AD9775EB datasheet(27/48 Pages) AD

27 / 48 page

REV. 0

AD9775

–27–

MODULATION, INTERPOLATION = 2

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

polation rate of the AD9775 is 2

×. Modulation is achieved by

multiplying successive samples at the interpolation filter output

by the sequence (1, –1). Figures 26a–26d represent the spectral

response of the AD9775 DAC output with 2

× interpolation in

the various modulation modes to a narrow band baseband signal

(again, the tall rectangles in the graphic). The advantage of

interpolation becomes clear in Figures 26a–26d, where it can

be seen that the images that would normally appear in the spec-

trum around the input data rate frequency are suppressed by

>70 dB. Another significant point is that the interpolation filter-

ing is done previous to the digital modulator. For this reason, as

Figures 26a–26d show, the pass band of the interpolation

filters can be frequency shifted, giving the equivalent of a

high pass digital filter.

Note that when using the fS/4 modulation mode, there is no true

stop band as the band edges coincide with each other. In the fS/8

modulation mode, amplitude scaling occurs over only a portion of

the digital filter pass band due to constructive addition over just

that section of the band.

Figure 26. Effects of Digital Modulation on DAC Output Spectrum, Interpolation = 2

OUT (

fDATA)

0.5

–100

–80

–60

–40

–20

1.0

1.5

2.0

Figure 26a. 2

× Interpolation, Modulation = Disabled

fOUT ( fDATA)

0.5

–100

–80

–60

–40

–20

1.0

1.5

2.0

Figure 26b. 2

× Interpolation, Modulation = fDAC/2

fOUT ( fDATA)

0.5

–100

–80

–60

–40

–20

1.0

1.5

2.0

Figure 26c. 2

× Interpolation, Modulation = fDAC/4

fOUT ( fDATA)

0.5

–100

–80

–60

–40

–20

1.0

1.5

2.0

Figure 26d. 2

× Interpolation, Modulation = fDAC/8

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