Using fully differential op amps as attenuators

Using fully differential op amps as attenuatorsTexas Instrunents IncorporatedAmplifiers: Op Ampsis accomplished by keeping the parallel combination equal provides the same results as those shown in Figure 3 ofto Rg while simultaneously setting the voltage divider toPart 1. To download and install the free TINA-TI softwareprovide the input common-mode voltage at no loadvisitwww.ti.com/tina-tiandclickthedownloadbuttonDesign Examples 6a and 6bAnother way to eliminate the problem with inputFor Example 6a, we will use the same approach as forcommon-mode voltage is to use the input attenuator toExample 5, with RF-I kQ2, and calculate RG-4 ks2(thethe fda as the circuits attenuator as described earliernearest standard 1% value is 4.02 kQ2)and Rr=2.6666 kQ2 Conclusion(the nearest standard 1% value is 2.67kS2). This makesWe have analyzed two approaches that attenuate and levelZIN R 4.02 kQ2, and SPICe shows il lo be inore on Che ordershift high-amplitude, sE unipolar signals to the input rangeof 4.46 kQ2 VREp Should be set to the common-mode voltof lower-voltage input ADCs. The primary differenceage of the input signal and is calculated bybetween the unipolar input design and the bipolar designsVsig min sig max 0V+20Vdescribed in Parts I and 2 is that a reference voltage to theREF=10Valternate input must be provided in the unipolar design tomake sure the output swing is symmetrical about theThe function is the same as before, but with this approachcommon-mode voltage. For the first approach(Example 5)the only freedom of choice given the design requirementswe chose input resistor values to provide the requiredis the value of rattenuation and to keep the noise gain of the FDa equalToseeanexampleExcelworksheetgotohttp://to 2 for stability. This approach allows the use of a lowerwww.ti.com/it/zip/slyt859andclickOpentoviewthevalue for VREF. The second approach(Example 6a) usesWinZip directory online (or click Save to download thethe gain-setting resistors of the FDA in much the sameWin Zip file for offline use). Then open the file FDA-way as using an inverting op amp, then a resistor is bootAttenuator_ Examples sE_ Unipolar_ Input. xls and selectstrapped across the inputs to provide a noise gain of 2.Wethe Unipolar sE fda re rg atten worksheet tab To seesaw in the simulation that this last approach caused a proba TiNA-TI simulation of the circuit in Example 6a, go tohttp://www.ti.com/it/zip/slyt359andclickOpentoviewlem with the input common-mode voltage going too high onthe Winzip directory online (or click Save to download theChe posilive peaks of the input signal, but this was easilycompensated for by splitting the R resistor and tying theWin Zip file for offline use). If you have the TINA- TI softcenter to ground (Example 6b). The two approaches yieldware installed, you can open the file fDA_ Attenuatorthe same voltage translation that is needed to accomplishExamples_sE_Unipolar_ Input. TSC lo view the examplethe interface task. Other performance metrics were not(the middle circuit labeled"Example 6a). To downloadanalyzed here, but Che lwo approaches have substantiallyandinstallthefreetiNa-tisoftware,visitwwwti.com/the same noise, bandwidth, and other AC and DC performtina-ti and click the download buttonance characteristics as long as the value of RF is the sameThe simulation waveforms for Example 6a show that theThe input -attenuator approach in Example 5 is moresignal is distorted. Further investigation will show that theomplex but allows the input impedance to be adjustedinput common-mode voltage range of the THS4509 usedindependently froin Che gainl-setling resistors used aroundin the simulation has been violated at the most positivethe FDA. At least to a certain degree, lower values canaks of the input signal, causing nonlinear operation. Ireasily be achieved if desired, but there is a maximum allowthis case the sPice model shows a problem; but care mustbe taken to double-check operation against the data sheetable Rs+ where larger values require the RG+ resistor to bea negative value. For example, setting rs=4 kQ2 results inas not all sPice models will show this error. For instanceRG+=0Q. The spreadsheet tool provided will generatereplacing the THS1509 model with the THS1520 will simu#NUM! errors for this input as it tries to calculate thelate fine, but the actual device has a similar input commonnearest standard value, which then replicates throughoutmode voltage rangethe rest of the cells that require a value for RG+; but thisOne way to correct the problem is to use pull-down resis-value will worktors from the FDa input pins to ground as described in theTHS4509 data sheet. In this case, instead of placing the fullThe approach in Examples 6a and 6b is easier, but theinput impedance is set as a multiplication of the feedbackvalue Rr across the inputs, we place half the value(1.33resistor and attenuation ZIN- x Rr x Attenuation. ThiskQ2)from each input to ground. These resistors will act topull down the inputs and bring the common-mode voltagedoes allow some design flexibility by varying the value ofRF, but the impact on noise, bandwidth, distortion, andback into linear operation. To see a TINA-TI simulation ofother performance characteristics should be consideredthiscorrectedcircuit(example6b),gotohttp://www.tiAlso, as mentioned before, voltages at the amplifier nodescommit/zip/slyt359 and click Open to view the WinZipshould be checked against data-sheet specificationsdirectory online (or click Save lo download the Win zip filebecause sPice models will not always show problemsfor offline use). If you have the TINA-TI software installedOne final nole: The source inpedance will affect theyou can open the file FDA- Attenuator_ Examples_ senput gain or attenuation of either circuit, and should heUnipolar_Input. TSC to view the example (the bottomincluded in the value of Rs, especially if it is significantcircuit labeled"Example 6b"). Note that the circuitAnalog Applications Journal 40 2009www.ti.comHigh-Performance Analog ProductsAmplifiers: Op AmpsTexas Instruments IncorporatedReferencesRelated web sitesFor more information related to this article, you can downamplifier.ti.comloadanacrobatReader(fileatwww-s.ti.com/sc/techlit/www.ti.com/se/device/partnumberlitmumber and replace "inumber "with the TI Lit.# for Replace partnumber with ADS8321; THS4509,orthe materials listed belowTIIS4520Document iitleTI Lit。#TINA TI and spreadsheet support files for examples1. Jim Karki, " Fully-Differential Amplifierswww.ti.com/lit/zip/slyt359Application ReportTo download TiNa-ti software2.im Karki, "Using Fully Differential Op Ampswww.ti.com/tina-tias Attenuators, Part 1: Differential Bipolarnput Signals, Analog Applications Journal(2Q2009).3. Jin Karki, "Using Fully Differential Op Aipsas Attenuators, Part 2: Single-Ended BipolarInput Signals, Analog Applications Journal(3Q2009)High-Performance Analog Productswww.ti.com/aaj402009Analog Applications JournalIMPORTANT NOTICETexas Instruments Incorporated and its subsidiaries (Ti reserve the right to make corrections, modifications, enhancementsimprovements, and other changes to its products and services at any time and to discontinue any product or service without noticeCustomers should obtain the latest relevant information before placing orders and should verify that such information is current andcomplete All products are sold subject to TI's terms and conditions of sale supplied at the time of order acknowledgmentTI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI's standardwarranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. 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