AD9691 Datasheet, PDF (36/72 Pages) Analog Devices – 14-Bit, 1.25 GSPS JESD204B, Dual Analog-to-Digital Converter

AD9691
Data Sheet
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 61) 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 17 shows the different bandwidth options by including
different half-band filters. In all cases, the DDC filtering stage of
the AD9691 provides less than −0.001 dB of pass-band ripple
and greater than 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 (Register 0x310, Register 0x330,
Register 0x350, and Register 0x370).
Table 17. DDC Filter Characteristics
ADC
Sample
Rate
(MSPS)
DDC Decimation
Ratio
Real Output
Sample Rate
(MSPS)
1250
2 (HB1)
1250
4 (HB1 + HB2)
625
8 (HB1 + HB2 + HB3) 312.5
16 (HB1 + HB2 + HB3 + 156.25
HB4)
Complex (I/Q)
Output Sample Rate
(MSPS)
625 (I) + 625 (Q)
312.5 (I) + 312.5 (Q)
156.25 (I) + 156.25 (Q)
78.125 (I) + 78.125 (Q)
Alias
Protected
Bandwidth
(MHz)
481.3
240.6
120.3
60.2
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 18. DDC Filter Alias Rejection
Alias
Pass-Band Ripple/
Rejection (dB) Cutoff Point (dB)
Alias Protected Bandwidth for
Real (I) Outputs¹
>100
90
85
63.3
25
19.3
10.7
<−0.001
<−0.001
<−0.001
<−0.006
−0.5
−1.0
−3.0
<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
1 _{fOUT = ADC input sample rate (fS) ÷ DDC decimation ratio.}
Rev. 0 | Page 36 of 72