2-BIT, HIGH-SPEED ATTENUATOR

8-TO-1 VIDEO MULTIPLEXER

50W

OPA4872

R

R

R

R

2R

2R

2R

To50

Load

W

523W

523W

IN0

IN1

IN2

IN3

V

IN

A0 A1

SD

EN

V

OUT

Logic

4-INPUT RGB ROUTER

R =69W

O

OPA4872

To75

Load

W

Logic

R = 69W

O

OPA4872

Logic

523W

523W

523W

523W

EN

SD

EN

SD

R =Z ||(

+R +R )

O

F

G

O

O

R

1+

=2

R

F

R

G

(1)

75W

+5V

-5V

OPA4872

To75

Load

W

Logic

R

2

R

1

R

3

R

4

RedOut

A0 A1

523W

523W

EN

SD

R +R

=1046

R =R

W

F

G

F

G

(2)

OPA4872-EP

www.ti.com........................................................................................................................................................................................... SBOS444 – DECEMBER 2008

In

contrast

to

the

PGA,

a

two-bit

high-speed

Two OPA4872s can be used together to form an

attenuator can be implemented by using an R-2R

8-input video multiplexer. The multiplexer is shown in

ladder together with the OPA4872. Figure 29 shows

Figure 31.

such an implementation.

Channel 0 sees the full input signal amplitude, where

as channel 1 sees 1/2 VIN, channel 2 see 1/4 VIN and

channel 3 sees 1/8 VIN.

Figure 29. 2-Bit, High-Speed Attenuator,

500MHz Channel Bandwidth

Three OPA4872s can be used together to form a

four-input RGB router. The router for the red

component is shown in Figure 30. Identical stages

would be used for the green and blue channels.

Figure 31. 8-to-1 Video Multiplexer

When

connecting

OPA4872

outputs

together,

maintain a gain of +1V/V at the load. The OPA4872

configuration shown is a gain of +6 dB; thus, the

matching resistance must be selected to achieve

–6 dB.

The set of equations to solve is shown in Equation 1

and Equation 2. Here, the impedance of interest is

75

Ω.

Figure 30. 4-Input RGB Router

(Red Channel Shown)

Copyright © 2008, Texas Instruments Incorporated

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