THS7353_15 Datasheet, PDF (25/57 Pages) Texas Instruments – 3-Channel Low Power Video Buffer with I2C Control, Selectable Filters, External Gain Control, 2:1 Input MUX, and Selectable Input Modes

THS7353
www.ti.com
SLOS484B – NOVEMBER 2005 – REVISED AUGUST 2012
INPUT MODES OF OPERATION – AC SYNC TIP CLAMP
The last input mode of operation is the ac with sync-tip-clamp (STC) which also requires a capacitor in series
with the input. Note that while the term sync-tip-clamp is used throughout this document, the THS7353 is better
termed as a dc restoration circuit based on the way this function is performed. This circuit is an active clamp
circuit and not a passive diode clamp function. This function should be used when ac coupling is desired with
signals that have sync signals embedded such as CVBS, Y’, and G’ signals.
The input to the THS7353 has an internal control loop which sets the lowest input applied voltage to clamp at
approximately 250 mV. If the input signal tries to go below the 250-mV level, the internal control loop of the
THS7353 sources up to 2 mA of current to increase the input voltage level on the THS7353 input side of the
coupling capacitor. As soon as the voltage goes above the 250-mV level, the loop stops sourcing current.
One of the concerns about the sync-tip-clamp level is how the clamp reacts to a sync edge that has overshoot –
common in VCR signals or reflections found in poor PCB layouts or poor cables. Ideally the STC should not react
to the overshoot voltage of the input signal. Otherwise, this could result in clipping on the rest of the video signal
because there may be too much increase of the bias voltage.
To help minimize this input signal overshoot problem, the patent-pending internal STC control loop in the
THS7353 has an I²C selectable low-pass filter as shown in Figure 63. This filter can be selected to be about
500 kHz, 2.5 MHz, or 5 MHz. The 500-kHz filter is useful when the THS7353’s 5^th-order low pass filter is selected
for 9-MHz operation. The effect of this filter is to slow down the response of the control loop so as not to clamp
on the input overshoot voltage, but rather the flat portion of the sync signal when the ringing should be settled
out. The 2.5-MHz filter is best suited for use in conjunction with the 16-MHz signal LPF to account for the faster
sync times associated with the higher rate video signals. For HDTV signals, the 5-MHz STC filter should be
selected to allow for the faster sync rates to properly set the clamp level. Any STC filter can be selected by the
user regardless of the signal or system filter.
As a result of this selectable delay, the sync has an apparent voltage shift occurring between 150 ns and 2 μs
after the sync falling edge – depending on the STC LPF. The amount of shift is dependant upon the amount of
droop in the signal as dictated by the input capacitor and the STC input bias current selection. Because the sync
is primarily for timing purposes with syncing occurring on the edge of the sync signal, this shift is transparent in
most systems. Note that if the source signal is known to be good, selecting the 5-MHz STC LPF is recommended
for all sources.
While this feature may not fully eliminate overshoot issues on the input signal in case of really bad overshoot
and/or ringing, the STC system should help minimize improper clamping levels. As an additional method to help
minimize this issue, an external capacitor (example: 10 pF to 47 pF) to ground in parallel with the external
termination resistors can help filter overshoot problems.
It should be noted that this STC system is dynamic and does not rely upon timing in any way. It only depends on
the voltage appearing at the input pin at any given point in time. The STC filtering helps minimize level shift
problems associated with switching noises or very short spikes on the signal line. This helps ensure a robust
STC system.
Input
Input
Pin
0.1 mF
V_S+ V_S+
250 mV _STC
LPF
Comparator
1.6 mA STC
5.6 mA Bias
7.6 mA ^Select
Internal
Circuitry
Figure 63. Equivalent AC Sync Tip Clamp Input Mode Circuit
Copyright © 2005–2012, Texas Instruments Incorporated
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