AD9518-2ABCPZ datasheet(29/64 Pages) AD

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Data Sheet

AD9518-2

Rev. C | Page 29 of 64

By using combinations of the DM and FD modes, the AD9518

can achieve values of N all the way down to N = 1 and up to

N = 262,175. Table 27 shows how a 10 MHz reference input

can be locked to any integer multiple of N.

Note that the same value of N can be derived in different ways, as

illustrated by the case of N = 12. The user can choose a fixed divide

mode of P = 2 with B = 6, use the dual modulus mode of 2/3 with

A = 0, B = 6, or use the dual modulus mode of 4/5 with A = 0,

B = 3.

The maximum frequency into the prescaler in 2/3 dual-modulus

mode is limited to 200 MHz. There are only two cases where

this frequency limitation limits the flexibility of that N divider:

N = 7 and N = 11. In these two cases, the maximum frequency

into the prescaler is 300 MHz and is achieved by using the P = 1

FD mode. In all other cases, the user can achieve the desired N

divider value by using the other prescaler modes.

A and B Counters

The B counter must be ≥3 or bypassed, and, unlike the R counter,

A = 0 is actually zero.

When the prescaler is in dual-modulus mode, the A counter

must be less than the B counter.

The maximum input frequency to the A/B counter is reflected

in the maximum prescaler output frequency (~300 MHz) that is

specified in Table 2. This is the prescaler input frequency (VCO or

CLK) divided by P. For example, a dual modulus mode of P = 8/9

is not allowed if the VCO frequency is greater than 2400 MHz

because the frequency going to the A/B counter is too high.

When the AD9518 B counter is bypassed (B = 1), the A counter

should be set to 0, and the overall resulting divide is equal to the

prescaler setting, P. The possible divide ratios in this mode are

1, 2, 3, 4, 8, 16, and 32. This mode is useful only when an

external VCO/VCXO is used because the frequency range of the

internal VCO requires an overall feedback divider greater than 32.

Although manual reset is not normally required, the A/B counters

have their own reset bit. Alternatively, the A and B counters can be

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

that these reset bits are not self-clearing.

R, A, and B Counters—SYNC Pin Reset

The R, A, and B counters can also be reset simultaneously through

the SYNC pin. This function is controlled by Register 0x019[7:6]

(see Table 44). The SYNC pin reset is disabled by default.

R and N Divider Delays

Both the R and N dividers feature a programmable delay cell.

These delays can be enabled to allow adjustment of the phase

relationship between the PLL reference clock and the VCO or

CLK. Each delay is controlled by three bits. The total delay

range is about 1 ns. See Register 0x019 in Table 44.

Table 27. Using a 10 MHz Reference Input to Generate Different VCO Frequencies

fREF

(MHz)

fVCO

(MHz)

Mode

Comments/Conditions

P = 1, B = 1 (A and B counters are bypassed).

P = 2, B = 1 (A and B counters are bypassed).

A counter is bypassed.

Maximum frequency into prescaler in P = 2/3 mode is 200 MHz.

If N = 7 or N = 11 is desired for prescaler input frequency of 200 MHz

to 300 MHz, use P = 1, and N = 7 or 11, respectively.

150

1500

151

1510

151

1510

200

2000

270

2700

P = 8 is not allowed (2700 ÷ 8 > 300 MHz).

P = 32 is not allowed (A > B is not allowed).

2710

P = 32, A = 22, B = 84.

P = 16 is also permitted.

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