APPLICATIONS INFORMATION

2:1 HIGH-SPEED VIDEO MULTIPLEXER

LOGO INSERTER

RGB VIDEO INVERTER

75W

75W

75W

+5V

-5V

EN

Channel

Select

1/3OPA3875

x1

x1

V

IN_1

V

OUT

V

IN_2

402W

402W

OPA3875

SBOS341D – DECEMBER 2006 – REVISED AUGUST 2008 ............................................................................................................................................ www.ti.com

that the 75

Ω input matching impedance is set here by

the parallel combination of 92

Ω and 402Ω. In order

not to disturb the sync, color burst, and blanking if

OPERATION

present, the inverting amplifiers are only switched on

The OPA3875 can be used as a triple 2:1 high-speed

during active video.

video multiplexer, as illustrated in the front page

schematic for an RGB signal. Figure 26 shows a

simplified version of the front page schematic in

which one output is shown with its input and output

Figure 28 illustrates the principle of overlaying a

impedance matching resistors.

picture in a picture. The picture comes through U1;

the signal to be overlayed comes through U2. Here

we have a reference voltage of 0.714V in channel 2

indicating that we will highlight a section of the picture

Figure 27 illustrates an extension of the previously

with white (for NTSC-related RGB video). How much

shown RGB switching circuit with a noninverting

white comes through depends on the combination of

signal going through channel 1 and an inverted signal

select 1 and select 2 pins as well as the series output

going

through

channel

two.

Here,

the

output

resistance of each OPA3875. To match the 75

Ω

impedance of the OPA3875 is set to 75

Ω. Looking at

output impedance of the video cable, the parallel

the input part of this circuit, we see that the RGB

combination of the series output resistance (R and

signal is inverted with an OPA3693 fixed gain set in

nR) needs to be 75

Ω. The two select pins gives us 2

an inverting configuration with a reference voltage on

bits of control. By selecting n = 2, you have the

the noninverting node. The reference voltage, set

capability of a 0% highlight (full original video signal),

here at 0.714V, has a gain of 1 at the output of the

33% highlight, 66% highlight, and 100% highlight (all

OPA3691 as the input signal is AC-coupled (not

white). By selecting n = 3, you have 0%, 25%, 75%,

represented here). This bias voltage is required to

and 100% highlight capabilities, etc.

prevent the video from swinging negative. Note also

Figure 26. Triple 2:1 High-Speed Video Multiplexer

10

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Product Folder Link(s): OPA3875