IOUT and IOUT. 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.
The value of the load resistance is limited by the output
compliance specification of the DAC900. To maintain speci-
fied linearity performance, the voltage for IOUT and IOUT
should not exceed the maximum allowable compliance range.
The two single-ended output voltages can be combined to
find the total differential output swing:
(2 • Code – 1023)
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.
VOUTDIFF = VOUT – VOUT
=
• IOUTFS • RLOAD
(7)
1024
ANALOG OUTPUTS
The DAC900 provides two complementary current outputs,
IOUT and IOUT. The simplified circuit of the analog output
stage representing the differential topology is shown in
Figure 2. The output impedance of 200kΩ || 12pF for IOUT
and IOUT results from the parallel combination of the differ-
ential switches, along with the current sources and associ-
ated parasitic capacitances.
INPUT CODE (D9 - D0)
IOUT
IOUT
11 1111 1111
10 0000 0000
00 0000 0000
20mA
10mA
0mA
0mA
10mA
20mA
TABLE I. Input Coding vs Analog Output Current.
OUTPUT CONFIGURATIONS
The current output of the DAC900 allows for a variety of
configurations, some of which are illustrated below. As
mentioned previously, utilizing the converter’s differential
outputs will yield the best dynamic performance. Such a
differential output circuit may consist of an RF transformer
(see Figure 3) or a differential amplifier configuration (see
Figure 4). The transformer configuration is ideal for most
applications with ac coupling, while op amps will be suitable
for a DC-coupled configuration.
+VA
DAC900
The single-ended configuration (see Figure 6) may be consid-
ered for applications 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 volt-
age signal. To improve on the DC linearity, an I-to-V con-
verter can be used instead. This will result in a negative signal
excursion and, therefore, requires a dual supply amplifier.
IOUT
RL
IOUT
RL
FIGURE 2. Equivalent Analog Output.
DIFFERENTIAL WITH TRANSFORMER
The signal voltage swing that may develop at the two
outputs, IOUT and IOUT, 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 DAC900, or even
cause permanent damage. With the full-scale output set to
20mA, the positive compliance equals 1.25V, operating with
+VD = 5V. Note that the compliance range decreases to
about 1V for a selected output current of IOUTFS = 2mA.
Care should be taken that the configuration of DAC900 does
not exceed the compliance range to avoid degradation of the
distortion performance and integral linearity.
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 DAC900 to a 50Ω load. This results in a 25Ω
load for each of the outputs, IOUT and IOUT. The output
signals are ac coupled and inherently isolated because of the
transformer's magnetic coupling.
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 DAC900 by selecting a suitable
transformer while maintaining optimum voltage levels at
DAC900
12
SBAS093B
TI [ TEXAS INSTRUMENTS ]
