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AD6636 Datasheet(PDF) 32 Page - Analog Devices |
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AD6636 Datasheet(HTML) 32 Page - Analog Devices |
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32 / 80 page  AD6636 Rev. A | Page 32 of 80 6-Tap Fixed Coefficient Filter (FIR2) Following the first cascade of the FIR1 and HB1 filters is the second cascade of the FIR2 and HB2 filters. The 6-tap, fixed- coefficient FIR2 filter is useful in providing extra alias protection for the decimating HB2 filter in certain filter configurations. It is a simple sum-of-products FIR filter with six filter taps and 5-bit fixed coefficients. Note that this filter does not decimate. The normalized coefficients used in the implementation and the 5-bit decimal equivalent value of the coefficients are listed in Table 16. Table 16. 6-Tap FIR2 Filter Coefficients Coefficient Number Normalized Coefficient Decimal Coefficient (5-Bit) C0, C5 −0.125 −2 C1, C4 +0.1875 +3 C2, C3 +0.9375 +15 The user can either use or bypass this filter. Writing Logic 0 to the FIR2 enable bit in the FIR-HB control register bypasses this fixed-coefficient filter. The filter is useful in certain filter configurations only and bypassing it for other applications results in power savings. The filter is especially useful in increasing the stop-band attenuation of the HB2 filter that follows. Therefore, it is optimal to use both FIR2 and HB2 in a configuration. This filter runs at a sample rate given by one of the following equations: If HB1 is bypassed, fFIR2 = fHB1 If HB1 is not bypassed, fFIR2 = fHB1/2, where: fHB1 is the input rate of the HB1 filter. fFIR2 is the input rate of the FIR2 filter. The maximum input and output rate for this filter is 75 MHz. The response of the FIR2 filter is shown in Figure 34. FRACTION OF FIR2 INPUT SAMPLE RATE 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 –8.33 –16.67 –25.00 –33.33 –41.67 –50.00 –58.33 –66.67 –75.00 –83.33 –91.67 –100.00 –30 0.39 FIR2 RESPONSE 0.61 Figure 34. FIR2 Filter Response to the Input Rate of the Filter Decimate-by-2, Half-Band Filter (HB2) The second stage of the second cascade of the FIR-HB block is a decimate-by-2, half-band filter. The 27-tap, symmetric, fixed- coefficient HB2 filter has low power consumption due to its polyphase implementation. The filter has 20 bits of input and output data with 12-bit coefficients. The normalized coefficients used in the implementation and the 10-bit decimal equivalent value of the coefficients are listed in Table 17. Other coefficients are 0s. Table 17. HB2 Filter Fixed Coefficients Coefficient Number Normalized Coefficient Decimal Coefficient (12-Bit) C1, C27 +0.00097656 +2 C3, C25 −0.00537109 −11 C5, C23 +0.015 +32 C7, C21 −0.0380859 −78 C9, C19 +0.0825195 +169 C11, C17 −0.1821289 −373 C13, C15 +0.6259766 +1282 C14 +1 +2048 Similar to the HB1 filter, the user can either use or bypass this filter. Writing Logic 0 to the HB1 enable bit in the FIR-HB control register bypasses this fixed-coefficient HB filter. The filter is useful in certain filter configurations only and bypassing it for other applications results in power savings. For example, the filter is useful in narrow-band applications in which more filtering is required, as compared to wide-band applications, in which a higher output rate may prohibit the use of a decimating filter. The response of the HB2 filter is shown in Figure 35. |
Número de pieza similar - AD6636_15 |
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Descripción similar - AD6636_15 |
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