AD9773BSVRL Analog Devices Inc, AD9773BSVRL Datasheet - Page 46

• Manufacturer Part Number: AD9773BSVRL
• Description: IC,D/A CONVERTER,DUAL,12-BIT,CMOS,TQFP,80PIN
• Manufacturer: Analog Devices Inc
• Series: TxDAC®r

Specifications of AD9773BSVRL

• Rohs Status: RoHS non-compliant
• Settling Time: 11ns
• Number Of Bits: 12
• Data Interface: Serial, SPI™
• 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: AD9773-EBZ - BOARD EVALUATION AD9773

AD9773
APPLYING THE OUTPUT CONFIGURATIONS
The following sections illustrate typical output configurations
for the AD9773. Unless otherwise noted, it is assumed that
I
optimum dynamic performance, a differential output
configuration is suggested. A simple differential output can be
achieved by converting I
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 configuration 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 AD9773 and
the equivalent circuit. A typical application where this information
may be useful is when designing an interface filter between
the AD9773 and the Analog Devices AD8345 quadrature
modulator.
OUTFS
is set to a nominal 20 mA. For applications requiring
OUTA
I
OUTFS
V
and/or I
SOURCE
Figure 97. DAC Output Equivalent Circuit
× (R
p-p
A
+ R
=
OUTA
AD9773
OUTB
B
OUTA
)
I
I
or I
OUTB
OUTA
is connected to a load resistor, R
and I
R
OUTB
A
R
A
+ R
OUTB
are maintained at ground or
B
OUTA
(DIFFERENTIAL)
to a voltage output by
R
B
V
or I
OUT
V
V
OUTB
OUT
OUT
–
+
into a
LOAD
Rev. D | Page 46 of 60
,
For the typical situation, where I
both equal 50 Ω, the equivalent circuit values become
Note that the output impedance of the AD9773 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
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 (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 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 AD9773. 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 approximately
half the signal power dissipates across R
DIFF
OUTA
, may be inserted in applications where the output of the
V
R
and I
OUT
SOURCE
OUTA
= 100 Ω
Figure 98. Transformer-Coupled Output Circuit
OUTB
and I
= 2 V p-p
(that is, V
DAC
I
I
OUTA
OUTB
OUTB
. The complementary voltages appearing
OUTA
MINI-CIRCUITS
DIFF
and V
T1-1T
OUTFS
is determined by the
= 20 mA and R
OUTB
DIFF
LOAD
.
) swing symmetrically
R
, via a passive
LOAD
A
and R
B