AD9545 datasheet(116/157 Pages) AD

116 / 157 page

AD9545

Data Sheet

Rev. A | Page 116 of 157

As such, it takes a minimum of N samples for the processor to

compute the desired average, after which it continues to update

the average one sample at a time. The skew measurement

processor provides the user with an indication that it has

captured the first block of N samples by setting the skew offset

full window bit in Bit D7 of Register 0x3A33 or the skew drift

full window bit in Bit D7 of Register 0x3A3B. These bits are

Logic 0 for the first N − 1 measurements in a sequence and

Logic 1 for the Nth measurement. As such, when the user reads

the skew offset or skew drift bit field, the measurement is fully

averaged if the corresponding skew offset full window or skew

drift full window bit is set. When these bits are cleared, offset or

drift readings captured are not fully averaged but are still

useable. The results, however, exhibit greater variance than a

fully averaged measurement.

Unlike the results from the user time stamp processor, the skew

measurement processor results are in absolute units of picosec-

onds. To access the time skew measurement processor results,

use the skew offset and skew drift bit fields.

The Skew offset bit field comprises a 62-bit signed integer (twos

complement) in Register 0x3A2C to Register 0x3A33. The skew

offset bit field represents time in units of 2−16 picoseconds (a

range of approximately ±35 sec). Skew offset measurements

assume the reference and measurement sources are of nearly the

same frequency. That is, skew is meaningless for two sources

with differing frequency, because the skew constantly drifts in

such a case. The skew measurement processor measures skew

relative to the user assigned skew reference source (the

processor computes a coarse average of the reference period).

The skew measurement processor determines the polarity of the

skew offset measurement by considering a time window

spanning ±½ unit interval (UI), where 1 UI is the measured

period of the reference source. A negative value means time

stamps from the measurement source lead time stamps from

the reference source by as much as ½ UI, whereas a positive

value means time stamps from the measurement source lag

time stamps from the reference source by as much as ½ UI.

Note that jitter can affect the polarity of the skew measurement

result.

Consider a skew offset value of 0x FFFFFF8A67324B5, which is

−31,567,174,475 decimal. Scale this by 2−16 ps to yield

−481,676.8566131591796875 ps. This result implies the

measurement source leads the reference source by

approximately 482 ns.

The skew drift bit field comprises a 62-bit signed integer (twos

complement) in Register 0x3A34 to Register 0x3A3B. The skew

drift bit field has units of picoseconds per unit interval (ps/UI).

That is, the value is relative to the measured period of the

reference source. Skew drift is a measure of the rate at which the

skew offset varies cycle by cycle.

Consider two signals with a nominal frequency of 100 Hz. A

skew drift value of 0x3FFFFFF8A67324B5 is −31,567,174,475

decimal. Scale this value by 2−16 ps/UI to yield

−481,676.8566131591796875 ps/UI. Because the reference

period is 10 ms, this result indicates the measurement source

period is decreasing 482 ns every 10 ms, implying a frequency

offset of −48.2 ppm (−482 ns/10 ms = −48.2 × 10−6).

Two signals with exactly the same frequency have no drift and

must yield a drift measurement of zero. However, if the

frequency difference is too great, it exceeds the limits of the

skew measurement processor, which is 1/16th UI. Upon

reaching this limit, the skew measurement processor sets the

skew limit exceeded bit in Bit D5 of Register 0x300C.

TAGGED SKEW MEASUREMENT TIME STAMPS

The skew measurement processor can operate on tagged time

stamps from the various time stamp sources shown in

Figure 90. When the selected skew reference and/or skew

measurement time stamp source is generating tagged time

stamps, the user can program the skew measurement processor

to only pay attention to the tagged time stamps, while ignoring

untagged time stamps. To enable tagged skew reference time

stamp processing, program the skew reference tags only bit in

Bit D5 of Register 0x2A15 to Logic 1. To enable tagged skew

measurement time stamp processing program the skew measure

tags only bit in Bit D5 of Register 0x2A16 to Logic 1.

AD9545 Datasheet(PDF) 116 Page - Analog Devices

AD9545 Datasheet(HTML) 116 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL