OPA680 datasheet(13/22 Pages) TI1

13 / 22 page

OPA680

Ω

82.5

Ω

Ω Cable

Ω

Load

RG-59

82.5

Ω

402

Ω

340

Ω

Video 1

+5V

–5V

OPA680

Ω

402

Ω

340

Ω

Video 2

–5V

+5V

DIS

DC gain for this circuit is equal to RF. At high frequencies,

the DAC output capacitance will produce a zero in the

noise gain for the OPA680 that may cause peaking in the

closed-loop frequency response. CF is added across RF to

compensate for this noise gain peaking. To achieve a flat

transimpedance frequency response, the pole in the feed-

back network should be set to:

1/2

πR

FCF = √GBP/4πRFCD

which will give a closed-loop transimpedance bandwidth

f–3dB, of approximately:

f–3dB = √GBP/(2πRFCD)

FIGURE 6. Two-Channel Video Multiplexer.

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

OP680 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 OPA680 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 do 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 grounded 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

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

FIGURE 5. DAC Transimpedance Amplifier.

OPA680

High Speed

DAC

O = IO RF

GBP

→ Gain Bandwidth

Product (Hz) for the OPA680

Ω

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