AD9777_15 Datasheet, PDF (34/60 Pages) Analog Devices – 16-Bit, 160 MSPS 2/4/8

AD9777
MODULATION, NO INTERPOLATION
With Control Register 01h, Bit 7 and Bit 6 set to 00, the
interpolation function on the AD9777 is disabled. Figure 59 to
Figure 62 show the DAC output spectral characteristics of the
AD9777 in the various modulation modes, all with the
interpolation filters disabled. The modulation frequency is
determined by the state of Control Register 01h, Bits 5 and 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 characteris-
tics required for the DAC reconstruction filter. Note also, per
the previous discussion on amplitude modulation, that the
spectral components (where modulation is set to fS/4 or fS/8) are
scaled by a factor of 2. In the situation where the modulation is
fS/2, the modulated spectral components add constructively,
and there is no scaling effect.
The Effects of Digital Modulation on DAC Output Spectrum, Interpolation Disabled
0
0
–20
–20
–40
–40
–60
–60
–80
–80
–100
0
0.2
0.4
0.6
0.8
1.0
f_OUT (×f_DATA)
Figure 59. No Interpolation, Modulation Disabled
0
–100
0
0.2
0.4
0.6
0.8
1.0
f_OUT (×f_DATA)
Figure 61. No Interpolation, Modulation = fDAC/4
0
–20
–20
–40
–40
–60
–60
–80
–80
–100
0
0.2
0.4
0.6
0.8
1.0
f_OUT (×f_DATA)
Figure 60. No Interpolation, Modulation = fDAC/2
–100
0
0.2
0.4
0.6
0.8
1.0
f_OUT (×f_DATA)
Figure 62. No Interpolation, Modulation = fDAC/8
Rev. C | Page 34 of 60