OPA2673 datasheet(22/40 Pages) TI

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APPLICATION INFORMATION

WIDEBAND VIDEO MULTIPLEXING

WIDEBAND CURRENT-FEEDBACK

2kW

82.5W

Cable

RG-59

82.5W

75W

475W

402W

Video1

+5V

-5V

1/2

OPA2673

1/2

OPA2673

2kW

75W

475W

402W

Video2

-5V

+5V

DIS

Power-supply

decouplingnotshown.

DIS

1/2

OPA2673

+6V

-6V

Load

50W

Source

133W

402W

6.8 F

0.1 F

6.8 F

0.1 F

-VS

OPA2673

SBOS382A – JUNE 2008 – REVISED OCTOBER 2008..................................................................................................................................................... www.ti.com

OPERATION

One common application for video speed amplifiers

that include a disable pin is to wire multiple amplifier

The OPA2673 gives the exceptional ac performance

outputs together, then select one of several possible

of a wideband current-feedback op amp with a highly

video inputs to source onto a single line. This simple

linear, high-power output stage. Requiring 16mA/ch

wired-OR

video

multiplexer

can

easily

quiescent current, the OPA2673 swings to within 1.1V

implemented using the OPA2673, as Figure 77

of either supply rail and delivers in excess of 460mA

illustrates.

at room temperature. This low output headroom

requirement, along with supply voltage independent

biasing,

gives

remarkable

dual

(±6V)

supply

operation.

The

OPA2673

delivers

greater

than

450MHz bandwidth driving a 2VPP output into 100Ω

on a single +12V supply. Previous boosted output

stage amplifiers typically suffer from very poor

crossover distortion as the output current goes

through zero. The OPA2673 achieves a comparable

power gain with much better linearity. The primary

advantage of a current-feedback op amp over a

voltage-feedback op amp is that ac performance

(bandwidth and distortion) is relatively independent of

signal gain. Figure 76 shows the dc-coupled, gain of

+4V/V, dual power-supply circuit configuration used

as the basis of the ±6V Electrical Characteristics and

Typical Characteristics. For test purposes, the input

impedance is set to 50

Ω with a resistor to ground and

the output impedance is set to 50

Ω with a series

output resistor. Voltage swings reported in the

Figure 77. Two-Channel Video Multiplexer

Electrical Characteristics are taken directly at the

input and output pins, while load powers (dBm) are

Typically, channel switching is performed either on

defined at a matched 50

Ω load. For the circuit of

sync or retrace time in the video signal. The two

Figure 76, the total effective load is 100

Ω || 402Ω =

inputs are approximately equal at this time. The

Ω.

make-before-break

disable

characteristic

the

OPA2673 ensures that there is always one amplifier

controlling the line when using a wired-OR circuit

similar to that shown in Figure 77. Because both

inputs may be on for a short period during the

transition

between

channels,

the

outputs

are

combined through the output impedance matching

resistors (82.5

Ω in this case). When one channel is

disabled, its feedback network forms part of the

output impedance and slightly attenuates the signal in

getting out onto the cable. The gain and output

matching resistors have been slightly increased to get

a signal gain of +1V/V at the matched load and

provide a 75

Ω output impedance to the cable. The

video multiplexer connection (as shown in Figure 77)

also ensures that the maximum differential voltage

across the inputs of the unselected channel do not

exceed the rated ±1.2V maximum for standard video

signal levels. The active-off line circuitry integrated

Figure 76. DC-Coupled, G = +4V/V, Bipolar

within the OPA2673 ensures that the off-channel will

Supply, Specification and Test Circuit

stay off independently of the signal amplitude present

at the output.

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