AD9775BSVRL Analog Devices Inc, AD9775BSVRL Datasheet - Page 42

• Manufacturer Part Number: AD9775BSVRL
• Description: IC DAC 14BIT DUAL 160MSPS 80TQFP
• Manufacturer: Analog Devices Inc
• Series: TxDAC+®r

Specifications of AD9775BSVRL

• Rohs Status: RoHS non-compliant
• Settling Time: 11ns
• Number Of Bits: 14
• Data Interface: Parallel
• Number Of Converters: 2
• Voltage Supply Source: Analog and Digital
• Power Dissipation (max): 410mW
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 80-TQFP Exposed Pad, 80-eTQFP, 80-HTQFP, 80-VQFP
• For Use With: AD9775-EBZ - BOARD EVALUATION FOR AD9775

AD9775
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
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
negative unipolar voltage. This configuration provides the best
DAC dc linearity as I
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
OUTFS
OUTA
× (R
V
and/or I
SOURCE
Figure 97. DAC Output Equivalent Circuit
A
+ R
p-p
B
=
OUTA
)
OUTB
I
I
OUTA
OUTB
or I
R
is connected to a load resistor, R
A
+ R
OUTB
B
are maintained at ground or
V
(DIFFERENTIAL)
OUT
OUTA
V
V
OUT
OUT
or I
+
–
OUTB
into a
LOAD
Rev. E | Page 42 of 56
,
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.
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.
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
reconstruction filter or cable. R
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
OUTA
, can be inserted in applications where the output of the
V
R
SOURCE
OUT
and I
OUTA
=
Figure 98. Transformer-Coupled Output Circuit
OUTB
100
=
and I
2
I
DAC
I
OUTA
OUTB
(that is, V
Ω
V p-p
OUTB
. The complementary voltages appearing
MINI-CIRCUITS
OUTA
T1-1T
DIFF
and V
OUTFS
is determined by the
= 20 mA and R
OUTB
LOAD
R
) swing symmetrically
LOAD
, via a passive
DIFF
.
A
and R
B