DAC900-Q1 Datasheet, PDF (15/26 Pages) Texas Instruments – 10-BIT 165-MSPS DIGITAL-TO-ANALOG CONVERTER

DAC900-Q1
www.ti.com
SBAS505 – JUNE 2010
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 +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 DAC900 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 DAC900 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.
Table 1. Input Coding vs Analog Output Current
INPUT CODE (D9 - D0)
11 1111 1111
10 0000 0000
00 0000 0000
I_OUT
20mA
10mA
0mA
I_OUT
0mA
10mA
20mA
Output Configurations
The current output of the DAC900 allows for a variety of configurations, some of which are illustrated in the
following sections. 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 considered 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 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 impedance requirements. The
differential transformer configuration 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, I_OUT and
I_OUT. The output signals are ac coupled and inherently isolated because of the transformer's magnetic coupling.
As shown in Figure 3, the transformer's center tap is connected to ground. This forces the voltage swing on I_OUT
and I_OUT to be centered at 0V. In this case the two resistors, R_S, may be replaced with one, R_DIFF, or omitted
altogether. This approach should only be used if all components are close to each other, and if the V_SWR is not
important. A complete power transfer from the DAC output to the load can be realized, but the output compliance
range should be observed. Alternatively, if the center tap is not connected, the signal swing will be centered at
R_S × I_OUTFS / 2. However, in this case, the two resistors (R_S) must be used to enable the necessary DC-current
flow for both outputs.
Copyright © 2010, Texas Instruments Incorporated
Product Folder Link(s): DAC900-Q1
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