EL4421CN Elantec Semiconductor, Inc. (acquired by Intersil), EL4421CN Datasheet - Page 13

• Manufacturer Part Number: EL4421CN
• Description: Multiplexed-input Video Amplifier
• Manufacturer: Elantec Semiconductor, Inc. (acquired by Intersil)

Applications Information
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
resistor will limit peaking to 2 5 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
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
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 For a 3 pF
parasitic this requires that the feedback divider
have less than 265
two 510
Alternatively a small capacitor across R
used to create more of a frequency-compensated
divider The value of the capacitor should match
the parasitic capacitance at the
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
resistors when a gain of
to 51
should suffice A 22
impedance equivalent to
to de-Q the inputs
b
input It is also
a
2 is desired
EL4421C 22C 41C 42C 43C 44C
Contd
F
can be
series
b
Multiplexed-Input Video Amplifiers
in-
13
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
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 R
R
tor is 0
a gain of
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 3 58 MHz
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
Feedthrough of EL4441 and EL4443 at 3 58 MHz
Inputs
Select
IN
A1A0
Channel Select
Input A0
In unity-gain connection the feedback resis-
Feedthrough of EL4421 at 3 58 MHz
00
01
10
11
and very little crosstalk is induced For
a
2 the crosstalk is about
b
Selected
b
b
101 dB
In1
80 dB
96 dB
The unselected input’s signal
0
1
Selected
b
b
b
In2
77 dB
76 dB
84 dB
Selected
b
In1
93 dB
Selected
b
b
b
In3
90 dB
77 dB
66 dB
b
60 dB
Selected
b
Selected
In2
b
b
b
88 dB
In4
92 dB
90 dB
66 dB
F