AD9512BCPZ datasheet(29/48 Pages) AD

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AD9512

Rev. A | Page 29 of 48

Divider Phase Offset

The phase of each output may be selected, depending on the

divide ratio chosen. This is selected by writing the appropriate

values to the registers, which set the phase and start high/low

bit for each output. These are the odd numbered registers from

4Bh to 53h. Each divider has a 4-bit phase offset <3:0> and a

start high or low bit <4>.

Following a sync pulse, the phase offset word determines how

many fast clock (CLK1 or CLK2) cycles to wait before initiating

a clock output edge. The Start H/L bit determines if the divider

output starts low or high. By giving each divider a different

phase offset, output-to-output delays can be set in increments of

the fast clock period, tCLK.

Figure 25 shows three dividers, each set for DIV = 4, 50% duty

cycle. By incrementing the phase offset from 0 to 2, each output

is offset from the initial edge by a multiple of tCLK.

123456

789

11 12

CLOCK INPUT

CLK

DIVIDER OUTPUTS

DIV = 4, DUTY = 50%

START = 0,

PHASE = 0

START = 0,

PHASE = 1

START = 0,

PHASE = 2

tCLK

× t

CLK

Figure 25. Phase Offset—All Dividers Set for DIV = 4, Phase Set from 0 to 2

For example:

CLK1 = 491.52 MHz

tCLK1 = 1/491.52 = 2.0345 ns

For DIV = 4

Phase Offset 0 = 0 ns

Phase Offset 1 = 2.0345 ns

Phase Offset 2 = 4.069 ns

The three outputs may also be described as:

OUT1 = 0°

OUT2 = 90°

OUT3 = 180°

Setting the phase offset to Phase = 4 results in the same relative

phase as the first channel, Phase = 0° or 360°.

In general, by combining the 4-bit phase offset and the Start

H/L bit, there are 32 possible phase offset states (see Table 13).

Table 13. Phase Offset—Start H/L Bit

4Bh to 53h

Phase Offset

(Fast Clock

Rising Edges)

Phase Offset <3:0>

Start H/L <4>

The resolution of the phase offset is set by the fast clock period

(tCLK) at CLK1 or CLK2. As a result, every divide ratio does not

have 32 unique phase offsets available. For any divide ratio, the

number of unique phase offsets is numerically equal to the

divide ratio (see Table 13):

DIV = 4

Unique Phase Offsets Are Phase = 0, 1, 2, 3

DIV= 7

Unique Phase Offsets Are Phase = 0, 1, 2, 3, 4, 5, 6

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