AD9773-EB datasheet(34/60 Pages) AD

34 / 60 page

AD9773

Rev. D | Page 34 of 60

MODULATION, NO INTERPOLATION

With Control Register 01h, Bit 7 and Bit 6 set to 00, the

interpolation function on the AD9773 is disabled. Figure 59 to

Figure 62 show the DAC output spectral characteristics of the

AD9773 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 characteristics required

for the DAC reconstruction filter. Note also, per the previous

discussion on amplitude modulation, that the spectral com-

ponents (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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