AD9518_1_15 Datasheet, PDF(28/64 Page) Analog Devices – 6-Output Clock Generator with Integrated 2.5 GHz VCO

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AD9518_1_15 Datasheet, PDF (28/64 Pages) Analog Devices – 6-Output Clock Generator with Integrated 2.5 GHz VCO

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AD9518-1

In differential mode, the reference input pins are internally self-

biased so that they can be ac-coupled via capacitors. It is possible

to dc couple to these inputs. If the differential REFIN is driven

by a single-ended signal, the unused side (REFIN) should be

decoupled via a suitable capacitor to a quiet ground. Figure 34

shows the equivalent circuit of REFIN.

REF1

85kâ¦

REFIN

10kâ¦ 12kâ¦

150â¦

REFIN

150â¦

10kâ¦ 10kâ¦

REF2

85kâ¦

Figure 34. REFIN Equivalent Circuit

Reference Switchover

The AD9518 supports dual single-ended CMOS inputs, as well

as a single differential reference input. In the dual single-ended

reference mode, the AD9518 supports automatic and manual

PLL reference clock switching between REF1 (on Pin REFIN)

and REF2 (on Pin REFIN). This feature supports networking

and other applications that require smooth switching of redundant

references. When used in conjunction with the automatic holdover

function, the AD9518 can achieve a worst-case reference input

switchover with an output frequency disturbance as low as 10 ppm.

When using reference switchover, the single-ended reference

inputs should be dc-coupled CMOS levels and never be allowed

to go to high impedance. If these inputs are allowed to go to high

impedance, noise may cause the buffer to chatter, causing

a false detection of the presence of a reference.

Reference switchover can be performed manually or auto-

matically. Manual switchover is performed either through

requires the presence of a clock on the reference input that is

being switched to, or that the deglitching feature be disabled

(Register 0x01C[7]). The reference switching logic fails if this

condition is not met, and the PLL does not reacquire.

Data Sheet

Automatic revertive switchover relies on the REFMON pin to

indicate when REF1 disappears. By programming Register 0x01B =

0xF7 and Register 0x01C = 0x26, the REFMON pin is programmed

to be high when REF1 is invalid, which commands the switch to

REF2. When REF1 is valid again, the REFMON pin goes low, and

the part again locks to REF1. It is also possible to use the STATUS

pin for this function, and REF2 can be used as the preferred

reference.

A switchover deglitch feature ensures that the PLL does not

receive rising edges that are far out of alignment with the newly

selected reference.

Automatic nonrevertive switching is not supported.

Reference Divider R

The reference inputs are routed to the reference divider, R.

R (a 14-bit counter) can be set to any value from 0 to 16,383

by writing to Register 0x011 and Register 0x012. (Both R = 0 and

R = 1 give divide-by-1.) The output of the R divider goes to one

of the PFD inputs to be compared with the VCO frequency

divided by the N divider. The frequency applied to the PFD

must not exceed the maximum allowable frequency, which

depends on the antibacklash pulse setting (see Table 2).

The R counter has its own reset. The R counter can be reset

using the shared reset bit of the R, A, and B counters. It can

also be reset by a SYNC operation.

VCXO/VCO Feedback Divider NâP, A, B, R

The N divider is a combination of a prescaler (P) and two

counters, A and B. The total divider value is

N = (P Ã B) + A

where the value of P can be 2, 4, 8, 16, or 32.

Prescaler

The prescaler of the AD9518 allows for two modes of operation:

a fixed divide (FD) mode of 1, 2, or 3, and a dual modulus (DM)

mode where the prescaler divides by P and (P + 1) {2 and 3,

4 and 5, 8 and 9, 16 and 17, or 32 and 33}. The prescaler modes

of operation are given in Table 44, Register 0x016[2:0]. Not all

modes are available at all frequencies (see Table 2).

When operating the AD9518 in dual modulus mode (P//P + 1),

the equation used to relate input reference frequency to VCO

output frequency is

fVCO = (fREF/R) Ã (P Ã B + A) = fREF Ã N/R

However, when operating the prescaler in an FD mode of 1, 2,

or 3, the A counter is not used (A = 0) and the equation

simplifies to

fVCO = (fREF/R) Ã (P Ã B) = fREF Ã N/R

When A = 0, the divide is a fixed divide of P = 2, 4, 8, 16, or 32,

in which case the previous equation also applies.

Rev. C | Page 28 of 64

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