AD9775 Analog Devices, AD9775 Datasheet - Page 33

• Manufacturer Part Number: AD9775
• Manufacturer: Analog Devices

Specifications of AD9775

• Resolution (bits): 14bit
• Dac Update Rate: 400MSPS
• Dac Settling Time: 11ns
• Max Pos Supply (v): +3.5V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

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.
The Effects of the Digital Modulation on the DAC Output Spectrum, Interpolation = 2×
–100
–100
–20
–40
–60
–80
–20
–40
–60
–80
0
0
0
0
Figure 63. 2× Interpolation, Modulation = Disabled
Figure 64. 2× Interpolation, Modulation = f
0.5
0.5
f
f
OUT
OUT
1.0
1.0
(×
(×
f
f
DATA
DATA
)
)
1.5
1.5
DAC
/2
2.0
2.0
Rev. E | Page 33 of 56
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 f
true stop band as the band edges coincide with each other. In
the f
portion of the digital filter pass band due to constructive
addition over just that section of the band.
–100
–100
–20
–40
–60
–80
S
–20
–40
–60
–80
/8 modulation mode, amplitude scaling occurs over only a
0
0
0
0
Figure 65. 2× Interpolation, Modulation = f
Figure 66. 2× Interpolation, Modulation = f
0.5
0.5
S
f
f
/4 modulation mode, there is no
OUT
OUT
1.0
1.0
(×
(×
f
f
DATA
DATA
)
)
1.5
1.5
DAC
DAC
/4
/8
AD9775
2.0
2.0