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AD9684 View Datasheet(PDF) - Analog Devices

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AD9684 Datasheet PDF : 64 Pages
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AD9684
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FIR FILTERS
GENERAL DESCRIPTION
There are four sets of decimate by 2, low-pass, half-band, FIR
filters (labeled HB1 FIR, HB2 FIR, HB3 FIR, and HB4 FIR in
Figure 58) following 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 16 shows the different bandwidth options by including
different half-band filters. In all cases, the DDC filtering stage of
the AD9684 provides less than −0.001 dB of pass-band ripple
and greater than 100 dB of stop band alias rejection.
Table 17 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 (Register 0x310,
Register 0x330, Register 0x350, and Register 0x370).
Table 16. DDC Filter Characteristics
ADC Sample DDC Decimation
Rate (MSPS) Ratio
Real Output
Sample Rate
(MSPS)
1000
2 (HB1)
1000
4 (HB1 + HB2)
500
8 (HB1 + HB2 + HB3) 250
16 (HB1 + HB2 +
125
HB3 + HB4)
Complex (I/Q)
Output Sample
Rate (MSPS)
500 (I) + 500 (Q)
250 (I) + 250 (Q)
125 (I) + 125 (Q)
62.5 (I) + 62.5 (Q)
Alias Protected
Bandwidth
(MHz)
385.0
192.5
96.3
48.1
Ideal SNR
Improvement1
(dB)
1
4
7
10
Pass-Band
Ripple
(dB)
<−0.001
Alias
Rejection
(dB)
>100
1 The ideal SNR improvement due to oversampling and filtering = 10log(bandwidth/(fS/2)).
Table 17. DDC Filter Alias Rejection
Alias Rejection (dB)
>100
90
85
63.3
25
19.3
10.7
Pass-Band Ripple/Cutoff Point (dB)
<−0.001
<−0.001
<−0.001
<−0.006
−0.5
−1.0
−3.0
1 fOUT = ADC input sample rate/DDC decimation ratio.
Alias Protected Bandwidth for
Real (I) Outputs1
<38.5% × fOUT
<38.7% × fOUT
<38.9% × fOUT
<40% × fOUT
44.4% × fOUT
45.6% × fOUT
48% × fOUT
Alias Protected Bandwidth for
Complex (I/Q) Outputs1
<77% × fOUT
<77.4% × fOUT
<77.8% × fOUT
<80% × fOUT
88.8% × fOUT
91.2% × fOUT
96% × fOUT
Rev. 0 | Page 40 of 64
 

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