Datasheet

AD9704/AD9705/AD9706/AD9707 Data Sheet
Rev. B | Page 40 of 44
APPLICATIONS INFORMATION
OUTPUT CONFIGURATIONS
The following sections illustrate some typical output
configurations for the AD9704/AD9705/AD9706/AD9707.
Unless otherwise noted, it is assumed that I
OUTFS
is set to a
nominal 2 mA. For applications requiring the optimum
dynamic performance, a differential output configuration is
suggested. A differential output configuration can consist of
either an RF transformer or a differential op amp configuration.
The transformer configuration provides the optimum high
frequency performance and is recommended for any application
that allows ac coupling. The differential op amp configuration is
suitable for applications requiring dc coupling, signal gain,
and/or a low output impedance.
A single-ended output is suitable for applications where low
cost and low power consumption are primary concerns.
DIFFERENTIAL COUPLING USING A TRANSFORMER
An RF transformer can be used to perform a differential-to-single-
ended signal conversion, as shown in Figure 88. The distortion
performance of a transformer typically exceeds that available from
standard op amps, particularly at higher frequencies. Transformer
coupling provides excellent rejection of common-mode distortion
(that is, even-order harmonics) over a wide frequency range. It
also provides electrical isolation and can deliver voltage gain
without adding noise. Transformers with different impedance
ratios can also be used for impedance matching purposes. The
main disadvantages of transformer coupling are the low frequency
roll-off, lack of power gain, and the higher output impedance.
AD9704/AD9705
AD9706/AD9707
IOUTB
IOUTA
20
21
OPTIONAL R
DIFF
R
LOAD
05926-095
Figure 88. Differential Output Using a Transformer
The center tap on the primary side of the transformer must be
connected to a voltage that keeps the voltages on IOUTA and
IOUTB within the output common voltage range of the device.
Note that the dc component of the DAC output current is equal
to I
FS
/2 and flows out of both IOUTA and IOUTB. The center
tap of the transformer should provide a path for this dc current.
In many applications, AGND provides the most convenient
voltage for the transformer center tap. The complementary
voltages appearing at IOUTA and IOUTB (that is, V
IOUTA
and
V
IOUTB
) swing symmetrically around AGND and should be
maintained with the specified output compliance range of the
AD9704/AD9705/AD9706/AD9707.
A differential resistor, R
DIFF
, can be inserted in applications
where the output of the transformer is connected to the load,
R
LOAD
, via a passive reconstruction filter or cable. R
DIFF
, as
reflected by the transformer, is chosen to provide a source
termination that results in a low VSWR. Note that approxi-
mately half the signal power is dissipated across R
DIFF
.
SINGLE-ENDED BUFFERED OUTPUT USING AN OP
AMP
An op amp, such as the ADA4899-1, can be used to perform a
single-ended current-to-voltage conversion, as shown in Figure 89.
The AD9704/AD9705/AD9706/AD9707 are configured with a
pair of series resistors, R
S
, off each output. The feedback resistor,
R
FB
, determines the peak signal swing by the following formula:
2
FS
FB
OUT
I
RV ×=
The common-mode voltage of the output is determined by the
following formula:
OUT
B
FB
REF
CM
V
R
R
VV −
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+×= 1
The maximum and minimum voltages out of the amplifier are,
respectively, the following:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+×=
B
FB
REF
MAX
R
R
VV 1
V
MIN
= V
MAX
− I
FS
× R
FB
+5V
AD9704/AD9705
AD9706/AD9707
IOUTA
IOUTB
20
R
FB
V
OUT
REFIO
23
21
R
S
OTCM
19
C
F
C
R
S
R
B
05926-100
+
–
ADA4899-1
–5V
Figure 89. Single-Supply Single-Ended Buffer