HMC7043LP7FE datasheet(23/44 Pages) AD

23 / 44 page

HMC7043

Data Sheet

Rev. B | Page 22 of 43

Clock Grouping, Skew, and Crosstalk

Although the output channels are logically independent, for

physical reasons, they are first grouped into pairs, called clock

groups. Each clock group shares a reference, an input buffer,

and a SYNC retime flip flop originating from the clock

distribution network.

The second level of grouping is according to the supply pin. Clock

Group 1 (Channel 2 and Channel 3) is on an independent supply,

and the other supply pins are each responsible for two clock groups.

As the output channels are more tightly coupled (by sharing a

clock group or by sharing a supply pin), the skew is minimized.

However, the isolation between those channels suffers.

Table 15 shows the clock grouping by location, and Table 16

show the typical skew and isolation that can be expected and how

it scales with distance between output channels.

Isolation improves as either the aggressor or the affected

frequencies decrease. Nevertheless, for particularly important

clock channels where spurious tones must be minimized, carefully

consider their frequency and channel configurations to isolate

continuously running frequencies onto different supply domains.

Channels configured as pulse generators are normally not an

issue, because they are disabled during normal operation.

Table 15. Supply Pin Clock Grouping by Location

Supply Pin

Location

Clock Group

Channel

VCC2_OUT

Southwest

VCC3_OUT

South

VCC6_OUT

Northeast

VCC7_OUT

Northwest

Table 16. Typical Skew and Isolation vs. Distance

Distance

Typical

Skew (ps)

1 GHz Isolation,

Differential (dB)

Distant Supply Group

±20

90 to 100

Closest Neighbor on

Different Supply Group

±15

Shared Supply

±10

Same Clock Group

±10

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