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Photograph A3 shows the same circuit, with the counter

running at 25MHz. This turns out to be close to the limit of

the TTL counter used in the breadboard, rather than the limit

of the EL4331. Here the different glitches are easily

recognizable - a small glitch for one of the 4 input EL4331s

A/B switching, somewhat larger glitches when two banks

switch together, and the biggest glitch when all three banks

switch. Photograph A4 shows the big glitch in detail. A good

PCB and equal length and matched traces would clean up

these glitches.

Note: No supply bypass capacitors shown. Only one of three

channels shown.

Figure 4 shows one of the three channels of a component

video, 8:1 multiplexer. In this example, the power-down

capability is used to save on EL4331s, but as can be seen,

the control part does become more complicated. Using the

power-down mode for multiplexing does, of course, slow

down the speed with which one can select a given input

channel. However, if input channel selection can be done

during a blanking period, the couple of microseconds that it

takes to power-down one chip may be no problem. Note that

some external logic is needed in this application, both to

select the appropriate amplifier, and also to force a break-

before-make action by pulling the T_OFF line low. All this

logic would best be incorporated inside a PAL or gate array,

and is shown in gate form just to illustrate the idea. Note that

the BIT0 line would have the 3ns response time, since it is

switching the muxamps directly.

8-to-1 Multiplexer using Power-Down

FIGURE 4. A SIMPLE 8:1 COMPONENT VIDEO

MULTIPLEXER

TABLE 2. CHANNEL SELECTION TABLE

BIT2

BIT1

BIT0

OUTPUT

000

SIG0

001

SIG1

010

SIG2

011

SIG3

100

SIG4

101

SIG5

110

SIG6

111

SIG7

EL4331