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DAC908 Fiches technique(PDF) 11 Page - Texas Instruments

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No de pièce DAC908
Description  8-Bit, 165MSPS DIGITAL-TO-ANALOG CONVERTER
Download  23 Pages
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Fabricant  TI1 [Texas Instruments]
Site Internet  http://www.ti.com
Logo TI1 - Texas Instruments

DAC908 Fiches technique(HTML) 11 Page - Texas Instruments

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DAC908
11
SBAS109B
The value of the load resistance is limited by the output
compliance specification of the DAC908. 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:
(7)
ANALOG OUTPUTS
The DAC908 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 200k
Ω || 12pF for I
OUT
and IOUT results from the parallel combination of the differ-
ential switches, along with the current sources and associ-
ated parasitic capacitances.
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.
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.
FIGURE 2. Equivalent Analog Output.
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 DAC908, 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 DAC908 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 50
Ω doubly-terminated load and
a 20mA full-scale output current. A variety of loads can be
adapted to the output of the DAC908 by selecting a suitable
transformer while maintaining optimum voltage levels at
OUTPUT CONFIGURATIONS
The current output of the DAC908 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.
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 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 DAC908 to a 50
Ω 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.
INPUT CODE (D7 - D0)
IOUT
IOUT
1111 1111
20mA
0mA
1000 0000
10mA
10mA
0000 0000
0mA
20mA
TABLE I. Input Coding vs Analog Output Current.
VV
V
Code
IR
OUTDIFF
OUT
OUT
OUTFS
LOAD
== •
••
(–
)
2
255
256
I
OUT
I
OUT
DAC908
R
L
R
L
+V
A


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