THS7376IPW datasheet(43/61 Pages) TI

43 / 61 page

THS7376

www.ti.com

SBOS692 – JUNE 2013

When the ac STC operation is used, there must also be some finite amount of discharge bias current. As

previously described, if the input signal goes below the 0-V clamp level, the internal loop of the THS7376

sources current to increase the voltage appearing at the input pin. As the difference between the signal level and

the 0-V reference level increases, the amount of source current increases proportionally, thus supplying up to 6

mA of current. Therefore, the time to re-establish the proper STC voltage can be very fast. If the difference is

very small, then the source current is also very small to account for minor voltage droop.

However, what happens if the input signal goes above the 0-V input level? The problem is that the video signal is

always above this level and must not be altered in any way. Thus, if the sync level of the input signal is above

this 0-V level, then the internal discharge (sink) current reduces the ac-coupled bias signal to the proper 0-V

level.

This discharge current must not be large enough to alter the video signal appreciably or picture quality issues

may arise. This effect is often seen by comparing the tilt (droop) of a constant luma signal being applied and the

resulting output level. The associated change in luma level from the beginning and end of the video line is the

amount of line tilt (droop).

If the discharge current is very small, the amount of tilt is very low, which is a generally a good thing. However,

the amount of time for the system to capture the sync signal could be too long. This effect is also termed hum

rejection. Hum arises from the ac line voltage frequency of 50 Hz or 60 Hz. The value of the discharge current

and the ac-coupling capacitor combine to dictate the hum rejection and the amount of line tilt.

To allow for both dc- and ac-coupling in the same device, the THS7376 incorporates an 800-k

Ω resistor to

ground. Although a true constant-current sink is preferred over a resistor, there can be issues when the voltage

is near ground. This configuration can cause the current sink transistor to saturate and cause potential problems

with the signal. The 800-k

Ω resistor is large enough to not impact a dc-coupled DAC termination. For discharging

an ac-coupled source, Ohm’s Law is used. If the video signal is 1 V, then there is (1 V / 800 k

Ω = 1.25-μA) of

discharge current. If more hum rejection is desired or there is a loss of sync occurring, then simply decrease the

0.1-

μF input coupling capacitor. A decrease from 0.1 μF to 0.047 μF increases the hum rejection by a factor of

2.1. Alternatively, an external pull-down resistor to ground may be added that decreases the overall resistance

and ultimately increases the discharge current.

To ensure proper stability of the ac STC control loop, the source impedance must be less than 1 k

Ω with the

input capacitor in place. Otherwise, there is a possibility of the control loop ringing, which may appear on the

device output. Because most DACs or encoders use resistors (typically less than 300

Ω) to establish the voltage,

meeting the less than 1-k

Ω requirement is easily done. However, if the source impedance is very high, then

simply adding a 1-k

Ω resistor to GND ensures proper device operation.

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