AD9551 Datasheet, PDF (24/40 Pages) Analog Devices – Multiservice Clock Generator

AD9551
As in Step 4, use long division to convert the fraction, X/Y,
to an integer, N, and a proper fraction, R/Y (R and Y are
integers). The same caution given in Step 4 applies here,
regarding the need to use long division rather than a
calculator or a math program.
Given the example of X = 14,867,712 and Y = 616,200,
long division yields the following: N = 24 and R/Y =
78,912/616,200, which reduces to R/Y = 3,288/25,675. The
only caveats are that N must meet the constraints for NA
and NB given in the Output/Input Frequency Relationship
section and that Y < 2¹⁹ (524,288).
Next, use R and Y to compute the following:
Q = 2R − Y
Using R = 3288 and Y = 25,675 from the previous example
yields
Q = 2 × 3288 − 25,675 = −19,099
These values of N, Q, and Y constitute the respective input
divider values: Nx = 24, FRACx = −19,099, and MODx =
25,675.
In the example, FRACx is nonzero, so the division value is
an integer plus the fractional component, FRACx/MODx.
This implies that the input SDM is necessary as part of the
input divider. If FRACx = 0, then the input division factor is
an integer and the SDM is not required (it can be bypassed).
The choice of MODx affects the jitter performance of the
input section in a manner similar to the feedback dividers.
However, the spectral spacing of the spurs for the input
SDMs is as follows:
Δf _x
=
(MOD_x
f _REFx
)× (2N _x + 1) +
FRAC _x
The input SDMs are similar to the feedback SDM in that it
is desirable to scale MODx and FRACx by the integer part of
2¹⁹, divided by the value of MODx that was calculated pre-
viously in Step 5. In the example calculation, the value of
MODx is 25,675, which leads to a scale factor of 20 (the integer
part of 2¹⁹/25,675). A scale factor of 20 yields the following
results: FRACx = −19,099 × 20 = −381,980 and MODx =
25,675×20 = 513,500.
However, these FRACx and MODx values are different from
those that appear in Table 13 (A[3:0] = 0010). The reason is
that a scale factor of 6 (instead of 20) was found to yield the
most acceptable overall performance. A scale factor of 6
results in the following Table 13 values: FRACx = −19,099 ×
6 = −114,594, and MODx = 25,675 × 6 = 154,050.
LOW DROPOUT (LDO) REGULATORS
The AD9551 is powered from a single 3.3 V supply and contains
on-chip LDO regulators for each function to eliminate the need
for external LDOs. To ensure optimal performance, each LDO
output should have a 0.47 μF capacitor connected between its
access pin and ground.
Note that for best performance, the LDO bypass capacitors
must be placed in close proximity to the device.
Rev. B | Page 24 of 40