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AD6620ASZ Datenblatt(PDF) 3 Page - Analog Devices |
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AD6620ASZ Datenblatt(HTML) 3 Page - Analog Devices |
3 / 44 page AD6620 –3– REV. A Following CIC2 is the second fixed-coefficient decimating filter. This filter, CIC5, further reduces the sample rate by a program- mable ratio from 1 to 32. The data rate out of CIC5, fSAMP5, is determined by the decimation factors of MCIC5 and MCIC2. Each CIC stage is a FIR filter whose response is defined by the decimation rate. The purpose of these filters is to reduce the data rate of the incoming signal so that the final filter stage, a FIR RAM coefficient sum-of-products filter (RCF), can calculate more taps per output. As shown in Figure 1, on-chip multiplex- ers allow both CIC filters to be bypassed if a multirate clock is used. The fourth stage is a sum-of-products FIR filter with program- mable 20-bit coefficients, and decimation rates programmable from 1 to 32. The RAM Coefficient FIR Filter (RCF in Figure 1) can handle a maximum of 256 taps. The overall filter response for the AD6620 is the composite of all three cascaded decimating filters: CIC2, CIC5, and RCF. Each successive filter stage is capable of narrower transition band- widths but requires a greater number of CLK cycles to calculate the output. More decimation in the first filter stage will minimize overall power consumption. Data comes out via a parallel port or a serial interface. Figure 2 illustrates the basic function of the AD6620: to select and filter a single channel from a wide input spectrum. The frequency translator “tunes” the desired carrier to baseband. CIC2 and CIC5 have fixed order responses; the RCF filter provides the sharp transitions. More detail is provided in later sections of the data sheet. –fS/2 –3fS/8 –5fS/16 –fS/4 –3fS/16 –fS/8 –fS/16 DC fS/16 fS/8 3fS/16 fS/4 5fS/16 fS/2 3fS/8 SIGNAL OF INTEREST SIGNAL OF INTEREST "IMAGE" WIDEBAND INPUT SPECTRUM (–fsamp/2 TO fsamp/2) D' C' B' A' A C B D Figure 2a. Wideband Input Spectrum (e.g., 30 MHz from High-Speed ADC) –fS/2 –3fS/8 –5fS/16 –fS/4 –3fS/16 –fS/8 –fS /16 DC fS/16 fS/8 3fS/16 fS/4 5fS/16 fS/2 3fS/8 AFTER FREQUENCY TRANSLATION NCO "TUNES" SIGNAL TO BASEBAND A B C D D' C' B' A' Figure 2b. Frequency Translation (e.g., Single 1 MHz Channel Tuned to Baseband) 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 –120 –130 CIC2, CIC5, AND RCF FREQUENCY Figure 2c. Baseband Signal is Decimated and Filtered by CIC2, CIC5, RCF |
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