AD9963BCPZRL Analog Devices Inc, AD9963BCPZRL Datasheet - Page 43

• Manufacturer Part Number: AD9963BCPZRL
• Description: Dual 16B, 200 MSPS D-A Converter
• Manufacturer: Analog Devices Inc

Specifications of AD9963BCPZRL

• Package / Case: 72-VFQFN, CSP Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The circuit shown in Figure 60 shows a typical output circuit
configuration that provides a non zero bias voltage at the
TXCML pin. Resistance values of 499 Ω for R
R
1.0 V output common-mode voltage and a voltage of 0.5 V
supplied to the TXCML pin. The 2 mA full-scale current flows
through the 249 Ω R
decoupling capacitor, assures a low ac driving impedance for
the TXCML pin.
Transmit DAC Output Circuit Configurations
The following section illustrates some typical output configu-
rations for the AD9961/AD9963 transmit DACs. Unless
otherwise noted, it is assumed that I
2.0 mA. For applications requiring the optimum dynamic
performance, a differential output configuration is suggested.
A differential output configuration can consist of either an RF
transformer or a differential op amp configuration. The trans-
former configuration provides the optimum high frequency
performance and is recommended for any application that
allows ac coupling. The differential op amp configuration is
suitable for applications requiring dc coupling, signal gain,
and/or a low output impedance.
A single-ended output is suitable for applications where low
cost and low power consumption are primary concerns.
Differential Coupling Using a Transformer
An RF transformer can be used to perform a differential-to-
single-ended signal conversion, as shown in Figure 61. The
distortion performance of a transformer typically exceeds
that available from standard op amps, particularly at higher
frequencies. Transformer coupling provides excellent rejection
of common-mode distortion (that is, even-order harmonics)
over a wide frequency range. It also provides electrical isolation
and can deliver voltage gain without adding noise. Transformers
with different impedance ratios can also be used for impedance
matching purposes. The main disadvantages of transformer
coupling are low frequency roll-off, lack-of-power gain, and
high output impedance.
CML
produces a 2 V p-p differential output voltage swing with a
AD9961/AD9963
Figure 60. Circuit for Setting TXCML Level Using R
TXCML
CML
TXIN
TXIP
creating the 0.5 V TXCML voltage. The
65
66
62
C
OUTFS
R
R
L
L
is set to a nominal
R
R
L
CML
and 249 Ω for
V
+
–
OUT
CML
Rev. 0 | Page 43 of 60
The center tap on the primary side of the transformer must be
connected to a voltage that keeps the voltages on TXIP and
TXIN within the output common-mode voltage range of the
device. Note that the dc component of the DAC output current
is equal to I
center tap of the transformer should provide a path for this dc
current. In most applications, AGND provides the most conve-
nient voltage for the transformer center tap. The complementary
voltages appearing at TXIP and TXIN (that is, V
V
maintained with the specified output compliance range of the
AD9961/AD9963.
A differential resistor, R
where the output of the transformer is connected to the load,
R
reflected by the transformer, is chosen to provide a source
termination that results in a low voltage standing wave ratio
(VSWR). Note that approximately half the signal power is
dissipated across R
Differential Buffered Output Using an Op Amp
A dual op amp (see the circuit shown in Figure 62) can be used
in a differential version of the single-ended buffer shown in
Figure 63. The same R-C network is used to form a one-pole,
differential, low-pass filter to isolate the op amp inputs from
the high frequency images produced by the DAC outputs.
The feedback resistor, R
to-peak signal swing by the formula
The minimum single-ended voltages out of the amplifier are,
respectively,
The common-mode voltage of the differential output is
determined by the formula
LOAD
IOUTN
V
V
V
, via a passive reconstruction filter or cable. R
AD9961/AD9963
) swing symmetrically around AGND and should be
OUT
MIN
CM
= V
= V
= 2 × R
Figure 61. Differential Output Using a Transformer
OUTFS
MAX
MAX
and flows out of both TXIP and TXIN. The
− R
FB
− R
TXIN
TXIP
DIFF
× I
FB
FB
.
65
66
FS
× I
× I
DIFF
FB
FS
, determines the differential peak-
FS
, can be inserted in applications
OPTIONAL R
DIFF
AD9961/AD9963
IOUTP
DIFF
R
and
LOAD
, as