Datasheet

AD9785/AD9787/AD9788
Rev. A | Page 38 of 64
DIGITAL DATAPATH
The AD9785/AD9787/AD9788 digital datapath consists of
three 2× half-band interpolation filters, a quadrature modulator,
and an inverse sinc filter. A 32-bit NCO provides the sine and
cosine carrier signals required for the quadrature modulator.
INTERPOLATION FILTERS
The AD9785/AD9787/AD9788 contain three half-band filters
that can be bypassed. This allows the device to operate with 2×,
4×, or 8× interpolation rates, or without interpolation. The
interpolation filters have a linear phase response. The coefficients
of the low-pass filters are given in Table 28, Table 29, and
Table 30. Spectral plots for the filter responses are shown in
Figure 55, Figure 56, and Figure 57.
In 2×, 4×, or 8× interpolation mode, the usable bandwidth of
the interpolation filter is 80% of the complex input data rate.
The usable bandwidth has a pass-band ripple of less than
0.0005 dB and a stop-band attenuation of greater than 85 dB.
The center frequency of the interpolation filter is set by the
NCO frequency tuning word (Register 0x0A, Bits [31:0]), so
baseband input signals are always centered in the interpolation
filter pass band.
10
–100
–4 4
f
OUT
(× Input Data Rate)
ATT ENUATION (d B)
–3–2–10123
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
07098-010
Figure 55. 2× Interpolation, Low-Pass Response to ±4× Input Data Rate
(Dotted Lines Indicate 1 dB Roll-Off)
10
–100
–4
4
f
OUT
(× Input Data Rate)
ATTENUATION (dB)
–3–2–10123
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
07098-011
Figure 56. 4× Interpolation, Low-Pass Response to ±4× Input Data Rate
(Dotted Lines Indicate 1 dB Roll-Off)
10
–100
–4
4
f
OUT
(× Input Data Rate)
ATTENUATION (dB)
–3–2–10123
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
07098-012
Figure 57. 8× Interpolation, Low-Pass Response to ±4× Input Data Rate
(Dotted Lines Indicate 1 dB Roll-Off)