Datasheet

LMH1981
SNLS214H –APRIL 2006–REVISED MARCH 2013 www.ti.com
Input Termination
The video source should be load terminated with a 75Ω resistor to ensure correct video signal amplitude and
minimize signal distortion due to reflections. In extreme cases, the LMH1981 can handle unterminated or double-
terminated input conditions, assuming 1 V
PP
signal amplitude for normal terminated video.
Input Coupling Capacitor
The input signal should be AC coupled to the V
IN
(pin 4) of the LMH1981 with a properly chosen coupling
capacitor, C
IN
.
The primary consideration in choosing C
IN
is whether the LMH1981 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 as prescribed in the next section. Other
considerations such as HSync jitter performance and start-up time are practically fixed by the limited range of
small C
IN
values. It’s important to note that video sources with AC-coupled outputs will introduce video-
dependent jitter that cannot be remedied by the sync separator; moreover, this type of jitter is not prevalent in
sources with DC-coupled input/output stages.
When only DC-coupled video sources are expected, a larger C
IN
value can be chosen to minimize voltage droop
and thus improve HSync jitter at the expense of increased start-up time as explained in START-UP TIME. A
typical C
IN
value such as 1 µF will give excellent jitter performance and reasonable start-up time using a
broadcast-quality DC-coupled video generator. For applications where low HSync jitter is not critical, C
IN
can be
a small value to reduce start-up time.
START-UP TIME
When there is a significant change to the video input signal, such as sudden signal switching, signal attenuation
(i.e.: additional termination via loop through) or signal gain (i.e.: disconnected end-of-line termination), the
quiescent operation of the LMH1981 will be disrupted. During this dynamic input condition, the LMH1981 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. Because
C
IN
determines the input settling time, its capacitance value is critical when minimizing overall start-up time.
For example, a settling time of 8 ms can be expected for a typical C
IN
value of 1 µF when switching in a standard
NTSC signal with no prior input. A smaller value yields shorter settling time at the expense of increased line
droop voltage and consequently higher HSync jitter, whereas a larger one gives lower jitter but longer settling
time. Settling time is proportional to the value of C
IN
, so doubling C
IN
will also double the settling time.
The value of C
IN
is a tradeoff between start-up time and jitter performance and therefore should be evaluated
based on the application requirements. Figure 11 shows a graph of typical input-referred HSync jitter vs. C
IN
values to use as a guideline. Refer to Horizontal Sync Output for more about jitter performance.
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