AD9775EB AD [Analog Devices], AD9775EB Datasheet - Page 27

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

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
REV. 0
Figure 26a. 2 × Interpolation, Modulation = Disabled
Figure 26b. 2 × Interpolation, Modulation = f
–100
–100
–20
–40
–60
–80
–20
–40
–60
–80
0
0
0
0
Figure 26. Effects of Digital Modulation on DAC Output Spectrum, Interpolation = 2 ×
0.5
0.5
f
f
OUT
OUT
1.0
(
1.0
(
f
f
DATA
DATA
)
)
1.5
1.5
DAC
2.0
2.0
/2
–27–
>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 f
stop band as the band edges coincide with each other. In the f
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 26d. 2 × Interpolation, Modulation = f
Figure 26c. 2 × Interpolation, Modulation = f
–100
–100
–20
–40
–60
–80
–20
–40
–60
–80
0
0
0
0
0.5
0.5
S
/4 modulation mode, there is no true
f
f
OUT
OUT
1.0
(
1.0
(
f
f
DATA
DATA
)
)
1.5
1.5
AD9775
DAC
DAC
2.0
2.0
/4
/8
S
/8