AD9706 Analog Devices, AD9706 Datasheet - Page 40

• Manufacturer Part Number: AD9706
• Manufacturer: Analog Devices

Specifications of AD9706

• Resolution (bits): 12bit
• Dac Update Rate: 175MSPS
• Dac Settling Time: 11ns
• Max Pos Supply (v): +3.6V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

AD9704/AD9705/AD9706/AD9707
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
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.
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
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
V
maintained with the specified output compliance range of the
AD9704/AD9705/AD9706/AD9707.
IOUTB
FS
AD9704/AD9705
AD9706/AD9707
/2 and flows out of both IOUTA and IOUTB. The center
) swing symmetrically around AGND and should be
Figure 88. Differential Output Using a Transformer
IOUTB
IOUTA
20
21
OPTIONAL R
OUTFS
DIFF
is set to a
IOUTA
R
LOAD
and
Rev. B | Page 40 of 44
A differential resistor, R
where the output of the transformer is connected to the load,
R
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
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
pair of series resistors, R
R
The common-mode voltage of the output is determined by the
following formula:
The maximum and minimum voltages out of the amplifier are,
respectively, the following:
LOAD
FB
, determines the peak signal swing by the following formula:
AD9704/AD9705/AD9706/AD9707
V
AD9704/AD9705
AD9706/AD9707
V
V
V
, via a passive reconstruction filter or cable. R
MIN
OUT
CM
MAX
= V
=
=
=
V
R
V
REF
MAX
Figure 89. Single-Supply Single-Ended Buffer
FB
REF
IOUTA
IOUTB
REFIO
OTCM
×
×
− I
×
⎛
⎜
⎜
⎝
I
2
1
FS
⎛
⎜
⎜
⎝
FS
1
21
20
23
19
+
× R
+
R
R
DIFF
S
R
FB
R
, off each output. The feedback resistor,
B
FB
R
FB
B
, can be inserted in applications
S
⎞
⎟
⎟
⎠
R
⎞
⎟
⎟
⎠
−
S
V
OUT
R
C
B
are configured with a
ADA4899-1
–
+
R
+5V
–5V
C
FB
F
Data Sheet
DIFF
DIFF
.
, as
V
OUT