10

Expanding the Multiplexer

In Figure 29, a 3:1 multiplexer circuit is shown. The

expansion to more inputs is very straight forward. Since the

EL4332 has a fixed gain of 2, interstage attenuators may be

required as shown in Figure 28. The truth table for the 3:1

multiplexer select lines is:

When interstage attenuators are used, the values should be

kept down in the region of 50

Ω–300Ω. This is to prevent a

combination of circuit board stray capacitance and the

EL4332’s input capacitance forming a significant pole. For

example, if instead of 100

Ω as shown, resistors of 1kΩ had

been used, and assuming 3pF of stray and 3pF of input

capacitance, a pole would be formed at about 53MHz.

TABLE 1.

XY

MUX OUTPUT

0

0

R3, G3, B3

0

1

R2, G2, B2

1

X

R1, G1, B1

FIGURE 29. TYPICAL CONNECTION FOR A 3:1 COMPONENT VIDEO MULTIPLEXER

EL4332