AD9775EB datasheet(29/48 Pages) AD

29 / 48 page

REV. 0

AD9775

–29–

fOUT ( fDATA)

–100

–80

–60

–40

–20

Figure 28a. 8

× Interpolation, Modulation Disabled

fOUT ( fDATA)

–100

–80

–60

–40

–20

Figure 28b. 8

× Interpolation, Modulation = fDAC/2

fOUT ( fDATA)

–100

–80

–60

–40

–20

Figure 28c. 8

× Interpolation, Modulation = fDAC/4

fOUT ( fDATA)

–100

–80

–60

–40

–20

Figure 28d. 8

× Interpolation, Modulation = fDAC/8

Figure 28. Effect of Digital Modulation on DAC Output Spectrum, Interpolation = 8

MODULATION, INTERPOLATION = 8

With Control Register 01h, Bits 7 and 6 set to “11,” the

interpolation rate of the AD9775 is 8

×. Modulation is achieved

by multiplying successive samples at the interpolation filter

output by the sequence (0, 0.707, 1, 0.707, 0, –0.707, –1, 0.707).

Figures 28a–28d represent the spectral response of the AD9775

DAC output with 8

× interpolation in the various modulation

modes to a narrow band baseband signal.

Looking at Figures 26–29, the user can see how higher interpola-

tion rates reduce the complexity of the reconstruction filter needed

at the DAC output. It also becomes apparent that the ability to

modulate by fS/2, fS/4, or fS/8 adds a degree of flexibility in

frequency planning.

ZERO STUFFING

(Control Register 01h, Bit 3)

As shown in Figure 29, a “0” or null in the output frequency

response of the DAC (after interpolation, modulation, and DAC

reconstruction) occurs at the final DAC sample rate (fDAC).

This is due to the inherent SIN(x)/x roll-off response in the digital-

to-analog conversion. In applications where the desired frequency

content is below fDAC/2, this may not be a problem. Note that at

fDAC/2 the loss due to SIN(x)/x is 4 dB. In direct RF applica-

tions, this roll-off may be problematic due to the increased

pass band amplitude variation as well as the reduced amplitude

of the desired signal.

Consider an application where the digital data into the AD9775

represents a baseband signal around fDAC/4 with a pass band of

fDAC/10. The reconstructed signal out of the AD9775 would

experience only a 0.75 dB amplitude variation over its pass band.

However, the image of the same signal occurring at 3

× fDAC/4

will suffer from a pass-band flatness variation of 3.93 dB. This

image may be the desired signal in an IF application using one

of the various modulation modes in the AD9775. This roll-off

of image frequencies can be seen in Figures 25 through 28,

where the effect of the interpolation and modulation rate is

apparent as well.

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