Part Number: LMH1981
Hi all,
My customer is using LMH1981MTX/NOPB as a sync separator in our product and would like to clarify some input-related specifications.
(Background)
We observed the following behavior:
)Jitter (~70 µs on HSOUT) occurs when the video input is not terminated (open condition)
)The issue appears only with tri-level sync formats (1080/25p, 24p, 23.98p)
)The issue does not appear when the input is properly terminated with 75 ohm
)The behavior is reproducible across multiple signal sources
In addition, we found that:
)Adjusting the DC offset voltage before the AC coupling capacitor at VIN eliminates the issue
This suggests that input DC conditions and termination may affect the device operation.
We could not find a clear specification in the datasheet regarding the allowable DC offset range before the coupling capacitor.
Could you clarify:
)Recommended or allowable DC offset range at the input (before AC coupling)
)Any constraints related to proper operation of the sync slicing or clamp circuit
(2) Internal clamp behavior
We would also like to better understand the input clamp:
)What is the typical VIN clamp voltage and its variation?
)Are there any limitations or operating conditions where the clamp may not function correctly?
)How sensitive is the device to DC offset when processing tri-level sync signals?
(3)Operation without proper 75 ohm termination
Since the issue appears only when the input is open:
)Are there any restrictions or limitations when operating without a 75ohm termination?
)Is proper termination required to guarantee correct sync extraction, especially for tri-level sync?
Based on our observations, we suspect that the issue may be related to the interaction between input amplitude, DC offset conditions, and the internal sync slicing mechanism.
Any clarification or design recommendations would be greatly appreciated.
Best Regards,
Ryusuke
