AD9551SLASHPCBZ datasheet(24/40 Pages) AD

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AD9551

Rev. B | Page 24 of 40

en in Step 4 applies here,

he constraints for NA

elationship

the previous example

lues of N, Q, and Y constitute the respective input

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:

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 a

integers). The same caution giv

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 t

and NB given in the Output/Input Frequency R

hat Y < 219 (524,288).

section and t

Next, use R and Y to compute the following:

= 2R − Y

Using R = 3288 and Y = 25,675 from

yields

= 2 × 3288 − 25,675 = −19,099

These va

divider values: Nx = 24, FRACx = −19,099, and MODx =

25,675.

In the example, FRACx is nonzero, so the division value i

an integer plus the fractional component, FRACx/MODx.

This implies that the input SDM is necessary as part of the

REF

FRAC

MOD

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.

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