AD9755 Datasheet, PDF (17/28 Pages) Analog Devices – 14-Bit, 300 MSPS High Speed TxDAC+ D/A Converter

AD9755
I_OUTA
I_OUTB
MINI-CIRCUITS
T1-1T
R_LOAD
Figure 20. Differential Output Using a Transformer
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 AD9755 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.
AD9755
I_OUTA
I_OUTB
25⍀
C_OPT
225⍀
225⍀
500⍀
25⍀
500⍀
AD8047
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 differen-
tial 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 AD9755, 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 capa-
bilities 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
AD9755 and the op amp, is also used to level-shift the differ-
ential output of the AD9755 to midsupply (i.e., AVDD/2). The
AD8041 is a suitable op amp for this application.
AD9755
I_OUTA
I_OUTB
225⍀
C_OPT
225⍀
500⍀
AD8041
1k⍀
25⍀
25⍀
500⍀
AVDD
AD9755
SINGLE-ENDED UNBUFFERED VOLTAGE OUTPUT
Figure 23 shows the AD9755 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 compli-
ance 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.
AD9755
I_OUTA
I_OUTB
I_OUTFS = 20mA
25⍀
50⍀
V_OUTA = 0V TO 0.5V
50⍀
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 AD9755
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 Outputs section. Although this single-ended configuration
typically provides the best dc linearity performance, its ac distor-
tion performance at higher DAC update rates may be limited by
the op amp’s slewing capabilities. The op amp provides 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 subsequently be reduced.
AD9755
I_OUTA
I_OUTB
C_OPT
R_FB
200⍀
200⍀
V_OUT = I_OUTFS ؋ R_FB
Figure 24. Unipolar Buffered Voltage Output
Figure 22. Single-Supply DC Differential Coupled Circuit
REV. B
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