AD9775EB View Datasheet(PDF) - Analog Devices
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AD9775EB Datasheet PDF : 48 Pages
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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.
0
AD9775
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.
0
–20
–20
–40
–40
–60
–80
–100
0
1
2
3
4
fOUT (؋fDATA)
Figure 28a. 8 × Interpolation, Modulation Disabled
0
–60
–80
–100
0
1
2
3
4
5
6
7
8
fOUT (؋f DATA)
Figure 28c. 8 × Interpolation, Modulation = fDAC/4
0
–20
–20
–40
–40
–60
–60
–80
–80
–100
0
1
2
3
4
fOUT (؋fDATA)
Figure 28b. 8 × Interpolation, Modulation = fDAC/2
–100
0
1
2
3
4
5
6
7
8
fOUT (؋fDATA)
Figure 28d. 8 × Interpolation, Modulation = fDAC/8
Figure 28. Effect of Digital Modulation on DAC Output Spectrum, Interpolation = 8 ×
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.
REV. 0
–29–
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