THS7360 datasheet(24/43 Pages) TI

24 / 43 page

THS7360

SLOS674 – JUNE 2010

www.ti.com

Note that the Y’ term is used for the luma channels

INPUT MODE OF OPERATION: DC

throughout this document rather than the more

The inputs to the THS7360 allow for both ac- and

common luminance (Y) term. This usage accounts for

dc-coupled inputs. Many DACs or video encoders can

the definition of luminance as stipulated by the

be dc-connected to the THS7360. One of the

International Commission on Illumination (CIE). Video

drawbacks to dc-coupling is when 0 V is applied to

departs from true luminance because a nonlinear

the input. Although the input of the THS7360 allows

term, gamma, is added to the true RGB signals to

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

form R’G’B’ signals. These R’G’B’ signals are then

of a traditional amplifier cannot yield a 0-V signal,

used to mathematically create luma (Y’). Thus,

resulting in possible clipping. This limitation is true for

luminance

(Y)

not

maintained,

providing

any

single-supply

amplifier

because

the

difference in terminology.

characteristics of the output transistors. Neither

This rationale is also used for the chroma (C’) term.

CMOS nor bipolar transistors can achieve 0 V while

Chroma is derived from the nonlinear R’G’B’ terms

sinking current. This transistor characteristic is also

and, thus, it is nonlinear. Chominance (C) is derived

the same reason why the highest output voltage is

from linear RGB, giving the difference between

always less than the power-supply voltage when

chroma

(C’)

and

chrominance

(C).

The

color

sourcing current.

difference signals (P’B/P’R/U’/V’) are also referenced

This output clipping can reduce the sync amplitudes

in this manner to denote the nonlinear (gamma

(both horizontal and vertical sync) on the video

corrected) signals.

signal. A problem occurs if the video signal receiver

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

uses an automatic gain control (AGC) loop to account

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

for losses in the transmission line. Some video AGC

professional video systems. The Society of Motion

circuits

derive

gain

from

the

horizontal

sync

Picture

and

Television

Engineers

(SMPTE)

amplitude. If clipping occurs on the sync amplitude,

component

standard

stipulates

that

the

luma

then the AGC circuit can increase the gain too

information is placed on the first channel, the blue

much—resulting in too much luma and/or chroma

color difference is placed on the second channel, and

amplitude gain correction. This correction may result

the red color difference signal is placed on the third

in a picture with an overly bright display with too

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

much color saturation.

nomenclature. Because the luma channel (Y') carries

Other AGC circuits use the chroma burst amplitude

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

for amplitude control; reduction in the sync signals

carries the sync information, it makes logical sense

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

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

good engineering design practice to ensure that

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

saturation/clipping does not take place. Transistors

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

always take a finite amount of time to come out of

makes logical sense to place the B' signal on the

saturation. This saturation could possibly result in

second channel and the R' signal on the third

timing delays or other aberrations on the signals.

channel, respectfully. Thus, hardware compatibility is

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

To eliminate saturation or clipping problems, the

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

THS7360 has an input level shift feature. The

on all three channels, but this configuration may not

resulting output with a 0-V applied input signal is

always be the case in all systems.

approximately 120 mV. The THS7360 rail-to-rail

output stage can create this output level while

connected to a typical video load. This configuration

ensures that no saturation or clipping of the sync

signals occur. This shift is constant, regardless of the

input signal. For example, if a 0.1-V input is applied

to the SD channel, the output is 0.1V X 5.6 V/V +

0.12V = 0.68 V.

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