Datasheet

LMH1980
www.ti.com SNLS263A –JULY 2007–REVISED MARCH 2013
Input Coupling Capacitor
The input signal should be AC coupled to the V
IN
(pin 4) of the LMH1980 with a properly chosen coupling
capacitor, C
IN
.
The primary consideration in choosing C
IN
is whether the LMH1980 will interface with video sources using an
AC-coupled output stage. If AC-coupled video sources are expected in the end-application , then it’s
recommended to choose a small C
IN
value such as 0.01 µF to avoid missing sync output pulses due to average
picture level changes. It’s important to note that video sources with an AC-coupled output will cause video-
dependent jitter at the HSync output of the sync separator. When only DC-coupled video sources are expected, a
larger value for C
IN
may be used without concern for missing sync output pulses. A smaller C
IN
value can be
used to increase rejection of source AC hum components and also reduce start-up time regardless of the video
source's output coupling type.
START-UP TIME
When there is a significant change to the video input signal, such as sudden signal switching in, signal
attenuation (i.e.: load termination added via loop through) or signal gain (i.e.: load termination removed), the
quiescent operation of the LMH1980 will be disrupted. During this dynamic input condition, the LMH1980 outputs
may not be correct but will recover to valid signals after a predictable start-up time, which consists of an
adjustable input settling time and a predetermined “sync lock time”.
Input Settling Time and Coupling Capacitor Selection
Following a significant input condition, the negative sync tip of the AC-coupled signal settles to the input clamp
voltage as the coupling capacitor, C
IN
, recovers a quiescent DC voltage via the dynamic clamp current through
V
IN
. Because C
IN
determines the input settling time, its capacitance value is critical when minimizing overall start-
up time. A smaller C
IN
value yields shorter settling time at the expense of increased line droop voltage, whereas
a larger one yields reduced line droop but longer settling time. Settling time is proportional to the value of C
IN
, so
doubling C
IN
will also double the settling time.
Sync Lock Time
In addition to settling time, the LMH1980 has a predetermined sync lock time, T
SYNC-LOCK
, before the outputs are
correct. Once the AC-coupled input has settled enough, the LMH1980 needs time to detect the valid video signal
and apply fixed-level sync slicing before the output signals are correct.
For practical values of C
IN
, T
SYNC-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.
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 within 1 video field
from when an NTSC signal is just applied to when the LMH1980 outputs are valid.
Figure 12. Typical Start-Up Time for NTSC Input to LMH1980 (C
IN
= 0.1 µF)
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