EL4422CS datasheet(13/16 Pages) ELANTEC

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EL4421C22C41C42C43C44C

Multiplexed-Input Video Amplifiers

Applications Information

Contd

Capacitive loads will cause peaking in the fre-

quency response If capacitive loads must be driv-

en a small-valued series resistor can be used to

isolate it 12

X to 51X should suffice A 22X series

resistor will limit peaking to 25 dB with even a

220 pF load

Input Connections

The input transistors can be driven from resistive

and capacitive sources but are capable of oscilla-

tion when presented with an inductive input It

takes about 80 nH of series inductance to make

the inputs actually oscillate equivalent to four

inches of unshielded wiring or about 6

of unter-

minated input transmission line The oscillation

has a characteristic frequency of 500 MHz

Often simply placing one’s finger (via a metal

probe) or an oscilloscope probe on the input will

kill the oscillation Normal high-frequency con-

struction obviates any such problems where the

input source is reasonably close to the mux-amp

input If this is not possible one can insert series

resistors of around 51

X to de-Q the inputs

Feedback Connections

A feedback divider is used to increase circuit

gain and some precautions should be observed

The first is that parasitic capacitance at the

in-

put will add phase lag to the feedback path and

increase frequency response peaking or even

cause oscillation One solution is to choose feed-

back resistors whose parallel value is low The

pole frequency of the feedback network should be

maintained above at least 200 MHz Fora3pF

parasitic this requires that the feedback divider

have less than 265

X impedance equivalent to

two 510

X resistors when a gain of a2 is desired

Alternatively a small capacitor across RF can be

used to create more of a frequency-compensated

divider The value of the capacitor should match

the parasitic capacitance at the

input It is also

practical to place small capacitors across both the

feedback resistors (whose values maintain the de-

sired gain) to swamp out parasitics For instance

two 10 pF capacitors across equal divider resis-

tors will dominate parasitic effects and allow a

higher divider resistance

The other major concern about the divider con-

cerns unselected-channel crosstalk The differen-

tial input impedance of each input stage is

around 200 K

X The unselected input’s signal

sources thus drive current through that input im-

pedance into the feedback divider inducing an

unwanted output The gain from unselected in-

put to output the crosstalk attenuation if RF

RIN In unity-gain connection the feedback resis-

tor is 0

X and very little crosstalk is induced For

a gain of a2 the crosstalk is about

60 dB

Feedthrough Attenuation

The channels have different crosstalk levels with

different inputs Here is the typical attenuation

for all combinations of inputs for the mux-amps

at 358 MHz

Feedthrough of EL4441 and EL4443 at 358 MHz

In1

In2

In3

In4

Inputs

Select

A1A0

Selected

77 dB

90 dB

92 dB

80 dB

Selected

77 dB

90 dB

101 dB

76 dB

Selected

66 dB

96 dB

84 dB

66 dB

Selected

Feedthrough of EL4421 at 358 MHz

In1

In2

Channel Select

Selected

88 dB

Input A0

93 dB

Selected

Switching Glitches

The output of the mux-amps produces a small

‘‘glitch’’ voltage in response to a logic input

change A peak amplitude of only about 90 mV

occurs and the transient settles out in 20 ns The

glitch does not change amplitude with different

gain settings

With the four-input multiplexers when two logic

inputs are simultaneously changed

the glitch

amplitude doubles The increase can be a avoided

by keeping transitions at least 6 ns apart This

can be accomplished by inserting one gate delay

in one of the two logic inputs when they are truly

synchronous

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