AD9755_15 Datasheet, PDF (16/28 Pages) Analog Devices – 14-Bit, 300 MSPS

AD9755
40
35
30
25
20
15
10
5
0
0
2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0
I_OUTFS (mA)
Figure 17. I_AVDD vs. I_OUTFS
20
18
16
14
300MSPS
12
10
200MSPS
8
6
100MSPS
50MSPS
4
25MSPS
2
0
0.001
0.01
0.1
1
RATIO (f_OUT/f_DAC)
Figure 18. I_DVDD vs. f_OUT/f_DAC Ratio
10
DIV SETTING 11
9
8
7
DIV SETTING 10
DIV SETTING 01
6
5
4
DIV SETTING 00
3
2
1
0
0 25 50 75 100 125 150 175 200 225 250 275 300
f_DAC (MHz)
Figure 19. PLLV_DD vs. f_DAC
APPLYING THE AD9755
OUTPUT CONFIGURATIONS
The following sections illustrate some typical output configurations
for the AD9755. Unless otherwise noted, it is assumed that I_OUTFS
is set to a nominal 20 mA. For applications requiring the optimum
dynamic performance, a differential output configuration is
suggested. A differential output configuration may consist of
either an RF transformer or a differential op amp configuration.
The transformer configuration provides the optimum high
frequency performance and is recommended for any application
allowing for ac coupling. The differential op amp configuration
is suitable for applications requiring dc coupling, a bipolar output,
signal gain, and/or level-shifting within the bandwidth of the
chosen op amp.
A single-ended output is suitable for applications requiring a
unipolar voltage output. A positive unipolar output voltage will
result if I_OUTA and/or I_OUTB are connected to an appropriately
sized load resistor, R_LOAD, referred to ACOM. This configuration
may be more suitable for a single-supply system requiring a dc-
coupled, ground referred output voltage. Alternatively, an amplifier
could be configured as an I-V converter, thus converting I_OUTA or
I_OUTB into a negative unipolar voltage. This configuration provides
the best dc linearity since I_OUTA or I_OUTB is maintained at a virtual
ground. Note that I_OUTA provides slightly better performance
than I_OUTB.
DIFFERENTIAL COUPLING USING A TRANSFORMER
An RF transformer can be used to perform a differential-to-
single-ended signal conversion, as shown in Figure 20. A
differentially-coupled transformer output provides the optimum
distortion performance for output signals whose spectral content
lies within the transformer’s pass band. An RF transformer such
as the Mini-Circuits T1–1T provides excellent rejection of
common-mode distortion (i.e., even-order harmonics) and
noise over a wide frequency range. When I_OUTA and I_OUTB are
terminated to ground with 50 Ω, this configuration provides
0 dBm power to a 50 Ω load on the secondary with a DAC full-
scale current of 20 mA. A 2:1 transformer such as the Coilcraft
WB2040-PC can also be used in a configuration in which I_OUTA
and I_OUTB are terminated to ground with 75 Ω. This configuration
improves load matching and increases power to 2 dBm into a
50 Ω load on the secondary. Transformers with different imped-
ance ratios may also be used for impedance matching purposes.
Note that the transformer provides ac coupling only.
The center tap on the primary side of the transformer must be
connected to ACOM to provide the necessary dc current path for
both I_OUTA and I_OUTB. The complementary voltages appearing
at I_OUTA and I_OUTB (i.e., V_OUTA and V_OUTB) swing symmetrically
around ACOM and should be maintained with the specified
output compliance range of the AD9755. A differential resistor,
R_DIFF, may be inserted into applications where the output of the
transformer is connected to the load, R_LOAD, via a passive
reconstruction filter or cable. R_DIFF is determined by the
transformer’s impedance ratio and provides the proper source
termination that results in a low VSWR.
–16–
REV. B