Datasheet

7
6
VSOUT
HSOUT
4
5
V
IN
HD
C
5
0.1 PF
R
3
10 k:
R
10
100:
TP1
HD Detect Flag Output
2
1
R
9
100:
C
2
560 pF
C
1
*OPT
Q1
MMBT3904
R
1
75:
BNC
J3
LMH1980
SNLS263A –JULY 2007–REVISED MARCH 2013 www.ti.com
– Composite NTSC & PAL (or Component 480I & 576I) without vertical serration & equalization pulses (i.e.:
from logical OR-ing of H & V signals)
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 LMH1980 can handle non-terminated or
double-terminated input conditions, assuming 1V
PP
signal amplitude for normally terminated video.
Input Filtering
An external filter is recommended if the video signal has large chroma amplitude that extends near the sync tip
and/or has considerable high-frequency noise, so they do not interfere with sync separation. A simple RC low-
pass chroma filter with a series resistor (R
9
) and a filter capacitor (C
2
) to ground can be used to sufficiently
attenuate chroma such that minimum peak of its amplitude is above the slicing level and also to improve the
overall signal-to-noise ratio. To achieve the desired filter cutoff frequency, it’s advised to vary C
2
and keep R
9
small (i.e.: 100Ω) to minimize sync tip clipping due to the voltage drop across R
9
. Keep in mind that as the cutoff
frequency decreases, the LMH1980 output propagation delays increase, which could affect the timing
relationship between the sync and video signals.
In applications where the chroma filter needs to be disabled when HD video is input, it is possible to use a
transistor switch (Q1) controlled by the HD flag (pin 5) to open C
2
’s connection to ground as shown in Figure 11.
When a HD tri-level sync input signal is applied, HD will output logic low (following a brief delay for auto format
detection) and Q1 will turn off to disable the chroma filter, which is intended for SD composite video only. When
a SD bi-level sync signal (i.e.: NTSC/PAL) is applied, HD will output logic high and Q1 will turn on to enable the
chroma filter.
Important: If the filter cutoff frequency (f
CO
) is set too low and HD video is applied, the filter can severely roll off
and attenuate the input's high-bandwidth tri-level sync pulses such that the LMH1980 cannot detect a valid HD
input signal. If the LMH1980 cannot detect a valid HD input, then the HD flag will never change from logic high to
low and the switch-controlled filter will never be disabled via Q1. In other words, f
CO
should not be set too low
that the filter impairs the LMH1980's ability to detect a valid HD input. The values of R
9
and C
2
shown in
Figure 11 give f
CO
=2.79 MHz (about -4 dB at 3.58 MHz NTSC subcarrier frequency) without impairing HD video
format detection.
Figure 11. External Switch-Controlled Chroma Filter
If a PC video input with bi-level sync is to be used, C
2
should be removed to disable chroma filtering. This is
necessary because HD will output logic high (like in the SD video input case) and enable the filter. A chroma
filter could severely band-limit a high-bandwidth PC video signal, which could roll-off and attenuate the sync
pulses such that the LMH1980 cannot detect a valid input signal.
If some high-frequency noise filtering is needed for all video inputs, a small capacitor (C
1
) may be optionally used
in parallel but outside of the transistor switch. When Q1 is turned on, then C
1
and C
2
will be connected in parallel
(C
1
+C
2
)
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