Data Sheet
AD9737A/AD9739A
Rev. | Page 55 of 64
ANALOG INTERFACE CONSIDERATIONS
ANALOG MODES OF OPERATION
code-dependent glitches that occur in a conventional two-switch
DAC and the quad-switch DAC. In the two-switch architecture,
a code-dependent glitch occurs each time the DAC switches to
a different state (that is, D1 to D2). This code-dependent glitching
causes an increased amount of distortion in the DAC. In quad-
switch architecture (no matter what the codes are), there are
always two switches transitioning at each half clock cycle, thus
eliminating the code-dependent glitches. However, a constant
glitch occurs at 2 × DACCLK_x because half the internal switches
change state on the rising DACCLK_x edge whereas the other
half change state on the falling DACCLK_x edge.
VG1
VDD
IOUTP
IOUTN
VG1
VG4
VG3
VG2
DACCLK_x
CLK
LATCHES
DBx[13:0]
VG2
VG3
VG4
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088
INPUT
DATA
DACCLK_x
TWO-SWITCH
DAC OUTPUT
FOUR-SWITCH
DAC OUTPUT
(NORMAL MODE)
t
D1
D2
D3
D4
D5
D6
D7
D8
D9 D10
D6
D7
D8
D9 D10
D1
D2
D3
D4
D5
D6
D7
D8
D9 D10
D1
D2
D3
D4
D5
t
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089
Figure 170. Two-Switch and Quad-Switch DAC Waveforms
Another attribute of the quad-switch architecture is that it also
enables the DAC core to operate in one of the following two
modes: normal mode and mix-mode. The mode is selected via
SPI Register 0x08, Bits[1:0], with normal mode being the default
value. In the mix-mode, the output is effectively chopped at the
DAC sample rate. This has the effect of reducing the power of
the fundamental signal while increasing the power of the images
centered around the DAC sample rate, thus improving the
output power of these images.
INPUT
DATA
DACCLK_x
FOUR-SWITCH
DAC OUTPUT
(
fS MIX MODE)
–D6
–D7
–D8
–D9
–D10
D6
D7
D8
D9
D10
–D1
–D2
–D3
–D4
–D5
D1
D2
D3
D4
D5
D6
D7
D8
D9 D10
D1
D2
D3
D4
D5
t
09616-
090
Figure 171. Mix-Mode DAC Waveforms
Figure 171 shows the DAC waveforms for mix-mode. This ability
to change modes provides the user the flexibility to place a
carrier anywhere in the first two Nyquist zones, depending
on the operating mode selected. Switching between the analog
modes reshapes the sinc roll-off that is inherent at the DAC output.
The maximum amplitude in both Nyquist zones is impacted by
this sinc roll-off, depending on where the carrier is placed (see
Figure 172). As a practical matter, the usable bandwidth in the
third Nyquist zone becomes limited at higher DAC clock rates
(that is, >2 GSPS) when the output bandwidth of the DAC core
and the interface network (that is, balun) contributes to
additional roll-off.
FREQUENCY (Hz)
0FS
1.50FS
1.25FS
1.00FS
0.75FS
0.50FS
0.25FS
–35
–30
–25
–20
–15
–10
–5
0
FIRST
NYQUIST ZONE
SECOND
NYQUIST ZONE
THIRD
NYQUIST ZONE
MIX MODE
NORMAL
MODE
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091
Figure 172. Sinc Roll-Off for Each Analog Operating Mode
C