Datasheet

AD9775
Rev. E | Page 32 of 56
MODULATION, NO INTERPOLATION
With Control Register 0x01, Bit 7 and Bit 6 set to 00, the
interpolation function on the AD9775 is disabled.
Figure 59
through
Figure 62 show the DAC output spectral characteristics
of the AD9775 in the various modulation modes, all with the
interpolation filters disabled. The modulation frequency is
determined by the state of Control Register 0x01, Bit 5 and Bit
4.
The tall rectangles represent the digital domain spectrum of a
baseband signal of narrow bandwidth. By comparing the digital
domain spectrum to the DAC sin(x)/x roll-off, an estimate can
be made for the characteristics required for the DAC recon-
struction filter.
Note also, per the previous discussion on amplitude
modulation, that the spectral components (where modulation is
set to f
S
/4 or f
S
/8) are scaled by a factor of 2. In the situation
where the modulation is f
S
/2, the modulated spectral
components add constructively, and there is no
scaling effect.
The Effects of the Digital Modulation on the DAC Output Spectrum, Interpolation Disabled
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
0 0.2 0.4 0.6 0.8 1.0
f
OUT
( × f
DATA
)
02858-059
Figure 59. No Interpolation, Modulation Disabled
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
0 0.2 0.4 0.6 0.8 1.0
f
OUT
( × f
DATA
)
02858-060
Figure 60. No Interpolation, Modulation = f
DAC
/2
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
0 0.2 0.4 0.6 0.8 1.0
f
OUT
( × f
DATA
)
02858-061
Figure 61. No Interpolation, Modulation = f
DAC
/4
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
0 0.2 0.4 0.6 0.8 1.0
f
OUT
( × f
DATA
)
02858-062
Figure 62. No Interpolation, Modulation = f
DAC
/8