The ADC32RF5x provides optional (enabled via SPI writes) analog out-of-band, large amplitude dither. It has a bandwidth of ~ 20 MHz located at DC and an adjustable amplitude with a maximum dither power of ~ -20 dBFS (PAR ~ 9 dB). The dither is completely rolled-off into the noise floor within ~ 100 MHz as illustrated in Figure 7-7. Since the dither is large amplitude, it is recommended for the signal input not to exceed -2.5 dBFS to avoid input saturation. The dither signal also couples to the input signal and, depending on input frequency, can degrade the close in phase noise.

Figure 7-7 Analog out-of-band dither

In the frequency domain the dither signal shows up like individual tones as shown in Figure 7-12. The dither update frequency can be adjusted with the dither divider setting. The dither update frequency is: F_S / 4 / 2047 / 'Dither Divider'. In the frequency spectrum there will be 2 larger dither spurs at F_IN +/- F_S / 4 / 'Dither Divider'.

By default, the divider is set to 50, which translates to a dither spur spacing of ~ 7 kHz. A divider setting of 32 translates to a dither spacing of ~ 11 kHz as shown in Figure 7-9. The lower the divider setting, the higher the dither tone frequency. Figure 7-9 also shows that the dither energy reduces as the offset frequency increases - less dither energy reduces the higher harmonic spur improvement.

Figure 7-8 Dither Close-Up

Figure 7-9 Dither vs Dither Divider Setting

The analog dither needs to be enabled in multiple locations. Different dither amplitudes should be used depending on internal averaging used as shown in Table 7-5.

The internal analog dither can be enabled via the following register writes. The dither divider is set in register 0xB1 as actual -1 (e.g. a divider of 48 would be programmed as 47, default is 0x00 which is divider = 50). See Table 7-9.