Datasheet

AD9761
–14–
AD9761
–15–
5MSPS
RATIO (f
OUT
/f
CLK
)
40
30
0.05 0.15
40MSPS
20
0 0.10
20MSPS
10MSPS
10
0
I
DVDD
(mA)
0.20
2.5MSPS
5
35
25
15
Figure 16. I
DVDD
vs. Ratio @ DVDD = 3 V
APPLYING THE AD9761
Output Congurations
The following sections illustrate some typical output congu-
rations for the AD9761. Unless otherwise noted, it is assumed
that I
OUTFS
is set to a nominal 10 mA. For applications requir-
ing the optimum dynamic performance, a differential output
conguration is suggested. A differential output conguration
may consist of either an RF transformer or a differential op amp
conguration. The transformer conguration provides the
optimum high frequency performance and is recommended
for any application allowing for ac coupling. The differential
op amp conguration 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 volt age
will result if IOUTA and/or IOUTB is connected to an
appropriately sized load resistor, R
LOAD
, referred to ACOM.
This conguration may be more suitable for a single-sup ply
system requiring a dc-coupled, ground referred output volt-
age. Alternatively, an amplier could be congured as an I-V
converter, thus converting I
OUTA
or I
OUTB
into a negative
unipolar voltage. This conguration provides the best dc
linearity since IOUTA or IOUTB is maintained at a virtual
ground.
Differential Coupling Using a Transformer
An RF transformer can be used to perform a differential-
to-single-ended signal conversion as shown in Figure 17. A
differentially coupled transformer output provides the op timum
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-or der
harmonics) and noise over a wide frequency range. It also
provides electrical isolation and the ability to deliver twice
the power to the load. Transformers with different imped ance
ratios may also be used for impedance matching purposes.
Note that the transformer provides ac coupling only.
OPTIONAL
R
DIFF
R
LOAD
MINI-CIRCUITS
T1-1T
IOUTA
IOUTB
AD9761
Figure 17. 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 cur rent
path for both I
OUTA
and I
OUTB
. The complementary voltages
appearing at IOUTA and IOUTB (i.e., V
OUTA
and V
OUTB
)
swing symmetrically around ACOM and should be main tained
with the specied output compliance range of the AD9761.
A differential resistor, R
DIFF
, may be inserted in applica tions
in which the output of the transformer is connected to the
load, R
LOAD
, via a passive reconstruction lter or cable
requiring double termination. R
DIFF
is determined by the
transformer’s impedance ratio and provides the proper
source termination, which results in a low VSWR. Note
that approximately half the signal power will be dissipated
across R
DIFF
.
Differential Coupling Using an Op Amp
An op amp can also be used to perform a differential
to single-ended conversion as shown in Figure 18. The
AD9761 is congured with two equal load resistors, R
LOAD
,
of 50 . The differential voltage developed across IOUTA
and IOUTB is converted to a single-ended signal via the
differential op amp conguration. An optional capacitor
can be installed across IOUTA and IOUTB forming a real
pole in a low-pass lter. The addition of this capacitor also
enhances the op amp’s distortion performance by prevent-
ing the DAC’s high slewing output from overloading the op
amp’s input.
C
OPT
200
500
IOUTA
IOUTB
AD9761
R
LOAD
50
200
AD8042
500
R
LOAD
50
Figure 18. DC Differential Coupling Using an Op Amp
The common-mode rejection of this conguration is typi cally
determined by the resistor matching. In this circuit, the
differential op amp circuit using the AD8042 is congured
to provide some additional signal gain. The op amp must
operate from a dual supply since its output is approxi mately
±1.0 V. A high speed amplier capable of preserving the
differential performance of the AD9761 while meeting other
system level objectives (i.e., cost, power) should be select ed.
The op amp’s differential gain, gain setting resistor val ues,
and full-scale output swing capabilities should all be con sid-
ered when optimizing this circuit.
REV. C
REV. C