THS7368IPW datasheet(24/45 Pages) TI2

24 / 45 page

Level

Shift

Internal

Circuitry

+3.3V

800kW

Input

0.1 F

800kW

800k

THS7368

SBOS497 – DECEMBER 2009

www.ti.com

However, what happens if the input signal goes

INPUT MODE OF OPERATION: AC BIAS

above the 0-V input level? The problem is the video

Sync-tip clamps work very well for signals that have

signal is always above this level and must not be

horizontal and/or vertical syncs associated with them;

altered in any way. Thus, if the sync level of the input

however, some video signals do not have a sync

signal is above this 0-V level, then the internal

embedded within the signal. If ac-coupling of these

discharge (sink) current reduces the ac-coupled bias

signals is desired, then a dc bias is required to

signal to the proper 0-V level.

properly set the dc operating point within the

This discharge current must not be large enough to

THS7368. This function is easily accomplished with

alter the video signal appreciably or picture quality

the THS7368 by simply adding an external pull-up

issues may arise. This effect is often seen by looking

resistor to the positive power supply, as shown in

at the tilt (droop) of a constant luma signal being

Figure 40.

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

Figure 40. AC-Bias Input Mode Circuit

part, the THS7368 incorporates an 800-k

Ω resistor to

Configuration

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

The dc voltage appearing at the input pin is equal to

cause the current sink transistor to saturate and

Equation 1:

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

(1)

source, Ohm’s Law is used. If the video signal is 1 V,

The THS7368 allowable input range is approximately

then there is 1 V/800 k

Ω = 1.25-μA of discharge

0 V to (VS+ – 1.5 V), allowing for a very wide input

current. If more hum rejection is desired or there is a

voltage range. As such, the input dc bias point is very

loss of sync occurring, then simply decrease the

flexible, with the output dc bias point being the

0.1-

μF input coupling capacitor. A decrease from

primary factor. For example, if the output dc bias

0.1

μF to 0.047 μF increases the hum rejection by a

point is desired to be 1.6 V on a 3.3-V supply, then

factor of 2.1. Alternatively, an external pull-down

the input dc bias point should be (1.6 V – 300 mV)/2

resistor to ground may be added that decreases the

= 0.65 V. Thus, the pull-up resistor calculates to

overall

resistance

and

ultimately

increases

the

approximately 3.3 M

Ω, resulting in 0.644 V. If the

discharge current.

output dc-bias point is desired to be 1.6 V with a 5-V

To ensure proper stability of the ac STC control loop,

power supply, then the pull-up resistor calculates to

the source impedance must be less than 1-k

Ω with

approximately 5.36 M

Ω.

the input capacitor in place. Otherwise, there is a

Keep in mind that the internal 800-k

Ω resistor has

possibility of the control loop ringing, which may

approximately

±20%

variance.

such,

the

appear on the output of the THS7368. Because most

calculations should take this variance into account.

DACs or encoders use resistors to establish the

For the 0.644-V example above, using an ideal

voltage, which are typically less than 300-

Ω, meeting

3.3-M

Ω

resistor,

the

input

bias

voltage

the less than 1-k

Ω requirement is easily done.

approximately 0.644 V ± 0.1 V.

However, if the source impedance looking from the

THS7368 input perspective is very high, then simply

The value of the output bias voltage is very flexible

adding a 1-k

Ω resistor to GND ensures proper

and is left to each individual design. It is important to

operation of the THS7368.

ensure that the signal does not clip or saturate the

video signal. Thus, it is recommended to ensure the

output bias voltage is between 0.9 V and (VS+ – 1 V).

For 100% color saturated CVBS or signals with

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