THS7364IPW datasheet(29/47 Pages) TI

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Level

Shift

Internal

Circuitry

800kW

Input

THS7364

www.ti.com

SBOS530 – AUGUST 2010

R’G’B’ (commonly mislabeled RGB) is also called

Other AGC circuits use the chroma burst amplitude

G’B’R’ (again commonly mislabeled as GBR) in

for amplitude control; reduction in the sync signals

professional video systems. The Society of Motion

does not alter the proper gain setting. However, it is

Picture

and

Television

Engineers

(SMPTE)

good engineering design practice to ensure that

component

standard

stipulates

that

the

luma

saturation/clipping does not take place. Transistors

information is placed on the first channel, the blue

always take a finite amount of time to come out of

color difference is placed on the second channel, and

saturation. This saturation could possibly result in

the red color difference signal is placed on the third

timing delays or other aberrations on the signals.

channel. This practice is consistent with the Y'/P'B/P'R

To eliminate saturation or clipping problems, the

nomenclature. Because the luma channel (Y') carries

THS7364 has a 150-mV input level shift feature. This

the sync information and the green channel (G') also

feature takes the input voltage and adds an internal

carries the sync information, it makes logical sense

+150-mV shift to the signal. Because the THS7364

that G' be placed first in the system. Because the

also has a gain of 6 dB (2 V/V), the resulting output

blue color difference channel (P'B) is next and the red

with a 0-V applied input signal is approximately 300

color difference channel (P'R) is last, then it also

mV. The THS7364 rail-to-rail output stage can create

makes logical sense to place the B' signal on the

this output level while connected to a typical video

second channel and the R' signal on the third

load. This configuration ensures that no saturation or

channel, respectfully. Thus, hardware compatibility is

clipping of the sync signals occur. This shift is

better achieved when using G'B'R' rather than R'G'B'.

constant, regardless of the input signal. For example,

Note that for many G'B'R' systems, sync is embedded

if a 1-V input is applied, the output is 2.3 V.

on all three channels, but this configuration may not

always be the case in all systems.

Because the internal gain is fixed at +6 dB, the gain

dictates what the allowable linear input voltage range

INPUT MODE OF OPERATION: DC

can be without clipping concerns. For example, if the

power supply is set to 3 V, the maximum output is

The inputs to the THS7364 allow for both ac- and

approximately 2.9 V while driving a significant amount

dc-coupled inputs. Many DACs or video encoders can

of current. Thus, to avoid clipping, the allowable input

be dc-connected to the THS7364. One of the

is ([2.9 V/2] – 0.15 V) = 1.3 V. This range is valid for

drawbacks to dc-coupling arises when 0 V is applied

up to the maximum recommended 5-V power supply

to the input. Although the input of the THS7364

that allows approximately a ([4.9 V/2] – 0.15 V) = 2.3

allows for a 0-V input signal without issue, the output

V input range while avoiding clipping on the output.

swing of a traditional amplifier cannot yield a 0-V

signal, resulting in possible clipping. This limitation is

The input impedance of the THS7364 in this mode of

true for any single-supply amplifier because of the

operation

dictated

the

internal,

800-k

Ω

characteristics of the output transistors. Neither

pull-down resistor, as shown in Figure 78. Note that

CMOS nor bipolar transistors can achieve 0 V while

the internal voltage shift does not appear at the input

sinking current. This transistor characteristic is also

pin; it only shows at the output pin.

the same reason why the highest output voltage is

always less than the power-supply voltage when

sourcing current.

This output clipping can reduce the sync amplitudes

(both horizontal and vertical sync) on the video

signal. A problem occurs if the video signal receiver

uses an automatic gain control (AGC) loop to account

for losses in the transmission line. Some video AGC

circuits

derive

gain

from

the

horizontal

sync

amplitude. If clipping occurs on the sync amplitude,

then the AGC circuit can increase the gain too

much—resulting in too much luma and/or chroma

amplitude gain correction. This correction may result

in a picture with an overly bright display with too

Figure 78. Equivalent DC Input Mode Circuit

much color saturation.

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