AD9775 datasheet(33/56 Pages) AD

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AD9775

Rev. E | Page 33 of 56

MODULATION, INTERPOLATION = 2×

With Control Register 0x01, Bit 7 and Bit 6 set to 01, the

interpolation rate of the AD9775 is 2×. Modulation is achieved

by multiplying successive samples at the interpolation filter

output by the sequence (+1, −1). Figure 63 through Figure 66

represent the spectral response of the AD9775 DAC output with

2× interpolation in the various modulation modes to a narrow-

band baseband signal (the tall rectangles in the graphic). The

advantage of interpolation becomes clear in Figure 63 through

Figure 66, where the images that would normally appear in the

spectrum around the input data rate frequency are suppressed

by >70 dB.

Another significant point is that the interpolation filtering is

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

Figure 63 through Figure 66 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.

The Effects of the Digital Modulation on the DAC Output Spectrum, Interpolation = 2×

–100

–80

–60

–40

–20

fOUT (×fDATA)

0.5

1.0

1.5

2.0

Figure 63. 2× Interpolation, Modulation = Disabled

–100

–80

–60

–40

–20

fOUT (×fDATA)

0.5

1.0

1.5

2.0

Figure 64. 2× Interpolation, Modulation = fDAC/2

–100

–80

–60

–40

–20

fOUT (×fDATA)

0.5

1.0

1.5

2.0

Figure 65. 2× Interpolation, Modulation = fDAC/4

–100

–80

–60

–40

–20

fOUT (×fDATA)

0.5

1.0

1.5

2.0

Figure 66. 2× Interpolation, Modulation = fDAC/8

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