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WM8974 数据表(PDF) 21 Page - Wolfson Microelectronics plc |
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WM8974 数据表(HTML) 21 Page - Wolfson Microelectronics plc |
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21 / 63 page ![]() Product Preview WM8974 w PP Rev 1.1 September 2004 21 PROGRAMMABLE NOTCH FILTER A programmable notch filter is provided. This filter has a variable centre frequency and bandwidth, programmable via two coefficients, a0 and a1. a0 and a1 are represented by the register bits NFA0[13:0] and NFA1[13:0]. Because these coefficient values require four register writes to setup there is an NFU (Notch Filter Update) flag which should be set only when all four registers are setup. REGISTER ADDRESS BIT LABEL DEFAULT DESCRIPTION 6:0 NFA0[13:7] 0 Notch Filter a0 coefficient, bits [13:7] 7 NFEN 0 Notch filter enable: 0=Disabled 1=Enabled R27 Notch Filter 1 8 NFU 0 Notch filter update. The notch filter values used internally only update when one of the NFU bits is set high. 6:0 NFA0[6:0] 0 Notch Filter a0 coefficient, bits [6:0] R28 Notch Filter 2 8 NFU] 0 Notch filter update. The notch filter values used internally only update when one of the NFU bits is set high. 6:0 NFA1[13:7] 0 Notch Filter a1 coefficient, bits [13:7] R29 Notch Filter 3 8 NFU 0 Notch filter update. The notch filter values used internally only update when one of the NFU bits is set high. 6:0 NFA1[6:0] 0 Notch Filter a1 coefficient, bits [6:0] R30 Notch Filter 4 8 NFU 0 Notch filter update. The notch filter values used internally only update when one of the NFU bits is set high. Table 7 Notch Filter Function The coefficients are calculated as follows: ) 2 / tan( 1 ) 2 / tan( 1 0 b b w w a + − = ) cos( ) 1 ( 0 0 1 w a a + − = Where: s c f f w / 2 0 π = s b b f f w / 2 π = fc = centre frequency in Hz, fb = -3dB bandwidth in Hz, fs = sample frequency in Hz The actual register values can be determined from the coefficients as follows: NFA0 = -a0 x 2 13 NFA1 = -a1 x 2 12 |
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