AD9751 Datasheet, PDF (16/26 Pages) Analog Devices – 10-Bit, 300 MSPS High-Speed TxDAC+ D/A Converter

AD9751
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 in applications
where the output of the transformer is connected to the load,
R_LOAD, via a passive reconstruction 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 converted 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
25⍀
C_OPT
225⍀
225⍀
500⍀
25⍀
500⍀
AD8047
AD9751
I_OUTA
I_OUTB
225⍀
C_OPT
225⍀
500⍀
AD8041
1k⍀
25⍀
25⍀
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 Output section of this data sheet. For optimum INL
performance, the single-ended, buffered voltage output configu-
ration is suggested.
AD9751
I_OUTA
I_OUTB
I_OUTFS = 20mA
25⍀
50⍀
V_OUTA = 0V TO 0.5V
50⍀
Figure 21. DC Differential Coupling Using an Op Amp
The 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, its gain setting resistor values, and full-scale
output swing capabilities should all be considered when opti-
mizing 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.
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 be
subsequently reduced.
–16–
REV. 0