OPA682 datasheet(13/22 Pages) TI1

13 / 22 page

OPA2680

transimpedance frequency response, the pole in each feed-

back network should be set to:

1/2

πR

FCF = √GBP/4πRFCD

which will give a corner frequency f–3dB of approximately:

f–3dB = √GBP/(2πRFCD)

WIDEBAND VIDEO MULTIPLEXING

One common application for video speed amplifiers which

include a disable pin is to wire multiple amplifier outputs

together, then select which one of several possible video

inputs to source onto a single line. This simple “Wired-OR

Video Multiplexer” can be easily implemented using the

OP2680N (SO-14 package only) as shown in Figure 6.

Typically, channel switching is performed either on sync or

retrace time in the video signal. The two inputs are approxi-

mately equal at this time. The “make-before-break” disable

characteristic of the OPA2680N ensures that there is always

one amplifier controlling the line when using a wired-OR

circuit like that shown in Figure 6. Since 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 resistor have

been slightly increased to get a signal gain of +1 at the

matched load and provide a 75

Ω output impedance to the

cable. The video multiplexer connection (Figure 6) also

insures that the maximum differential voltage across the

inputs of the unselected channel does not exceed the rated

±1.2V maximum for standard video signal levels.

The section on Disable Operation shows the turn-on and

turn-off switching glitches using a 0V input for a single

channel is typically less than

±50mV. Where two outputs are

switched (as shown in Figure 6), the output line is always

under the control of one amplifier or the other due to the

“make-before-break” disable timing. In this case, the switch-

ing glitches for two 0V inputs drop to <20mV.

FIGURE 6. Two-Channel Video Multiplexer (SO-14 package only).

FIGURE 5. High Speed DAC—Differential Transimpedance

Amplifier

1/2

OPA2680

1/2

OPA2680

High Speed

DAC

O = IO RF

GBP

→ Gain Bandwidth

Product (Hz) for the OPA2680

Ω

82.5

Ω

Ω Cable

Ω Load

RG-59

82.5

Ω

402

Ω

340

Ω

Video 1

+5V

–5V

1/2

OPA2680

1/2

OPA2680

Ω

402

Ω

340

Ω

Video 2

–5V

+5V

DIS

DISA

DISB

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