Figure 8-1 Analog Outputs

The DAC5662 provides two complementary current outputs, I_OUT1 and I_OUT2. The simplified circuit of the analog output stage representing the differential topology is shown in Figure 8-1. The output impedance of I_OUT1 and I_OUT2 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, I_OUT1 and I_OUT2, 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 I_OUTFS = 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 I_OUT1 and I_OUT2. 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.