AD9516-2SLASHPCBZ datasheet(35/84 Pages) AD

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AD9516-2

Rev. 0 | Page 35 of 84

REF1

REF2

REFIN

150Ω

10kΩ

12kΩ

10kΩ

REFIN

85kΩ

Figure 47. REFIN Equivalent Circuit

Reference Switchover

The AD9516 supports dual single-ended CMOS inputs, as well

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

reference mode, the AD9516 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 redundant references. 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.

There are several configurable modes of reference switchover.

The switchover can be performed manually or automatically.

The manual switchover is done either through a register setting

(0x1D), or by using the REF_SEL pin. The automatic switchover

occurs when REF1 disappears. There is also a switchover deglitch

feature which ensures that the PLL does not receive rising edges

that are far out of alignment with the newly selected reference.

There are two reference automatic switchover modes (0x1C):

• Prefer REF1: Switch from REF1 to REF2 when REF1

disappears. Return to REF1 from REF2 when REF1 returns.

• Stay on REF2: Automatically switch to REF2 if REF1 disappears,

but do not switch back to REF1 if it reappears. The reference

can be set back to REF1 manually at an appropriate time.

In automatic mode, REF1 is monitored by REF2. If REF1

disappears (two consecutive falling edges of REF2 without an

edge transition on REF1), REF1 is considered missing. Upon

the next subsequent rising edge of REF2, REF2 is used as the

reference clock to the PLL. If 0x1C<3> = 0b (default), when

REF1 returns (four rising edges of REF1 without two falling

edges of REF2 between the REF1 edges), the PLL reference

switchs back to REF1. If 0x1C<3> = 1b, the user can control

when to switch back to REF1. This is done by programming the

part to manual reference select mode (0x1C<4> = 0b) and by

ensuring that the Registers and/or REF_SEL pin are set to select

the desired reference. Auto mode can be re-enabled once REF1

is reselected.

Manual switchover requires the presence of a clock on the

reference input that is being switched to, or that the deglitching

feature be disabled (0x1C<7>).

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 16383 by

writing to 0x11 and 0x12. (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. 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 N: P, 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 × B) + A

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

Prescaler

The prescaler of the AD9516 allows for two modes of operation:

a fixed divide (FD) mode of 1, 2, or 3 and 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 53, 0x16<2:0>. Not all modes are

available at all frequencies (see Table 2).

When operating the AD9516 in dual modulus mode (P//P + 1),

the equation used to relate input reference frequency to VCO

output frequency is

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

However, when operating the prescaler in FD mode 1, 2, or 3,

the A counter is not used (A = 0) and the equation simplifies to

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

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