 particul ar 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 t heir use. analog devices reserves the right to change any "circuits from the lab" at an y time without notice, but is under no obligation to do so. trademarks and registered trademarks are the property of their re spective owners. ? 2008 C 2009 analog devices, inc. all rights reserved. trademarks a nd registered trademarks are the property of their respective owners. cn08446 - 0 - 8/09(a)
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