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AD9680 Datasheet(PDF) 57 Page - Analog Devices |
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AD9680 Datasheet(HTML) 57 Page - Analog Devices |
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57 / 97 page  Product Overview Online Documentation Design Resources Discussion Sample & Buy Data Sheet AD9680 Rev. C | Page 57 of 97 FIR FILTERS FIR FILTERS GENERAL DESCRIPTION There are four sets of decimate-by-2, low-pass, half-band, finite impulse response (FIR) filters (HB1 FIR, HB2 FIR, HB3 FIR, and HB4 FIR, shown in Figure 146). These filters follow the frequency translation stage. After the carrier of interest is tuned down to dc (carrier frequency = 0 Hz), these filters efficiently lower the sample rate while providing sufficient alias rejection from unwanted adjacent carriers around the bandwidth of interest. HB1 FIR is always enabled and cannot be bypassed. The HB2, HB3, and HB4 FIR filters are optional and can be bypassed for higher output sample rates. Table 17 shows the different bandwidth options by including different half-band filters. In all cases, the DDC filtering stage of the AD9680 provides less than −0.001 dB of pass-band ripple and >100 dB of stop-band alias rejection. Table 18 shows the amount of stop-band alias rejection for multiple pass-band ripple/cutoff points. The decimation ratio of the filtering stage of each DDC can be controlled individually through Bits[1:0] of the DDC control registers (0x310, 0x330, 0x350, and 0x370). Table 17. DDC Filter Characteristics ADC Sample Rate (MSPS) Half Band Filter Selection Real Output Complex (I/Q) Output Alias Protected Bandwidth (MHz) Ideal SNR Improvement (dB)1 Pass- Band Ripple (dB) Alias Rejection (dB) Decimation Ratio Output Sample Rate (MSPS) Decimation Ratio Output Sample Rate (MSPS) 1250 HB1 1 1250 2 625 (I) + 625 (Q) 481.25 1 <−0.001 >100 HB1 + HB2 2 625 4 312.5 (I) + 312.5 (Q) 240.62 4 HB1 + HB2 + HB3 4 312.5 8 156.25 (I) + 156.25 (Q) 120.31 7 HB1 + HB2 + HB3 + HB4 8 156.25 16 78.125 (I) + 78.125 (Q) 60.15 10 1000 HB1 1 1000 2 500 (I) + 500 (Q) 385.0 1 HB1 + HB2 2 500 4 250 (I) + 250 (Q) 192.5 4 HB1 + HB2 + HB3 4 250 8 125 (I) + 125 (Q) 96.3 7 HB1 + HB2 + HB3 + HB4 8 125 16 62.5 (I) + 62.5 (Q) 48.1 10 820 HB1 1 820 2 410 (I) + 410 (Q) 315.7 1 HB1 + HB2 2 410 4 205 (I) + 205 (Q) 157.8 4 HB1 + HB2 + HB3 4 205 8 102.5 (I) + 102.5 (Q) 78.9 7 HB1 + HB2 + HB3 + HB4 8 102.5 16 51.25 (I) + 51.25 (Q) 39.4 10 500 HB1 1 500 2 250 (I) + 250 (Q) 192.5 1 HB1 + HB2 2 250 4 125 (I) + 125 (Q) 96.3 4 HB1 + HB2 + HB3 4 125 8 62.5 (I) + 62.5 (Q) 48.1 7 HB1 + HB2 + HB3 + HB4 8 62.5 16 31.25 (I) + 31.25 (Q) 24.1 10 1 Ideal SNR improvement due to oversampling and filtering = 10log(bandwidth/(fS/2)). |
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