DAC902 Datasheet, PDF (12/24 Pages) Burr-Brown (TI) – 12-Bit, 165MSPS DIGITAL-TO-ANALOG CONVERTER

The value of the load resistance is limited by the output
compliance specification of the DAC902. To maintain speci-
fied linearity performance, the voltage for I_OUT and I_OUT
should not exceed the maximum allowable compliance range.
The two single-ended output voltages can be combined to
find the total differential output swing:
V_OUTDIFF
=
V_OUT
–
V_OUT
=
(2 • Code –
4096
4095)
• I_OUTFS
• R_LOAD
(7)
ANALOG OUTPUTS
The DAC902 provides two complementary current outputs,
I_OUT and I_OUT. The simplified circuit of the analog output
stage representing the differential topology is shown in
Figure 2. The output impedance of 200kΩ || 12pF for I_OUT
and I_OUT results from the parallel combination of the differ-
ential switches, along with the current sources and associ-
ated parasitic capacitances.
+V_A
DAC902
I_OUT
R_L
I_OUT
R_L
FIGURE 2. Equivalent Analog Output.
The signal voltage swing that may develop at the two
outputs, I_OUT and I_OUT, is limited by a negative and positive
compliance. The negative limit of –1V is given by the
breakdown voltage of the CMOS process, and exceeding it
will compromise the reliability of the DAC902, or even
cause permanent damage. With the full-scale output set to
20mA, the positive compliance equals 1.25V, operating with
+V_D = 5V. Note that the compliance range decreases to
about 1V for a selected output current of I_OUTFS = 2mA.
Care should be taken that the configuration of DAC902 does
not exceed the compliance range to avoid degradation of the
distortion performance and integral linearity.
Best distortion performance is typically achieved with the
maximum full-scale output signal limited to approximately
0.5V. This is the case for a 50Ω doubly-terminated load and
a 20mA full-scale output current. A variety of loads can be
adapted to the output of the DAC902 by selecting a suitable
transformer while maintaining optimum voltage levels at
I_OUT and I_OUT. Furthermore, using the differential output
configuration in combination with a transformer will be
instrumental for achieving excellent distortion performance.
Common-mode errors, such as even-order harmonics or
noise, can be substantially reduced. This is particularly the
case with high output frequencies and/or output amplitudes
below full-scale.
For those applications requiring the optimum distortion and
noise performance, it is recommended to select a full-scale
output of 20mA. A lower full-scale range down to 2mA may
be considered for applications that require a low power
consumption, but can tolerate a reduced performance level.
INPUT CODE (D11 - D0)
1111 1111 1111
1000 0000 0000
0000 0000 0000
I_OUT
20mA
10mA
0mA
I_OUT
0mA
10mA
20mA
TABLE I. Input Coding vs Analog Output Current.
OUTPUT CONFIGURATIONS
The current output of the DAC902 allows for a variety of
configurations, some of which are illustrated below. As men-
tioned previously, utilizing the converter’s differential out-
puts will yield the best dynamic performance. Such a differ-
ential output circuit may consist of an RF transformer or a
differential amplifier configuration. The transformer configu-
ration is ideal for most applications with ac coupling, while
op amps will be suitable for a DC-coupled configuration.
The single-ended configuration may be considered for appli-
cations requiring a unipolar output voltage. Connecting a
resistor from either one of the outputs to ground will convert
the output current into a ground-referenced voltage signal.
To improve on the DC linearity, an I-to-V converter can be
used instead. This will result in a negative signal excursion
and, therefore, requires a dual supply amplifier.
DIFFERENTIAL WITH TRANSFORMER
Using an RF transformer provides a convenient way of
converting the differential output signal into a single-ended
signal while achieving excellent dynamic performance (see
Figure 3). The appropriate transformer should be carefully
selected based on the output frequency spectrum and imped-
ance requirements. The differential transformer configura-
tion has the benefit of significantly reducing common-mode
signals, thus improving the dynamic performance over a
wide range of frequencies. Furthermore, by selecting a
suitable impedance ratio (winding ratio), the transformer can
be used to provide optimum impedance matching while
controlling the compliance voltage for the converter outputs.
The model shown in Figure 3 has a 1:1 ratio and may be used
to interface the DAC902 to a 50Ω load. This results in a 25Ω
load for each of the outputs, I_OUT and I_OUT. The output
signals are ac coupled and inherently isolated because of its
magnetic coupling.
12
DAC902
SBAS094B