LMH1981

SNLS214H – APRIL 2006 – REVISED MARCH 2013

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Input Termination

The video source should be load terminated with a 75

Ω 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 VPP signal amplitude for normal terminated video.

Input Coupling Capacitor

The input signal should be AC coupled to the VIN (pin 4) of the LMH1981 with a properly chosen coupling

capacitor, CIN.

The primary consideration in choosing CIN 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 CIN 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 CIN 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 CIN 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 CIN 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, CIN 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, CIN, recovers a quiescent DC voltage via the dynamic clamp current. Because

CIN 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 CIN 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 CIN, so doubling CIN will also double the settling time.

The value of CIN 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. CIN

values to use as a guideline. Refer to Horizontal Sync Output for more about jitter performance.

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