JAJSOU1B june 2022 – august 2023 ADC32RF54 , ADC32RF55
PRODUCTION DATA
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.
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: FS / 4 / 2047 / 'Dither Divider'. In the frequency spectrum there will be 2 larger dither spurs at FIN +/- FS / 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.
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.
Mode | Amplitude | Dither Amp1 | Dither Amp2 |
---|---|---|---|
1x AVG | +/-1024 codes | 0 | 0 |
1x AVG | +/-768 codes | 0 | -4 |
2x/4x AVG | +/-1024 codes | 3 | 0 |
2x/4x AVG | +/-768 codes | 0 | 0 |
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.
ADDR | DATA | DESCRIPTION | ADDR | DATA | DESCRIPTION | |
---|---|---|---|---|---|---|
0x05 | 0x40 | Select ANALOG page | 0xB1 | 0x00 |
Sets dither divider. 0x00 = /50 | |
0xA8 | 0x00 | DITHER AMP1: 3 = 0x80, 0 = 0x00 | 0xB2 | 0x00 | ||
0xCD | 0x00 | DITHER AMP2: -4 = 0x40, 0 = 0x00 | 0xAF | 0x18 | ||
0x04 | 0x01 | 0xAF | 0x10 | 0x10 = dither ENABLED, 0x90 = dither DISABLED | ||
0x20 | 0x04 | 0x04 | 0x01 | |||
0x91 | 0x40 | 0x20 | 0x00 | |||
0xAF | 0x10 | 0x04 | 0x00 |