Datasheet

AD9775
Rev. E | Page 42 of 56
APPLYING THE OUTPUT CONFIGURATIONS
The following sections illustrate typical output configurations
for the AD9775. Unless otherwise noted, it is assumed that
IOUTFS is set to a nominal 20 mA. For applications requiring
optimum dynamic performance, a differential output configu-
ration is suggested. A simple differential output may be
achieved by converting IOUTA and IOUTB to a voltage output
by terminating them to AGND via equal value resistors. This
type of configuration may be useful when driving a differential
voltage input device such as a modulator. If a conversion to a
single-ended signal is desired and the application allows for ac
coupling, an RF transformer may be useful, or if power gain is
required, an op amp may be used. The transformer configu-
ration provides optimum high frequency noise and distortion
performance. The differential op amp configuration is suitable
for applications requiring dc coupling, 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
results if I
OUTA
and/or I
OUTB
is connected to a load resistor, R
LOAD
,
referred to AGND. This configuration is most 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
DAC dc linearity as I
OUTA
or I
OUTB
are maintained at ground or
virtual ground.
UNBUFFERED DIFFERENTIAL OUTPUT,
EQUIVALENT CIRCUIT
In many applications, it may be necessary to understand the
equivalent DAC output circuit. This is especially useful when
designing output filters or when driving inputs with finite input
impedances.
Figure 97 illustrates the output of the AD9775 and
the equivalent circuit. A typical application where this
information may be useful is when designing an interface filter
between the AD9775 and Analog Devices’ AD8345 quadrature
modulator.
I
OUTA
I
OUTB
V
OUT
+
V
OUT
(DIFFERENTIAL)
V
SOURCE
=
I
OUTFS
× (R
A
+ R
B
)
p-p
V
OUT
–
R
A
+ R
B
02858-097
Figure 97. DAC Output Equivalent Circuit
For the typical situation, where I
OUTFS
= 20 mA and R
A
and R
B
both equal 50 Ω, the equivalent circuit values become
2
=
SOURCE
V V p-p
100=
OUT
R
Note that the output impedance of the AD9775 DAC itself is
greater than 100 kΩ and typically has no effect on the
impedance of the equivalent output circuit.
DIFFERENTIAL COUPLING USING A
TRANSFORMER
An RF transformer can be used to perform a differential-to-
single-ended signal conversion, as shown in
Figure 98. A dif-
ferentially 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 (that is, 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.
Transformers with different impedance ratios can also be used for
impedance matching purposes.
MINI-CIRCUITS
T1-1T
R
LOAD
I
OUTA
I
OUTB
DAC
02858-098
Figure 98. Transformer-Coupled Output Circuit
The center tap on the primary side of the transformer must be
connected to AGND to provide the necessary dc current path
for both I
OUTA
and I
OUTB
. The complementary voltages appearing
at I
OUTA
and I
OUTB
(that is, V
OUTA
and V
OUTB
) swing symmetrically
around AGND and should be maintained within the specified
output compliance range of the AD9775. A differential resistor,
R
DIFF
, can be inserted in 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. Note that approxi-
mately half the signal power dissipates across R
DIFF
.