
AD9776A/AD9778A/AD9779A
Rev. B | Page 33 of 56
INTERPOLATION FILTER ARCHITECTURE
The AD9776A/AD9778A/AD9779A can provide up to 8× inter-
polation, or the interpolation filters can be entirely disabled. It
is important to note that the input signal should be backed off
by approximately 0.01 dB from full scale to avoid overflowing
the interpolation filters. The coefficients of the low-pass filters
and
Table 18. Spectral plots for the filter responses are shown in
Table 15. Low-Pass Filter 1
Lower Coefficient
Upper Coefficient
Integer Value
H(1)
H(55)
4
H(2)
H(54)
0
H(3)
H(53)
+13
H(4)
H(52)
0
H(5)
H(51)
34
H(6)
H(50)
0
H(7)
H(49)
+72
H(8)
H(48)
0
H(9)
H(47)
138
H(10)
H(46)
0
H(11)
H(45)
+245
H(12)
H(44)
0
H(13)
H(43)
408
H(14)
H(42)
0
H(15)
H(41)
+650
H(16)
H(40)
0
H(17)
H(39)
1003
H(18)
H(38)
0
H(19)
H(37)
+1521
H(20)
H(36)
0
H(21)
H(35)
2315
H(22)
H(34)
0
H(23)
H(33)
+3671
H(24)
H(32)
0
H(25)
H(31)
6642
H(26)
H(30)
0
H(27)
H(29)
+20,755
H(28)
+32,768
Table 16. Low-Pass Filter 2
Lower Coefficient
Upper Coefficient
Integer Value
H(1)
H(23)
2
H(2)
H(22)
0
H(3)
H(21)
+17
H(4)
H(20)
0
H(5)
H(19)
75
H(6)
H(18)
0
H(7)
H(17)
+238
H(8)
H(16)
0
H(9)
H(15)
660
H(10)
H(14)
0
H(11)
H(13)
+2530
H(12)
+4096
Table 17. Low-Pass Filter 3
Lower Coefficient
Upper Coefficient
Integer Value
H(1)
H(15)
39
H(2)
H(14)
0
H(3)
H(13)
+273
H(4)
H(12)
0
H(5)
H(11)
1102
H(6)
H(10)
0
H(7)
H(9)
+4964
H(8)
+8192
Table 18. Inverse Sinc Filter
Lower Coefficient
Upper Coefficient
Integer Value
H(1)
H(9)
+2
H(2)
H(8)
4
H(3)
H(7)
+10
H(4)
H(6)
35
H(5)
+401
10
–100
–4
4
fOUT (× Input Data Rate)
AT
T
E
N
UAT
IO
N
(
d
B
)
–3
–2
–1
0123
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
06
45
2-
0
54
Figure 57. 2× Interpolation, Low-Pass Response to ±4× Input Data Rate
(Dotted Lines Indicate 1 dB Roll-Off)