Datasheet
REV. C
AD9751
–17–
The center tap on the primary side of the transformer must be
connected to ACOM 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 ACOM and should be maintained with the specified
output compliance range of the AD9751. A differential resistor,
R
DIFF
, may be inserted into 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.
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 21. The AD9751
is configured with two equal load resistors, R
LOAD
, of 25 Ω. The
differential voltage developed across I
OUTA
and I
OUTB
is con-
verted to a single-ended signal via the differential op amp
configuration. 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 high slewing output from
overloading the op amp’s input.
AD9751
I
OUTA
I
OUTB
C
OPT
500⍀
225⍀
225⍀
500⍀
25⍀25⍀
AD8047
Figure 21. DC Differential Coupling Using an Op Amp
T
he common-mode rejection of this configuration is typically
determined by the resistor matching. In this circuit, the dif-
ferential op amp circuit using the AD8047 is configured to
provide some additional signal gain. The op amp must operate
from a dual supply since its output is approximately ± 1.0 V.
A high speed amplifier capable of preserving the differential
performance of the AD9751, while meeting other system-
level objectives (i.e., cost, power), should be selected. The op
amp’s differential gain, gain setting resistor values, and full-
scale output swing capabilities should all be considered when
optimizing this circuit.
The differential circuit shown in Figure 22 provides the nec-
essary level-shifting required in a single-supply system. In this
case, AVDD, which is the positive analog supply for both the
AD9751 and the op amp, is also used to level-shift the differ-
ential output of the AD9751 to midsupply (i.e., AVDD/2). The
AD8041 is a suitable op amp for this application.
AD9751
I
OUTA
I
OUTB
C
OPT
500⍀
225⍀
225⍀
1k⍀25⍀25⍀
AD8041
1k⍀
AVDD
Figure 22. Single-Supply DC Differential Coupled Circuit
SINGLE-ENDED UNBUFFERED VOLTAGE OUTPUT
Figure 23 shows the AD9751 configured to provide a unipolar
output range of approximately 0 V to 0.5 V for a doubly-termi-
nated 50 Ω cable, since the nominal full-scale current, I
OUTFS
, of
20 mA flows through the equivalent R
LOAD
of 25 Ω. In this case,
R
LOAD
represents the equivalent load resistance seen by I
OUTA
or
I
OUTB
. The unused output (I
OUTA
or I
OUTB
) can be connected to
ACOM directly or via a matching R
LOAD
. Different values of
I
OUTFS
and R
LOAD
can be selected as long as the positive com-
pliance range is adhered to. One additional consideration in
this mode is the integral nonlinearity (INL), as discussed in the
Analog Outputs section. For optimum INL performance, the
single-ended, buffered voltage output configuration is suggested.
AD9751
I
OUTA
I
OUTB
50⍀
25⍀
50⍀
V
OUTA
= 0V TO 0.5V
I
OUTFS
= 20mA
Figure 23. 0 V to 0.5 V Unbuffered Voltage Output
SINGLE-ENDED BUFFERED VOLTAGE OUTPUT
Figure 24 shows a buffered single-ended output configuration in
which the op amp performs an I–V conversion on the AD9751
output current. The op amp maintains I
OUTA
(or I
OUTB
) at a
virtual ground, thus minimizing the nonlinear output impedance
effect on the DAC’s INL performance as discussed in the
Analog Output section. Although this single-ended configura-
tion typically provides the best dc linearity performance, its ac
distortion performance at higher DAC update rates may be
limited by the op amp’s slewing capabilities. The op amp pro-
vides a negative unipolar output voltage and its full-scale output
voltage is simply the product of R
FB
and I
OUTFS
. The full-scale
output should be set within the op amp’s voltage output swing
capabilities by scaling I
OUTFS
and/or R
FB
. An improvement in ac
distortion performance may result with a reduced I
OUTFS
, since
the signal current the op amp will be required to sink will subse-
quently be reduced.