AD9775EB datasheet(26/48 Pages) AD

26 / 48 page

REV. 0

AD9775

–26–

MODULATION, NO INTERPOLATION

With Control Register 01h, Bits 7 and 6 set to “00,” the inter-

polation function on the AD9775 is disabled. Figures 25a–25d

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 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 components (where modulation is set to fS/4 or fS/8)

are scaled by a factor of 2. In the situation where the modula-

tion is fS/2, the modulated spectral components add constructively

and there is no scaling effect.

fOUT ( fDATA)

0.2

–100

–80

–60

–40

–20

0.4

0.6

0.8

1.0

Figure 25a. No Interpolation, Modulation Disabled

fOUT ( fDATA)

0.2

–100

–80

–60

–40

–20

0.4

0.6

0.8

1.0

Figure 25b. No Interpolation, Modulation = fDAC/2

fOUT ( fDATA)

0.2

–100

–80

–60

–40

–20

0.4

0.6

0.8

1.0

Figure 25c. No Interpolation, Modulation = fDAC/4

fOUT ( fDATA)

0.2

–100

–80

–60

–40

–20

0.4

0.6

0.8

1.0

Figure 25d. No Interpolation, Modulation = fDAC/8

Figure 25. Effects of Digital Modulation on DAC Output Spectrum, Interpolation Disabled

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