Start- up Time

Settling

Time

Sync Lock Time

Signal

Before CIN

AC Coupled

Signal (VIN)

HSync

VSync

0

2

4

6

8

10

0

500

1000

1500

2000

2500

CIN (PF)

PAL

1080I

LMH1981

www.ti.com

SNLS214H – APRIL 2006 – REVISED MARCH 2013

Figure 11. Typical HSync Jitter vs. CIN Values

Sync Lock Time

In addition to settling time, the LMH1981 has a predetermined sync lock time, TSYNC-LOCK, before the outputs are

correct. Once the AC-coupled input has settled enough, the LMH1981 needs time to detect the valid video signal

and resolve the blanking & sync tip levels for 50% sync slicing before the output signals are correct.

For practical values of CIN, TSYNC-LOCK is typically less than 1 or 2 video fields in duration starting from the 1st

valid VSync output pulse to the valid HSync pulses beginning thereafter. VSync and HSync pulses are

considered valid when they align correctly with the input's vertical and horizontal sync intervals. Note that the

start-up time may vary depending on the video duty cycle, average picture level variations, and start point of

video relative to the vertical sync interval.

It is recommended for the outputs to be applied to the system after the start-up time is satisfied and outputs are

valid. For example, the oscilloscope screenshot in Figure 12 shows a typical start-up time of about 13.5 ms from

when an NTSC signal is switched in (no previous input) to when the LMH1981 outputs are valid.

Figure 12. Typical Start-Up Time for NTSC Input to LMH1981 via 1 µF Coupling Capacitor

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