AD9775EB AD [Analog Devices], AD9775EB Datasheet - Page 37

• Manufacturer Part Number: AD9775EB
• Description: 14-Bit, 160 MSPS 2X/4X/8X Interpolating Dual TxDAC+ D/A Converter
• Manufacturer: AD [Analog Devices]

APPLYING THE AD9775 OUTPUT CONFIGURATIONS
The following sections illustrate typical output configurations for
the AD9775. Unless otherwise noted, it is assumed that I
is set to a nominal 20 mA. For applications requiring optimum
dynamic performance, a differential output configuration is
suggested. A simple differential output may be achieved by con-
verting I
them to AGND via equal value resistors. This type of configura-
tion 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 configuration provides optimum
high frequency noise and distortion performance. The differen-
tial op amp configuration is suitable for applications requiring
dc-coupling, 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
R
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
tive unipolar voltage. This configuration provides the best DAC
dc linearity as I
tual 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 51 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.
For the typical situation, where I
both equal 50 Ω, the equivalent circuit values become:
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.
REV. 0
LOAD
, referred to AGND. This configuration is most suitable
OUTA
Figure 51. DAC Output Equivalent Circuit
OUTA
I
OUTFS
and I
OUTA
and/or I
V
(R
SOURCE
OUTB
or I
A
+ R
p-p
OUTB
OUTB
V
R
B
=
to a voltage output by terminating
)
SOURCE
OUT
I
I
OUTB
OUTA
is connected to a load resistor,
are maintained at ground or vir-
= 100 Ω
R
A
OUTFS
+ R
= 2 V p-p
B
OUTA
= 20 mA and R
V
(DIFFERENTIAL)
OUT
V
V
or I
OUT
OUT
+
–
OUTB
into a nega-
A
and R
OUTFS
B
–37–
DIFFERENTIAL COUPLING USING A TRANSFORMER
An RF transformer can be used to perform a differential-
to-single-ended signal conversion as shown in Figure 52. 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. 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.
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
at I
around AGND and should be maintained within the specified
output compliance range of the AD9775. A differential resistor,
R
transformer is connected to the load, R
struction filter or cable. R
impedance ratio and provides the proper source termination
that results in a low VSWR. Note that approximately half the
signal power will be dissipated across R
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 53. This has the added
benefit of providing signal gain as well. In Figure 53, the AD9775
is configured with two equal load resistors, R
differential voltage developed across I
to a single-ended signal via the differential op amp configura-
tion. An optional capacitor can be installed across I
I
this capacitor also enhances the op amp’s distortion performance
by preventing the DAC’s fast slewing output from overloading
the input of the op amp.
The common-mode (and second order distortion) rejection of this
configuration is typically determined by the resistor matching.
The op amp used must operate from a dual supply since its
output is approximately ± 1.0 V. A high speed amplifier, such as
the AD8021, capable of preserving the differential performance
OUTB
DIFF
OUTA
, may be inserted in applications where the output of the
, forming a real pole in a low pass filter. The addition of
Figure 52. Transformer-Coupled Output Circuit
and I
Figure 53. Op Amp-Coupled Output Circuit
OUTA
DAC
I
I
OUTB
OUTA
OUTB
and I
25
(i.e., V
DAC
I
OUTB
I
OUTB
OUTA
C
OPT
25
. The complementary voltages appearing
DIFF
OUTA
is determined by the transformer’s
225
225
MINI-CIRCUITS
and V
500
T1-T2
OUTA
OUTB
LOAD
DIFF
AD8021
) swing symmetrically
and I
500
R
225
.
R
, via a passive recon-
OPT
LOAD
LOAD
OUTB
AD9775
, of 25 Ω. The
is converted
AVDD
OUTA
and