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| 型號： | AD9775BSV |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | DAC |
| 英文描述： | 14-Bit, 160 MSPS 2X/4X/8X Interpolating Dual TxDAC+ D/A Converter |
| 中文描述： | PARALLEL, WORD INPUT LOADING, 0.011 us SETTLING TIME, 14-BIT DAC, PQFP80 |
| 封裝： | PLASTIC, MO-026-ADD-HD, TQFP-80 |
| 文件頁數(shù)： | 37/48頁 |
| 文件大?。?/td> | 6054K |
| 代理商： | AD9775BSV |
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REV. 0
AD9775
–37–
APPLYING THE AD9775 OUTPUT CONFIGURATIONS
The following sections illustrate typical output configurations for
the AD9775. Unless otherwise noted, it is assumed that I
OUTFS
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
OUTA
and I
OUTB
to a voltage output by terminating
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
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 nega-
tive unipolar voltage. This configuration provides the best DAC
dc linearity as I
OUTA
or I
OUTB
are maintained at ground or vir-
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.
I
OUTA
R
A
+ R
B
I
OUTB
V
(DIFFERENTIAL)
V
SOURCE
=
(R
A
B
)
I
OUTFS
p-p
V
OUT
+
V
OUT
–
Figure 51. DAC Output Equivalent Circuit
For the typical situation, where I
OUTFS
= 20 mA and R
A
and R
B
both equal 50
, the equivalent circuit values become:
V
SOURCE
= 2
V p-p
R
OUT
= 100
Note that the output impedance of the AD9775 DAC itself is
greater than 100 k
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 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.
I
OUTA
I
OUTB
DAC
R
LOAD
MINI-CIRCUITS
T1-T2
Figure 52. 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
(i.e., 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
, may be inserted in applications where the output of the
transformer is connected to the load, R
LOAD
, via a passive recon-
struction 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 approximately half the
signal power will be dissipated across R
DIFF
.
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
LOAD
, of 25
. The
differential voltage developed across I
OUTA
and I
OUTB
is converted
to a single-ended signal via the differential op amp configura-
tion. An optional capacitor can be installed across I
OUTA
and
I
OUTB
, forming a real pole in a low pass filter. The addition of
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.
I
OUTA
I
OUTB
DAC
25
AD8021
500
C
OPT
225
225
500
R
OPT
225
AVDD
25
Figure 53. Op Amp-Coupled Output Circuit
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
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