THS7360 datasheet(26/43 Pages) TI

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Level

Shift

Internal

Circuitry

+3.3V

800kW

Input

0.1 F

800kW

800k

THS7360

SLOS674 – JUNE 2010

www.ti.com

It should be noted that this STC system is dynamic

there is a loss of sync occurring, then simply

and does not rely upon timing in any way. It only

decrease the 0.1-mF input coupling capacitor. A

depends on the voltage that appears at the input pin

decrease from 0.1 mF to 0.047 mF increases the hum

at any given point in time. The STC filtering helps

rejection by a factor of 2.1. Alternatively, an external

minimize

level

shift

problems

associated

with

pull-down resistor to ground may be added that

switching noises or very short spikes on the signal

decreases the overall resistance and ultimately

line. This architecture helps ensure a very robust

increases the discharge current.

STC system.

To ensure proper stability of the ac STC control loop,

When the ac STC operation is used, there must also

the source impedance must be less than 1-k

Ω with

be some finite amount of discharge bias current. As

the input capacitor in place. Otherwise, there is a

previously described, if the input signal goes below

possibility of the control loop ringing, which may

the 0-V clamp level, the internal loop of the THS7360

appear on the output of the THS7360. Because most

sources current to increase the voltage appearing at

DACs or encoders use resistors to establish the

the input pin. As the difference between the signal

voltage, which are typically less than 300-

Ω, meeting

level and the 0-V reference level increases, the

the less than 1-k

Ω requirement is easily done.

amount

source

current

increases

However, if the source impedance looking from the

proportionally—supplying up to 6 mA of current.

THS7360 input perspective is very high, then simply

Thus, the time to re-establish the proper STC voltage

adding a 1-k

Ω resistor to GND ensures proper

can be very fast. If the difference is very small, then

operation of the THS7360.

the source current is also very small to account for

minor voltage droop.

INPUT MODE OF OPERATION: AC BIAS

However, what happens if the input signal goes

Sync-tip clamps work very well for signals that have

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

horizontal and/or vertical syncs associated with them;

video signal is always above this level and must not

however, some video signals do not have a sync

be altered in any way. As a result, if the sync level of

embedded within the signal. If ac-coupling of these

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

signals is desired, then a dc bias is required to

internal

discharge

(sink)

current

reduces

the

properly set the dc operating point within the

ac-coupled bias signal to the proper 0-V level.

THS7360. This function is easily accomplished with

the THS7360 by simply adding an external pull-up

This discharge current must not be large enough to

resistor to the positive power supply, as shown in

alter the video signal appreciably or picture quality

Figure 46.

issues may arise. This effect is often seen by looking

at 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.

Figure 46. AC-Bias Input Mode Circuit

To allow for both dc- and ac-coupling in the same

Configuration

part, the THS7360 incorporates an 800-k

Ω resistor to

ground. Although a true constant current sink is

The dc voltage appearing at the input pin is equal to

preferred over a resistor, there can be issues when

Equation 1:

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

(1)

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

0.5 V, then there is 0.5 V/800 k

Ω = 0.625-mA of

discharge current. If more hum rejection is desired or

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