1. Operation of Sync-tip-clamp
Input circuit will be explained. Sync-tip clamp circuit (below the clamp circuit) operates to keep a sync tip of the minimum
potential of the video signal. Clamp circuit is a circuit of the capacitor charging and discharging of the external input Cin. It is
charged to the capacitor to the external input Cin at sync tip of the video signal. Therefore, the potential of the sync tip is fixed.
And it is discharged charge by capacitor Cin at period other than the video signal sync tip. This is due to a small discharge
current to the IC.
In this way, this clamp circuit is fixed sync tip of video signal to a constant potential from charging of Cin and discharging of
Cin at every one horizontal period of the video signal.
The minute current be discharged an electrical charge from the input capacitor at the period other than the sync tip of video
signals. Decrease of voltage on discharge is dependent on the size of the input capacitor Cin.
If you decrease the value of the input capacitor, will cause distortion, called the H sag. Therefore, the input capacitor
recommend on more than 0.1uF.
< Clamp circuit >
A. Cin is large
B. Cin is small (H sag experience)
charge period charge period
< Waveform of input terminal >
2. Input impedance
The input impedance of the clamp circuit is different at the capacitor discharge period and the charge period.
The input impedance of the charging period is a few kΩ. On the other hand, the input impedance of the discharge period is
several MΩ. Because is a small discharge-current through to the IC.
Thus the input impedance will vary depending on the operating state of the clamp circuit.
3. Impedance of signal source
Source impedance to the input terminal, please lower than 200Ω. A high source impedance, the signal may be distorted. If
so, please to connect a buffer for impedance conversion.