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LTC1068-200CG 数据表(PDF) 12 Page - Linear Technology |
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LTC1068-200CG 数据表(HTML) 12 Page - Linear Technology |
12 / 30 page LTC1068 Series 12 1068fc MODES OF OPERATION Linear Technology’s universal switched-capacitor filters are designed for a fixed internal, nominal fCLK/fO ratio. The fCLK/fO ratio is 100 for the LTC1068, 200 for the LTC1068- 200, 50 for the LTC1068-50 and 25 for the LTC1068-25. Filter designs often require the fCLK/fOratioofeachsection to be different from the nominal ratio and in most cases different from each other. Ratios other than the nominal valuearepossiblewithexternalresistors.Operatingmodes useexternalresistors,connectedindifferentarrangements to realize different fCLK/fO ratios. By choosing the proper mode, the fCLK/fOratiocanbeincreasedordecreasedfrom the part’s nominal ratio. The choice of operating mode also effects the transfer function at the HP/N pins. The LP and BP pins always give the lowpass and bandpass transfer functions respectively, regardless of the mode utilized. The HP/N pins have a different transfer function depending on the mode used. Mode 1 yields a notch transfer function. Mode 3 yields a highpasstransferfunction.Mode2yieldsahighpassnotch transfer function (i.e., a highpass with a stopband notch). More complex transfer functions, such as lowpass notch, allpass or complex zeros, are achieved by summing two or more of the LP, BP or HP/N outputs. This is illustrated in sections Mode 2n and Mode 3a. Choosing the proper mode(s) for a particular application is not trivial and involves much more than just adjusting the fCLK/fO ratio. Listed here are four of the nearly twenty modes available. To make the design process simpler and quicker, Linear Technology has developed the FilterCAD for Widows design software. FilterCAD is an easy-to-use, powerful and interactive filter design program. The de- signer can enter a few filter specifications and the program produces a full schematic. FilterCAD allows the designer to concentrate on the filter’s transfer function and not get bogged down in the details of the design. Alternatively, those who have experience with the Linear Technology family of parts can control all of the details themselves. For a complete listing of all the operating modes, consult the appendices of the FilterCAD manual or the Help files in FilterCAD. FilterCAD can be obtained free of charge on the Linear Technology web site (www.linear.com) or you can order the FilterCAD CD-ROM by contacting Linear Technology Marketing. Mode 1 In Mode 1, the ratio of the external clock frequency to the center frequency of each 2nd order section is inter- nally fixed at the part’s nominal ratio. Figure 4 illustrates Mode 1 providing 2nd order notch, lowpass and band- pass outputs. Mode 1 can be used to make high order Butterworth lowpass filters; it can also be used to make low Q notches and for cascading 2nd order bandpass functions tuned at the same center frequency. Mode 1 is faster than Mode 3. PleaserefertotheOperatingLimitsparagraphunderApplica- tions Information for a guide to the use of capacitor CC. Mode 1b Mode 1b is derived from Mode 1. In Mode 1b (Figure 5) two additional resistors R5 and R6 are added to lower the amount of voltage fed back from the lowpass output into the input of the SA (or SB) switched-capacitor summer. This allows the filter’s clock-to-center frequency ratio to be adjusted beyond the part’s nominal ratio. Mode 1b maintains the speed advantages of Mode 1 and should be considered an optimum mode for high Q designs with fCLK to fCUTOFF (or fCENTER) ratios greater than the part’s nominal ratio. The parallel combination of R5 and R6 should be kept below 5k. PleaserefertotheOperatingLimitsparagraphunderApplica- tions Information for a guide to the use of capacitor CC. Figure 4. Mode 1, 2nd Order Filter Providing Notch, Bandpassing and Lowpass Outputs – + Σ AGND R1 N BP LP VIN 1068 F04 + – S R2 R3 CC fO = ; fn = fO Q = ; HON = – ; HOBP = – HOLP = HON R2 R1 R3 R1 R3 R2 fCLK RATIO DEVICE LTC1068 LTC1068-200 LTC1068-50 LTC1068-25 RATIO 100 200 50 25 |
类似零件编号 - LTC1068-200CG |
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类似说明 - LTC1068-200CG |
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