DAC5662 Datasheet, PDF (16/22 Pages) Texas Instruments – DUAL, 12-BIT 200 MSPS DIGITAL-TO-ANALOG CONVERTER

DAC5662
SLAS425 – JULY 2004
APPLICATION INFORMATION (continued)
The total differential output voltage is:
VOUTDIFF + VOUT1 * VOUT2
VOUTDIFF + ⁽²
Code * 4095)
4096
IOUTFS
RLOAD
Analog Outputs
AVDD
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(7)
(8)
S(1)
S(1)C
S(2)
S(2)C
S(N)
S(N)C
IOUT1
R_LOAD
IOUT2
R_LOAD
Current Source Array
Figure 20. Analog Outputs
The DAC5662 provides two complementary current outputs, IOUT1 and IOUT2. The simplified circuit of the
analog output stage representing the differential topology is shown in Figure 20. The output impedance of IOUT1
and IOUT2 results from the parallel combination of the differential switches, along with the current sources and
associated parasitic capacitances.
The signal voltage swing that may develop at the two outputs, IOUT1 and IOUT2, is limited by a negative and
positive compliance. The negative limit of –1 V is given by the breakdown voltage of the CMOS process and
exceeding it compromises the reliability of the DAC5662 or even causes permanent damage. With the full-scale
output set to 20 mA, the positive compliance equals 1.2 V. Note that the compliance range decreases to about 1
V for a selected output current of IOUTFS = 2 mA. Care should be taken that the configuration of DAC5662 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 0.5 Vpp. This is the case for a 50-Ω doubly terminated load and a 20-mA full-scale output current.
A variety of loads can be adapted to the output of the DAC5662 by selecting a suitable transformer while
maintaining optimum voltage levels at IOUT1 and IOUT2. 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.
For those applications requiring the optimum distortion and noise performance, it is recommended to select a
full-scale output of 20 mA. A lower full-scale range of 2 mA may be considered for applications that require low
power consumption, but can tolerate a slight reduction in performance level.
Output Configurations
The current outputs of the DAC5662 allow for a variety of configurations. As mentioned previously, utilizing the
converter’s differential outputs yield the best dynamic performance. Such a differential output circuit may consist
of an RF transformer or a differential amplifier configuration. The transformer configuration is ideal for most
applications with ac coupling, while op amps will be suitable for a dc-coupled configuration.
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