THS7364IPWR datasheet(31/47 Pages) TI

31 / 47 page

Level

Shift

Internal

Circuitry

+3.3V

800kW

Input

0.1 F

800kW

800k

THS7364

www.ti.com

SBOS530 – AUGUST 2010

This discharge current must not be large enough to

properly set the dc operating point within the

alter the video signal appreciably or picture quality

THS7364. This function is easily accomplished with

issues may arise. This effect is often seen by looking

the THS7364 by simply adding an external pull-up

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

resistor to the positive power supply, as shown in

applied and the resulting output level. The associated

Figure 80.

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

part, the THS7364 incorporates an 800-k

Ω resistor to

Figure 80. AC-Bias Input Mode Circuit

ground. Although a true constant current sink is

Configuration

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

The dc voltage appearing at the input pin is equal to

cause potential problems with the signal. The 800-k

Ω

Equation 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 1 V,

(1)

then there is 1 V/800 k

Ω = 1.25-mA of discharge

The THS7364 allowable input range is approximately

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

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

loss of sync occurring, then simply decrease the

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

0.1-mF input coupling capacitor. A decrease from

flexible, with the output dc bias point being the

0.1 mF to 0.047 mF increases the hum rejection by a

primary factor. For example, if the output dc bias

factor of 2.1. Alternatively, an external pull-down

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

resistor to ground may be added that decreases the

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

overall

resistance

and

ultimately

increases

the

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

discharge current.

approximately 3.3 M

Ω, resulting in 0.644 V. If the

To ensure proper stability of the ac STC control loop,

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

the source impedance must be less than 1-k

Ω with

power supply, then the pull-up resistor calculates to

the input capacitor in place. Otherwise, there is a

approximately 5.36 M

Ω.

possibility of the control loop ringing, which may

Keep in mind that the internal 800-k

Ω resistor has

appear on the output of the THS7364. Because most

approximately

±20%

variance.

such,

the

DACs or encoders use resistors to establish the

calculations should take this variance into account.

voltage, which are typically less than 300-

Ω, meeting

For the 0.644-V example above, using an ideal

the less than 1-k

Ω requirement is easily done.

3.3-M

Ω

resistor,

the

input

bias

voltage

However, if the source impedance looking from the

approximately 0.644 V ± 0.1 V.

THS7364 input perspective is very high, then simply

adding a 1-k

Ω resistor to GND ensures proper

The value of the output bias voltage is very flexible

operation of the THS7364.

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

ensure that the signal does not clip or saturate the

INPUT MODE OF OPERATION: AC BIAS

video signal. Thus, it is recommended to ensure the

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

Sync-tip clamps work very well for signals that have

For 100% color saturated CVBS or signals with

horizontal and/or vertical syncs associated with them;

Macrovision®, the CVBS signal can reach up to

however, some video signals do not have a sync

1.23 VPP at the input, or 2.46 VPP at the output of the

embedded within the signal. If ac-coupling of these

THS7364. In contrast, other signals are typically

signals is desired, then a dc bias is required to

1 VPP or 0.7 VPP at the input which translate to an

output voltage of 2 VPP or 1.4 VPP. The output bias

voltage must account for a worst-case situation,

depending on the signals involved.

Submit Documentation Feedback

Product Folder Link(s): THS7364

THS7364IPWR Datasheet(PDF) 31 Page - Texas Instruments

THS7364IPWR Datasheet(HTML) 31 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Datasheet Download

Link URL