GS1881_09 Datasheet, PDF (23/29 Pages) Gennum Corporation – Monolithic Video Sync Separators

As the source resistance rises, the video input sync tip starts to be clipped due to the
clamping current during the sync. This clamping current is relatively large due to the
non-symmetric duty cycle of video. To a good approximation the amount of sync clamp
current can be calculated as follows:
( I_CLAMPAVG) (T_SYNC) = ( I_DIS) (T_LINE - T_SYNC)
I_CLAMPAVG(4.7 μs) = (11μA) (63.5μs - 4.7μs)
... I_CLAMPAVG = 137.6μA
This clamp current flows in the source resistance causing a voltage drop equal to:
V_CLIP = ( I_CLAMPAVG) (R_S)
= (137.6μ) (R_S)
VIDEO
INPUT
I _CLAMP
R_S
-+
V_CLIP
75Ω
C_C
C_ƒ
8
2
6
4
680k _0.1μ
Figure 5-4: Simple Chrominance Filtering
Figure 5-5 shows the amount of sync clipping for a 560Ω source resistor. A graph of
V_CLIP versus R_S is shown in Figure 5-6, and Figure 5-7 shows the corresponding
capacitor value for a particular series resistor to provide a corner frequency of 500kHz.
In applications where signal levels are small the amount of attenuation should be
minimized. It follows from Figure 5-6 and Figure 5-7 that in order to minimize
attenuation a small series resistor and a larger capacitor to ground should be chosen.
This however, increases the capacitive loading of the signal source.
CH1
CH2
VIDEO
2
560W 0.1μF
75W
4
8
6
680k _0.1μ
Figure 5-5: Test Circuit 3 and Sync Clipping for a 650Ω Source Resistor
GS1881, GS4881, GS4981 Monolithic Video Sync
Separators
Data Sheet
6926 - 5
November 2009
23 of 29
Proprietary & Confidential