AD9772EB Analog Devices, AD9772EB Datasheet - Page 19

• Manufacturer Part Number: AD9772EB
• Description: 14-Bit/ 160 MSPS TxDAC+ with 2x Interpolation Filter
• Manufacturer: Analog Devices

APPLYING THE AD9772
OUTPUT CONFIGURATIONS
The following sections illustrate some typical output configura-
tions for the AD9772. Unless otherwise noted, it is assumed
that I
For applications requiring the optimum dynamic performance, a
differential output configuration is highly recommended. A
differential output configuration may consist of either an RF
transformer or a differential op amp configuration. The trans-
former configuration provides the optimum high frequency
performance and is recommended for any application allowing
for ac coupling. The differential op amp configuration is suitable
for applications requiring dc coupling, a bipolar output, signal
gain and/or level shifting.
A single-ended output is suitable for applications requiring a
unipolar voltage output. A positive unipolar output voltage will
result if IOUTA and/or IOUTB is connected to an appropriately
sized load resistor, R
tion may be more 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 convert-
ing IOUTA or IOUTB into a negative unipolar voltage. This
configuration provides the best dc linearity since IOUTA or
IOUTB is maintained at a virtual ground.
REV. 0
OUTFS
120
100
80
60
40
20
9
8
7
6
5
4
3
2
1
0
0
0
0
Figure 39. I
is set to a nominal 20 mA for optimum performance.
Figure 40. I
0.05
20
0.1
LOAD
40
DVDD
0.15
PLLVDD
, referred to ACOM. This configura-
60
RATIO – f
f
vs. Ratio @ DVDD = 3 V
DATA
f
0.2
f
f
f
I
f
f
DATA
DATA
DATA
CLKVDD
DATA
DATA
DATA
and I
80
– MSPS
OUT
= 100MSPS
= 150MSPS
= 125MSPS
= 25MSPS
0.25
= 65MSPS
= 50MSPS
/f
CLKVDD
DATA
100
0.3
I
PLLVDD
120
vs. f
0.35
DATA
140
0.4
0.45
160
–19–
DIFFERENTIAL COUPLING USING A TRANSFORMER
An RF transformer can be used to perform a differential-to-
single-ended signal conversion as shown in Figure 41. A
differentially-coupled transformer output provides the optimum
distortion performance for output signals whose spectral content
lies within the transformer’s passband. 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. Note that the transformer
provides ac coupling only and its linearity performance degrades
at the low end of its frequency range due to core saturation.
The center tap on the primary side of the transformer must be
connected to ACOM to provide the necessary dc current path
for both IOUTA and IOUTB. The complementary voltages
appearing at IOUTA and IOUTB (i.e., V
swing symmetrically around ACOM and should be maintained
with the specified output compliance range of the AD9772. A
differential resistor, R
which the output of the transformer is connected to the load,
R
mined by the transformer’s impedance ratio and provides the
proper source termination that results in a low VSWR (Voltage
Standing Wave Ratio). Note that approximately half the signal
power will be dissipated across R
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 42. The AD9772 is con-
figured with two equal load resistors, R
differential voltage developed across IOUTA and IOUTB is
converted to a single-ended signal via the differential op amp
configuration. An optional capacitor can be installed across
IOUTA and IOUTB, forming a real pole in a low-pass filter.
The addition of this capacitor also enhances the op amp’s dis-
tortion performance by preventing the DAC’s high slewing
output from overloading the op amp’s input.
The common-mode rejection of this configuration is typically
determined by the resistor matching. In this circuit, the differ-
ential op amp circuit using the AD8055 is configured to provide
LOAD
Figure 42. DC Differential Coupling Using an Op Amp
Figure 41. Differential Output Using a Transformer
, via a passive reconstruction filter or cable. R
AD9772
AD9772
IOUTA
IOUTB
IOUTA
IOUTB
DIFF
25
, may be inserted in applications in
OPTIONAL
R
DIFF
C
DIFF
OPT
.
MINI-CIRCUITS
25
225
225
T1-1T
LOAD
OUTA
500
, of 25
AD8055
and V
500
AD9772
R
LOAD
DIFF
OUTB
. The
is deter-
)