AD9775 datasheet(32/56 Pages) AD

32 / 56 page

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

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 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 the Digital Modulation on the DAC Output Spectrum, Interpolation Disabled

–100

–80

–60

–40

–20

0.2

0.4

0.6

0.8

1.0

fOUT (×fDATA)

Figure 59. No Interpolation, Modulation Disabled

–100

–80

–60

–40

–20

0.2

0.4

0.6

0.8

1.0

fOUT (×fDATA)

Figure 60. No Interpolation, Modulation = fDAC/2

–100

–80

–60

–40

–20

0.2

0.4

0.6

0.8

1.0

fOUT (×fDATA)

Figure 61. No Interpolation, Modulation = fDAC/4

–100

–80

–60

–40

–20

0.2

0.4

0.6

0.8

1.0

fOUT (×fDATA)

Figure 62. No Interpolation, Modulation = fDAC/8

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