Datasheet

Data Sheet AD9114/AD9115/AD9116/AD9117
Rev. C | Page 47 of 52
APPLICATIONS INFORMATION
OUTPUT CONFIGURATIONS
The following sections illustrate some typical output configu-
rations for the AD9114/AD9115/AD9116/AD9117. Unless
otherwise noted, it is assumed that I
xOUTFS
is set to a nominal
20 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 trans-
former 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 in which 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 102. 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 low frequency roll-off, lack of power gain, and
high output impedance.
AD9114/AD9115/
AD9116/AD9117
IOUTN
IOUTP
29
28
OPTIONAL R
DIFF
R
LOAD
07466-063
Figure 102. 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 IOUTP and
IOUTN within the output common-mode voltage range of the
device. Note that the dc component of the DAC output current
is equal to I
IOUTFS
and flows out of both IOUTP and IOUTN.
The center tap of the transformer should provide a path for this
dc current. In most applications, AGND provides the most
convenient voltage for the transformer center tap. The comple-
mentary voltages appearing at IOUTP and IOUTN (that is,
V
IOUTP
and V
IOUTN
) swing symmetrically around AGND and
should be maintained with the specified output compliance
range of the AD9114/AD9115/AD9116/AD9117.
A differential resistor, R
DIFF
, can be inserted in applications in
which 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 voltage standing wave ratio
(VSWR). Note that approximately 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 103.
Figure 103 is a simplified schematic. The REFIO pin must be
buffered to keep the load current less than 100 nA. The AD9114/
AD9115/AD9116/AD9117 are configured with a pair of series
resistors, R
S
, off each output. For best distortion performance, R
S
should be set to 0 Ω. The feedback resistor, R
FB
, determines the
peak-to-peak signal swing by the formula
V
OUT
= R
FB
× I
FS
The common-mode voltage of the output is determined by the
formula
2
1
FS
FB
B
FB
REF
CM
IR
R
R
VV
×
−








+×=
The maximum and minimum voltages out of the amplifier are,
respectively,








+×=
B
FB
REF
MAX
R
R
VV 1
V
MIN
= V
MAX
− I
FS
× R
FB
+5V
AD9114/AD9115/
AD9116/AD9117
IOUTP
IOUTN
29
R
FB
V
OUT
REFIO
34
28
R
S
AVSS
25
C
F
C
R
S
R
B
+
–
ADA4899-1
–5V
07466-064
Figure 103. Single-Supply, Single-Ended Buffer