Datasheet

AD9762
–15–
REV. B
I
OUTFS
– mA
30
0
2204 6 8 10 12141618
25
20
15
10
5
I
AVDD
– mA
Figure 47. I
AVDD
vs. I
OUTFS
Conversely, I
DVDD
is dependent on both the digital input wave-
form, f
CLOCK
, and digital supply DVDD. Figures 48 and 49
show I
DVDD
as a function of full-scale sine wave output ratios
(f
OUT
/f
CLOCK
) for various update rates with DVDD = 5 V and
DVDD = 3 V, respectively. Note, how I
DVDD
is reduced by more
than a factor of 2 when DVDD is reduced from 5 V to 3 V.
RATIO (f
OUT
/f
CLK
)
18
16
0
0.01 10.1
I
DVDD
– mA
8
6
4
2
12
10
14
5MSPS
25MSPS
50MSPS
100MSPS
125MSPS
Figure 48. I
DVDD
vs. Ratio @ DVDD = 5 V
RATIO (f
OUT
/f
CLK
)
8
0
0.01 10.1
I
DVDD
– mA
6
4
2
5MSPS
25MSPS
50MSPS
100MSPS
125MSPS
Figure 49. I
DVDD
vs. Ratio @ DVDD = 3 V
APPLYING THE AD9762
OUTPUT CONFIGURATIONS
The following sections illustrate some typical output configura-
tions for the AD9762. 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
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.
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 configura-
tion provides the best dc linearity since I
OUTA
or I
OUTB
is
maintained at a virtual ground. Note, 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 50. A
differentially coupled transformer output provides the optimum
distortion performance for output signals whose spectral content
lies within the transformer’s passband. 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. It also provides electrical isolation
and the ability to deliver twice the power to the load. Trans-
formers with different impedance ratios may also be used for
impedance matching purposes. Note that the transformer
provides ac coupling only.
R
LOAD
AD9762
22
21
MINI-CIRCUITS
T1-1T
OPTIONAL R
DIFF
IOUTA
IOUTB
Figure 50. 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 appear-
ing at I
OUTA
and I
OUTB
(i.e., V
OUTA
and V
OUTB
) swing symmetri-
cally around ACOM and should be maintained with the specified
output compliance range of the AD9762. A differential resistor,
R
DIFF
, may be inserted in applications in which 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 which results in a low VSWR. Note that approxi-
mately half the signal power will be dissipated across R
DIFF
.