AD9755 Analog Devices, AD9755 Datasheet - Page 17

• Manufacturer Part Number: AD9755
• Manufacturer: Analog Devices

Specifications of AD9755

• Resolution (bits): 14bit
• Dac Update Rate: 300MSPS
• Dac Settling Time: 11ns
• Max Pos Supply (v): +3.6V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

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
differential voltage developed across I
to a single-ended signal via the differential op amp configuration.
An optional capacitor can be installed across I
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.
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.
REV. B
Figure 22. Single-Supply DC Differential Coupled Circuit
Figure 21. DC Differential Coupling Using an Op Amp
Figure 20. Differential Output Using a Transformer
AD9755
AD9755
AD9755
I
I
OUTA
OUTB
I
I
OUTA
OUTB
I
I
OUTA
OUTB
25
25
C
OPT
C
OPT
25
225
225
25
MINI-CIRCUITS
225
225
OUTA
500
500
T1-1T
AD8041
and I
500
LOAD
1k
OUTA
OUTB
AD8047
, of 25 Ω. The
500
R
LOAD
is converted
and I
AVDD
OUTB
,
–17–
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
20 mA flows through the equivalent R
R
I
ACOM directly or via a matching R
I
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.
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
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
within the op amp’s voltage output swing capabilities by scaling
I
may result with a reduced I
amp will be required to sink will subsequently be reduced.
OUTB
OUTFS
OUTFS
LOAD
Figure 23. 0 V to 0.5 V Unbuffered Voltage Output
AD9755
. The unused output (I
represents the equivalent load resistance seen by I
and R
and/or R
Figure 24. Unipolar Buffered Voltage Output
AD9755
I
I
OUTA
OUTB
LOAD
I
I
OUTA
OUTB
FB
FB
. An improvement in ac distortion performance
can be selected as long as the positive compli-
and I
I
OUTFS
OUTFS
200
25
= 20mA
OUTFS
OUTA
. The full-scale output should be set
, since the signal current the op
50
or I
200
C
R
LOAD
OPT
FB
OUTB
LOAD
OUTA
. Different values of
) can be connected to
of 25 Ω. In this case,
V
OUTA
(or I
V
OUT
AD9755
50
= 0V TO 0.5V
OUTB
= I
OUTFS
OUTFS
) at a
OUTA
R
FB
, of
or