9

EL4331C

Triple 2:1 Mux-Amp AV = 1

significant output disturbance as one part turns on before

the other has fully turned off.

Single Supply Operation

Due to the fact that video signals often have negative

sync levels and invariably require ground to be within

the signal swing, running the EL4331 on a single supply

rail compromises many aspects of its performance. It is

difficult to generate a solid, clean, pseudo ground a few

volts away from ground without using more power, and

components than simply providing a negative power

rail. A signal ground has to be capable of handling all the

return currents from all the inputs, as well as the outputs,

from DC to frequencies in excess of 400 MHz. While

this is by no means impossible, a negative rail can be

generated from a standard +5V rail for a couple of dol-

lars and a square inch, or less, of board space. However,

a pseudo ground can be derived with for example an

LM336, to give an “AC ground” 2.5V above 0V. The

logic inputs will need some form of level shifting to

ensure that the logic “1” and “0” specifications can be

met. The pseudo ground must be well bypassed to the

real ground; note that the pseudo ground will have to

sink/source all the current that flows in the internal com-

pensation capacitors during slewing. This can easily be

several milliamps in a few nanoseconds. If the pseudo

ground “moves” because one channel is forcing current

into the derived ground, cross-talk into the other two

channels will become very significant.

Application Circuit #1

Figure 1 shows a very high speed RGB (or YUV) multi-

plexer. Two video sources can be displayed on one

monitor with the only stipulation that the video sources

have to be synchronous. An example is a picture-in-pic-

ture, or “window” is generated with one video source

(e.g. RGB TV) in a window, and a computer application

around it. Multiplexing synchronous RGB signals has

the advantage that the video signals do not have to be

digitized, and an image stored in RAM prior to being

displayed.

When the monitor is switched off, or goes into its

power-saving blanked mode, the EL4331 can be pow-

ered down to further save power. The input impedance

does not change appreciably between powered up and

down modes, although the bias current does drop to near

zero.

Figure 1. Two RGB Sources Multiplexed to One RGB Output