EL4331C
Triple 2:1 Mux-Amp AV = 1
significant output disturbance as one part turns on before
the other has fully turned off.
Figure 1. Two RGB Sources Multiplexed to One RGB Output
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.
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
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.
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ELANTEC [ ELANTEC SEMICONDUCTOR ]
