Datasheet

REV. B
–16–
AD9753
and is insensitive to f
CLOCK
. Conversely, I
DVDD
is dependent on
both the digital input waveform, f
CLOCK
, and digital supply,
DVDD. Figure 18 shows I
DVDD
as a function of the ratio (f
OUT
/
f
DAC
) for various update rates. In addition, Figure 19 shows the
effect that the speed of f
DAC
has on the PLLVDD current, given
the PLL divider ratio.
I
OUTFS
(mA)
40
20
0
20.010.00
I
AVDD
(mA)
35
10
30
25
15
5
2.5 5.0 7.5 12.5 15.0 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 AD9753
OUTPUT CONFIGURATIONS
The following sections illustrate some typical output configura-
tions for the AD9753. 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
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
is connected to an appropriately
sized load resistor, R
LOAD
, referred to ACOM. This configu-
ration 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 con-
verting 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 termi-
nated 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 configura-
tion 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.
R
LOAD
AD9753
MINI-CIRCUITS
T1-1T
I
OUTA
I
OUTB
Figure 20. Differential Output Using a Transformer
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 AD9753. A differ-
ential resistor, R
DIFF
, may be inserted in applications where the
output of the transformer is connected to the load, R
LOAD
, via a