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AD9777 View Datasheet(PDF) - Analog Devices

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AD9777 Datasheet PDF : 60 Pages
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AD9777
APPLYING THE OUTPUT CONFIGURATIONS
The following sections illustrate typical output configurations
for the AD9777. 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; if power gain is
required, an op amp may be used. The transformer configura-
tion 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 IOUTA and/or IOUTB is connected to a load resistor, RLOAD,
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 IOUTA or IOUTB into a
negative unipolar voltage. This configuration provides the best
DAC dc linearity as IOUTA or IOUTB 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 AD9777 and
the equivalent circuit. A typical application where this
information may be useful is when designing an interface filter
between the AD9777 and the Analog Devices AD8345
quadrature modulator.
IOUTA
IOUTB
VOUT+
VOUT
VSOURCE =
IOUTFS × (RA + RB)
p-p
RA + RB
VOUT
(DIFFERENTIAL)
Figure 97. DAC Output Equivalent Circuit
For the typical situation, where IOUTFS = 20 mA and RA and RB
both equal 50 Ω, the equivalent circuit values become
VSOURCE = 2 VP-P
ROUT = 100 Ω
Note that the output impedance of the AD9777 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.
IOUTA
DAC
IOUTB
MINI-CIRCUITS
T1-1T
RLOAD
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 IOUTA and IOUTB. The complementary voltages appearing
at IOUTA and IOUTB (that is, VOUTA and VOUTB) swing symmetrically
around AGND and should be maintained within the specified
output compliance range of the AD9777. A differential resistor,
RDIFF, can be inserted in applications where the output of the
transformer is connected to the load, RLOAD, via a passive
reconstruction filter or cable. RDIFF 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 RDIFF.
Rev. C | Page 46 of 60
 

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