AD9772A
DIFFERENTIAL COUPLING USING A TRANSFORMER
An RF transformer can be used to perform a differential-to-
single-ended signal conversion as shown in Figure 24. A
differentially-coupled transformer output provides the optimum
distortion performance for output signals whose spectral content
lies within the transformer’s passband. An RF transformer such
as the Mini-Circuits T1-1T provides excellent rejection of
common-mode distortion (i.e., even-order harmonics) and noise
over a wide frequency range. It also provides electrical isolation
and the ability to deliver twice the power to the load. Trans-
formers with different impedance ratios may also be used for
impedance matching purposes. Note that the transformer
provides ac coupling only and its linearity performance degrades
at the low end of its frequency range due to core saturation.
The common-mode rejection of this configuration is typically
determined by the resistor matching. In this circuit, the differ-
ential op amp circuit using the AD8055 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 AD9772A 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 26 provides the neces-
sary level shifting required in a single-supply system. In this
case, AVDD, the positive analog supply for both the AD9772A
and the op amp, is also used to level-shift the differential output
of the AD9772A to midsupply (i.e., AVDD/2). The AD8057 is
a suitable op amp for this application.
AD9772A
MINI-CIRCUITS
T1-1T
I
OUTA
OUTB
500ꢁ
OPTIONAL
AD9772A
R
LOAD
R
DIFF
225ꢁ
I
OUTA
I
AD8057
225ꢁ
I
OUTB
C
OPT
1kꢁ
Figure 24. Differential Output Using a Transformer
AVDD
25ꢁ
25ꢁ
1kꢁ
The center tap on the primary side of the transformer must be
connected to ACOM to provide the necessary dc current path
for both IOUTA and IOUTB. The complementary voltages appearing
at IOUTA and IOUTB (i.e., VOUTA and VOUTB) swing symmetrically
around ACOM and should be maintained with the specified
output compliance range of the AD9772A. A differential resis-
tor, RDIFF, may be inserted in applications in which the output
of the transformer is connected to the load, RLOAD, via a
passive reconstruction filter or cable. RDIFF is determined by the
transformer’s impedance ratio and provides the proper source
termination that results in a low VSWR (Voltage Standing Wave
Ratio). Note that approximately half the signal power will be dissi-
Figure 26. Single-Supply DC Differential Coupled Circuit
SINGLE-ENDED UNBUFFERED VOLTAGE OUTPUT
Figure 27 shows the AD9772A 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, IOUTFS, of
20 mA flows through the equivalent RLOAD of 25 Ω. In this case,
RLOAD represents the equivalent load resistance seen by IOUTA
.
The unused output (IOUTB) should be connected to ACOM
directly. Different values of IOUTFS and RLOAD can be selected as
long as the positive compliance range is adhered to. One addi-
tional 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, buff-
ered voltage output configuration is suggested.
pated across RDIFF
.
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 25. The AD9772A is
configured with two equal load resistors, RLOAD, of 25 Ω. The
differential voltage developed across IOUTA and IOUTB is converted
to a single-ended signal via the differential op amp configura-
tion. An optional capacitor can be installed across IOUTA and
IOUTB, 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.
AD9772A
I
= 20mA
OUTFS
V
= 0V TO 0.5V
OUTA
I
OUTA
50ꢁ
50ꢁ
I
OUTB
Figure 27. 0 V to 0.5 V Unbuffered Voltage Output
500ꢁ
AD9772A
SINGLE-ENDED BUFFERED VOLTAGE OUTPUT
225ꢁ
Figure 28 shows a buffered single-ended output configuration
in which the op amp U1 performs an I-V conversion on the
AD9772A output current. U1 maintains IOUTA (or IOUTB) at
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 configuration typi-
cally provides the best dc linearity performance, its ac distortion
performance at higher DAC update rates is often limited by
U1’s slewing capabilities. U1 provides a negative unipolar output
voltage and its full-scale output voltage is simply the product of
I
OUTA
AD8055
225ꢁ
I
OUTB
C
OPT
500ꢁ
25ꢁ
25ꢁ
Figure 25. DC Differential Coupling Using an Op Amp
–20–
REV. A
ADI [ ADI ]
