AD9775EB AD [Analog Devices], AD9775EB Datasheet - Page 29

• Manufacturer Part Number: AD9775EB
• Description: 14-Bit, 160 MSPS 2X/4X/8X Interpolating Dual TxDAC+ D/A Converter
• Manufacturer: AD [Analog Devices]

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.
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 (f
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 f
f
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.
REV. 0
DAC
/2 the loss due to SIN(x)/x is 4 dB. In direct RF applica-
Figure 28a. 8 × Interpolation, Modulation Disabled
Figure 28b. 8 × Interpolation, Modulation = f
–100
–100
–20
–40
–60
–80
–20
–40
–60
–80
0
0
0
0
DAC
/2, this may not be a problem. Note that at
Figure 28. Effect of Digital Modulation on DAC Output Spectrum, Interpolation = 8 ×
1
1
f
f
OUT
OUT
(
(
2
2
f
f
DATA
DATA
)
)
3
3
DAC
DAC
4
/2
4
).
–29–
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 f
frequency planning.
Consider an application where the digital data into the AD9775
represents a baseband signal around f
f
experience only a 0.75 dB amplitude variation over its pass band.
However, the image of the same signal occurring at 3 × f
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.
DAC
/10. The reconstructed signal out of the AD9775 would
Figure 28d. 8 × Interpolation, Modulation = f
Figure 28c. 8 × Interpolation, Modulation = f
–100
–100
–20
–40
–60
–80
–20
–40
–60
–80
0
0
0
0
S
/2, f
1
1
S
/4, or f
2
2
S
/8 adds a degree of flexibility in
3
3
f
f
OUT
OUT
(
(
4
4
f
f
DATA
DATA
DAC
)
)
5
5
/4 with a pass band of
6
6
AD9775
7
7
DAC
DAC
DAC
8
/4
8
/8
/4