AD9542BCPZ datasheet(49/61 Pages) AD

49 / 61 page

Data Sheet

AD9542

Rev. 0 | Page 49 of 61

Note that this formula ignores other contributors to phase error,

including jitter, frequency offset, and propagation delay variation.

If the user has a prior knowledge of the timing of an external

event, such as the switching of the reference input clock source

via an external mux, rather than using the phase transient step

detector, a more robust solution is to invalidate the associated

reference manually. To do so, force a reference fault condition

via the appropriate operational controls bit field. Using this

method imposes the least impact on the steady state operation of

the device. The only steady state impact is that the validation

timer of the associated reference must be set to a duration that

is longer (with suitable margin) than the duration between the

assertion of the force fault condition and the occurrence of the

external event.

SKEW ADJUSTMENT

Skew adjustment allows the user to associate a fixed phase offset

with a reference input, which, for example, is useful in applications

with redundant GNSS/GPS reference sources. That is, a user

may have two or more GNSS/GPS sources that have identical

frequency but may exhibit a fixed time offset due to a mismatch

between antenna cable lengths.

To activate the skew adjustment feature, program the 24-bit

signed Profile x phase skew bit field (where x is the profile number,

Profile 0 to Profile 7). The default value is zero, which disables

the skew adjustment feature. A nonzero value enables the skew

adjustment feature and denotes the desired time skew in units

of picoseconds per the following equation:

Time Skew = Profile x Phase Skew × 10

−12

As an example, determine the value of the Profile x phase skew

bit field necessary for a time skew of −35 ns. Solving the

previous equation for the Profile x phase skew yields

Profile x Phase Skew = (−35 × 10

−9)/10−12

= −35,000

= 0xFF7748 (hexadecimal)

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