差分滤波器设计

1、Application ReportSLWA053BNovember2007RevisedApril 2010Design of Differential Filters for High-Speed Signal ChainsKen Chan High Speed -Wireless InfrastructureABSTRACTDifferential filters have many desirable attributes. The task of designing differential filters can seem daunting at first. Single-end

2、ed filters designed in any filter design package can be converted to a differential implementation. This application report explores simple conversion techniques for low-pass, high-pass, and band-pass LC filters.1IntroductionDifferential signals have many desirable attributes in high-speed signal ch

3、ains:namely, common-mode rejection from a balanced signal path and matched filter response, immunity from single-ended component parasitic effects, decoupling the signal from requiring a ground reference, and rejection of even-order harmonics. However, most LC filter design information, techniques,

4、and software are for single-ended systems. This application report highlights some simple translations that can be made to convert the single-ended designs to fully differential designs. Some examples are used to demonstrate these translations.The design of single-ended passive analog LC filters has

5、 been simplified greatly by various pieces of software that can be obtained freely on the Internet. An Internet search for LC filter design yields many results, any of which can be used for this tutorial.2Single-Ended to Differential-Ended Filter Translation:2.1Low-Pass, High-Pass, and Band-Pass Fil

6、ter ImplementationsThe process of designing a differential filter requires the design of a single-ended filter that meets the filter requirements. Freely available tools may be used to generate a single-ended filter with the desirable frequency response. The low-pass, high-pass and band-pass differe

7、ntial filter implementations are discussed in the following sections.The single-ended LC low-pass filter can be converted to a differential filter by repeating and folding the design around GND to create the Vin+and Vin-. The GND is removed and the center capacitor values are recalculated:the C is h

8、alf-value (sumof two series C. The L in the horizontal parallel series paths are kept the same and the load resistor is doubled (sumof two series R. The process is illustrated in Figure 1.1SLWA053BNovember2007RevisedApril 2010Design of Differential Filters for High-Speed Signal Chains VinR26L2522HmR

9、25100WR28 L2622Hm100W22Hm R31L2822Hm100W 22HmWVout- Vin-L322HmR10100WV OUTV V R2100WL122HmOUTOUT- WL322HmR10100WV OUTV V R2100WL122HmR1200WOUT OUT-Single-Ended to Differential-Ended Filter Translation:Figure 1. Conversion Process From Single-Ended to a Differential FilterFigure 2. Shunt Capacitor In

10、put Low-Pass FilterFigure 3. Series Inductor Low-Pass FilterAll four of these produce the same response as shown in Figure 4.2Design of Differential Filters for High-Speed Signal Chains SLWA053BNovember2007RevisedApril 2010 WC21.1 nFR10V IN OUTV V R2C11.1 nFWFigure 4. Low-Pass Filter Response for Al

11、l Four Low-Pass Filters.Similar circuits can be translated for a simple LC high-pass filter. The circuit is folded along the GND point, the vertical series elements are added and the horizontal elements remain the same. This results in doubling the vertical inductors and load, similar to the approac

12、h used for the low-pass filters.Figure 5. Series Capacitor Input High-Pass Filter3SLWA053BNovember2007RevisedApril 2010Design of Differential Filters for High-Speed Signal Chains C41.1 nFR6V OUTV V R8C51.1 nFOUTOUT-Single-Ended to Differential-Ended Filter Translation:Figure 6. Shunt Inductor High-P

13、ass FilterAll four high-pass filters have the same frequency response as shown in Figure 7.Figure 7. Frequency Response for All Four High-Pass FiltersThis same principle holds for more complex BPF architectures. The single-ended filter is reflected around the GND point, the GND is removed, and the m

14、iddle elements are added together. Figure 8provides an example of a third-order Butterworth BPF.4Design of Differential Filters for High-Speed Signal Chains SLWA053BNovember2007RevisedApril 2010 V INOUTC7 55.5 pF V INOUTC8 55.5 pFV IN-OUT-Single-Ended to Differential-Ended Filter Translation:Figure 8. Example of Third-Order Butterworth Band-Pass FilterFigure 9. Frequency Response of the Third-Order Butterworth Band-Pass FilterFigure 10provides an example of a third-order Cauer BPF with some parallel ho