AD9551SLASHPCBZ datasheet(23/40 Pages) AD

23 / 40 page

AD9551

Rev. B | Page 23 of 40

Determine the feedback divider values for the output PLL.

Repeat this step for each ODF when multiple ODFs exist

(for example, 35, 36, and 40, in the case of Table 17).

To calculate the feedback divider values for a given ODF, use

the following equation:

ODF

OUT1

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

Note that the left side of the equation contains variables

with known quantities. Furthermore, the values are neces-

sarily rational, so the left side is expressible as a ratio of two

integers, X and Y. The following is an example equation:

⎟

⎠

⎜

⎝

⎦

⎣

1664

500

247

)

(

)

)(

(

625

⎟

⎞

⎜

⎛

⎥

⎤

⎢

⎡66

625

In the context of the AD9551, X/Y is always an improper

fraction. Therefore, it is expressible as the sum of an integer,

N, and the proper fraction, R/Y (R and Y are integers).

1664

500

247

Therefore, the example yields N = 148, Y = 1664, and R =

1228. To arrive at this result, use long division to convert the

improper fraction, X/Y, to an integer (N) and a proper fraction

(R/Y). Note that dividing Y into X by means of long division

yields an integer, N, and a remainder, R. The proper fraction

has a numerator (R, the remainder) and a denominator (Y,

the divisor), as follows:

–NY

= N +

Figure 23. Example Long Division

It is imperative to use long division to obtain the correct

results. Avoid the use of a calculator or math program

because these do not always yield correct results due to

internal rounding and/or truncation. Some calculators or

math programs may be up to the task if they can handle very

large integer operations, but such are not common.

In the example, N = 148 and R/Y = 1228/1664, which reduces

to R/Y = 307/416. These values of N, R, and Y constitute

the fol-lowing respective feedback divider values: N = 148,

FRAC = 307, and MOD = 416.

The only caveat is that N and MOD must meet the constraints

given in the Output/Input Frequency Relationship section.

In the example, FRAC is nonzero, so the division value is

an integer plus the fractional component, FRAC/MOD. This

implies that the feedback SDM is necessary as part of the feed-

back divider. If FRAC = 0, the feedback division factor is an

integer and the SDM is not required (it can be bypassed).

Although the feedback divider values obtained in this way

provide the proper feedback divide ratio to synthesize the

exact output frequency, they may not yield optimal jitter

performance at the final output. One reason for this is that

the value of MOD defines the period of the SDM, which has

a direct impact on the spurious output of the SDM. Spe-

cifically, the SDM spectrum has MOD at evenly spaced

spurs between dc and fIF. Thus, the spectral sepa-ration

(Δf) of the spurs associated with the feedback SDM is

MOD

Because the SDM is in the feedback path of the output PLL,

these spurs appear in the output signal as spurious compo-

nents offset by Δf from fOUT1. Therefore, a small MOD value

produces relatively large spurs, with relatively large frequency

offsets from fOUT1, whereas a large MOD value produces

smaller spurs but more closely spaced to fOUT1. Clearly, the

value of MOD has a direct impact on the spurious content

(that is, jitter) at OUT1.

Generally, the largest possible MOD value yields the smallest

spurs. Thus, it is desirable to scale MOD and FRAC by the

integer part of 220 divided by the value of MOD obtained

previously. In the example, the value of MOD is 416, yielding

a scale factor of 2520 (the integer part of 220/416). A scale

factor of 2520 leads to FRAC = 307 × 2520 = 773,640, and

MOD = 416 × 2520 = 1,048,320.

However, these FRAC and MOD values are different from

those that appear in Table 14 (Y[3:0] = 0100). The reason is

that a scale factor of 1120 (instead of 2520) was found to yield

the most accept-able overall performance. A scale factor of

1120 results in the following Table 14 values: FRAC =

343,840 and MOD = 465,920.

Determine the values of the REFA (or REFB) input dividers.

To calculate the feedback divider values, use the following

equation:

REF

Note that the left side of the equation contains variables

with known quantities. Furthermore, the values are neces-

sarily rational, so the left side is expressible as a ratio of two

integers, X and Y. The following is an example equation:

⎟

⎠

⎞

⎜

⎝

⎛

200

616

712

867

)

237

)(

(

100

)

239

(

62208

237

239

622

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