Datasheet

REV. C
AD9751
–16–
I
OUTFS
(mA)
40
20
0
20100
I
AVDD
(mA)
35
10
30
25
15
5
2.5 5 7.5 12.5 15 17.5
Figure 17. I
AVDD
vs. I
OUTFS
RATIO (f
OUT
/f
DAC
)
20
16
0
10.010.001
I
DVDD
(mA)
18
14
12
10
8
6
4
2
0.1
300MSPS
200MSPS
100MSPS
50MSPS
25MSPS
Figure 18. I
DVDD
vs. f
OUT
/f
DAC
Ratio
f
DAC
(MHz)
10
0
3001500
PLL_V
DD
(mA)
9
8
7
6
5
4
3
2
50 100 200 250
1
17525 75 125 225 275
DIV SETTING 00
DIV SETTING 11
DIV SETTING 10
DIV SETTING 01
Figure 19. PLLVDD vs. f
DAC
APPLYING THE AD9751
OUTPUT CONFIGURATIONS
The following sections illustrate some typical output configura-
tions for the AD9751. Unless otherwise noted, it is assumed
that I
OUTFS
is set to a nominal 20 mA. For applications requir-
ing 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 opti-
mum 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 band-
width 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
is connected to an appropriately
sized load resistor, R
LOAD
, referred to ACOM. This configura-
tion 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 per-
formance 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
0dBm 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 configura-
tion improves load matching and increases power to 2 dBm into
a 50 Ω load on the secondary. Transformers with different
impedance ratios may also be used for impedance matching
purposes. Note that the transformer provides ac coupling only.
R
LOAD
AD9751
MINI-CIRCUITS
T1-1T
I
OUTA
I
OUTB
Figure 20. Differential Output Using a Transformer