Datasheet

AD9549
Rev. D | Page 56 of 76
Register 0x0104—S-Divider (DPLL Feedback Divider)
Table 27.
Bits Bit Name Description
[7:0] S-divider Feedback divider. Divide ratio = 1 − 65,536. If the desired feedback ratio is greater than 65,536, or if
the feedback signal on FDBK_IN is greater than 400 MHz, then Bit 0 of Register 0x0106 must be set.
Note that the actual S-divider is the value in this register plus 1, so to have an R-divider of 1,
Register 0x0104 and Register 0x0105 must both be 0x00. Register 0x0104 is the least significant byte.
Register 0x0105—S-Divider (DPLL Feedback Divider) (Continued)
Table 28.
Bits Bit Name Description
[15:8] S-divider Feedback divider. Divide ratio = 1 − 65,536. If the desired feedback ratio is greater than 65,536, or if
the feedback signal on FDBK_IN is greater than 400 MHz, then Bit 0 of Register 0x0106 must be set.
Note that the actual S-divider is the value in this register plus 1, so to have an R-divider of 1,
Register 0x0104 and Register 0x0105 must both be 0x00. Register 0x0104 is the least significant byte.
Register 0x0106—S-Divider (DPLL Feedback Divider) (Continued)
Table 29.
Bits Bit Name Description
7 Falling edge triggered Setting this bit inverts the reference clock before S-divider.
[6:1] Reserved Reserved.
0 S-divider/2 Setting this bit enables an additional /2 prescaler. See the Feedback Divider (Divide-by-S) section.
If the desired feedback ratio is greater than 65,536, or if the feedback signal on FDBK_IN is greater
than 400 MHz, then this bit must be set. An example of this case is when the PLL is locking to an
image of the DAC output that is above the Nyquist frequency.
Register 0x0107—P-Divider
Table 30.
Bits Bit Name Description
[4:0] P-divider Divide ratio. Controls the ratio of DAC sample rate to loop filter sample rate. See the Digital Loop
Filter section. Loop filter sample rate = DAC sample rate/2^(divide ratio[4:0]). For the default case
of 1 GHz DAC sample rate, and P-divider[4:0] of 5, the loop filter sample rate is 31.25 MHz. Note that
the DAC sample rate is the same as system clock.
Register 0x0108—Loop Coefficients
See the Digital Loop Filter Coefficients section. Note that the AD9549 evaluation software derives these values.
Table 31.
Bits Bit Name Description
[7:0] Alpha-0 Linear coefficient for alpha coefficient.
Register 0x0109—Loop Coefficients (Continued)
Table 32.
Bits Bit Name Description
[11:8] Alpha-0 Linear coefficient for alpha coefficient.