Dual edge sampling modes (DES1X, DES2XL/H) outputs unique samples on both the rising and falling edge of CLK, doubling the sample rate for the same clock frequency compared to NRZ, RTZ or RF modes. DES1X mode requires input samples for both clock edges while DES2XL/H modes generate the falling edge samples by digital interpolation. The 2x DES interpolator has an 80% passband bandwidth, 55 dB stopband attenuation and can be configured as low pass or high pass (The response is shown in Figure 8-150). The DES interpolator is lowpass in DES2XL mode, passing the signal below 0.4*F_CLK and removing the image above 0.6*F_CLK. IN DES2XH mode signals above 0.6*F_CLK are passed and the image below 0.4*F_CLK is removed. In the transition band between 0.4*F_CLK and 0.6*F_CLK, the passband is attenuated by up to 6 dB and the image attenuation is significantly reduce.

Figure 8-1 DES Interpolator Frequency Response

A non-50% CLK duty cycle results in an image of the signal at F_CLK - F_OUT. The amplitude of the image in DES2XL/H modes compared to NRZ/RF modes is shown in Figure 8-2. DES2XL provides 30+ dB suppression over NRZ mode and DES2XH 20 to 30 dB suppression over RF mode. This reduces the analog filtering required after the DAC to remove the unwanted images.

Figure 8-2 DES2XL/H Image compare to NRZ and RF Modes

The input clock frequency and input data rate is the same for NRZ, RF, RTZ, DES2XL and DES2XH modes - only the output waveform generated by the DAC is changed (See Section 7.3.1). Changing between modes only requires a different setting of the MXMODE register.

Figure 8-57 shows a comparison of DES2XL and NRZ mode for a fullscale tone at 3497 MHz with 10 GSPS clock frequency. In addition to the reduction of the image at F_CLK - F_OUT = 6743 MHz, DES2XL mode can also suppress harmonics that in NRZ mode fold back below F_CLK/2. In the plot, HD2 has an image at 3006 MHz that is -65 dBc in NRZ mode and -80 dBc in DES2XL mode. Likewise, the HD3 image at 491 MHz improves from -70 dBc in NRZ mode to better than -90 dBc in DES2XL mode. SFDR between 0 and F_CLK/2 is limited by HD2 and therefore improves from 65 to 80 dBc. Note that the non-linearity specifications for DES2XL mode in Section 6 are measured between 0 to F_CLK/2 (as is NRZ mode), and for DES2XH mode between F_CLK/2 and F_CLK.

Figure 8-3 Output Spectra Comparing DES2XL and NRZ Modes

RF and DES2XH mode behave similarly. Figure 8-58 shows a tone at 7997 MHz with 10 GHz clock in RF and DES2XH modes. HD2 and HD3 have folded frequencies around 6 GHz in NRZ mode, these are suppressed > 10 dB in DES2XH mode.

Figure 8-4 Output Spectra Comparing DES2XH and RF Modes

One additional benefit of DES2XL compared to NRZ mode is an improvement in additive phase noise of approximately 6 dB in the 1/f region of the offset frequency (see Figure 8-5). This is due to DES2XL using both the rising and falling edges of the clock, which cancels some common mode noise in the clock path. Since RF mode also uses the falling edge to generate the inverse sample, there is no significant difference between RF mode and DES2XH mode.

fCLK = 10.24 GHz

Figure 8-5 Phase Noise vs Offset Frequency at 1 GHz

There are 3 small disadvantages of DES2XL and DES2XH modes to be aware of:

1. the attenuation of the signal in the transition band of the DES interpolation filter between 0.4*F_CLK and 0.6*F_CLK
2. An increased latency of 97 clock cycles. or 9.7 ns with a 10 GHz clock, due to the DES interpolation filter
3. An increase in power of 250 mW with a 10 GHz clock, due to the DES interpolation filter