AN2472

Video switch parameters

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1

Video switch parameters

1.1

The on-resistance of the switch determines the propagation delay as well as the losses

suffered by the incoming signal. The higher on-resistance of the switch increases the

insertion loss making the use of a buffer/gain-stage inevitable. Since the analog voltage

level for most video signals lies between 0 V and 1 V, the switch must provide a minimum

increase the propagation delay. Thus it is desirable to have the on-resistance of the video

switch only in the range of a few ohms. It is worth mentioning here that to achieve a lower

an important consideration in the design of an analog video switch.

If the on-resistance of the switch is higher, the need to use an amplifier is larger as there is a

higher voltage drop across the switch.

1.2

Bandwidth

The bandwidth of the video switch is an important parameter as it determines the signal

quality at the output. The higher bandwidth of the switch allows the signal at the input of the

switch to be reproduced at its output with minimum distortion on the edges and the

amplitude. The amplitude distortion is due to the losses through the switch, parasitic

resistances, and capacitances while the edge distortion comes mainly from the capacitance.

The high bandwidth of the switch maintains the high fidelity of the analog video signal.

The higher the bandwidth in the system, the higher is the detail in the video signal. The

highest frequency of the video signal depends on the rise/fall time of the signal. The

bandwidth of a video signal is a complex function depending on several factors like the

aspect ratio, number of vertical scan lines, frame rate or refresh rate and the ratio of total

horizontal/vertical pixels to active ones. The circuit that processes the video signal needs to

have more bandwidth than the actual bandwidth of the processed signal to minimize the

degradation of the signal and the resulting loss in picture quality. The amount of circuit

bandwidth needs to exceed the highest frequency in the signal to reproduce a high-quality

signal. Depending upon the attenuation of the signal at the output, the circuit bandwidth has

to be 3-6 times higher than the maximum frequency in the video signal. In addition to the

bandwidth, the circuit must slew fast enough to faithfully reproduce the video signal.

1.3

Cross-talk and off-isolation

It is seen during the crosstalk measurement that the termination on other ports can

significantly affect the crosstalk measured value on a port. When the unused ports are un-

terminated (left open) the value of the crosstalk measured is worse than when the unused

ports are terminated with proper 75 Ohm loads. Thus it is necessary to terminate the

unused ports with proper loads for an accurate crosstalk measurement (similar to a real

application environment). This also applies to the off-isolation parameter. The higher the off-

isolation value, the better the switch separates the active data from the non-active display

terminals.