EL4421C 22C 41C 42C 43C 44C
Multiplexed-Input Video Amplifiers
Applications Information
Contd
The other major concern about the divider con-
cerns unselected-channel crosstalk The differen-
tial input impedance of each input stage is
around 200 KX 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 R
F
R
IN
In unity-gain connection the feedback resis-
tor is 0X and very little crosstalk is induced For
a gain of
a
2 the crosstalk is about
b
60 dB
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 12X to 51X should suffice A 22X series
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 51X to de-Q the inputs
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
Feedthrough of EL4441 and EL4443 at 3 58 MHz
In1
00
Select
Inputs
A1A0
01
10
11
Selected
b
80 dB
b
101 dB
b
96 dB
In2
b
77 dB
In3
b
90 dB
b
77 dB
In4
b
92 dB
b
90 dB
b
66 dB
Selected
b
76 dB
b
84 dB
Selected
b
66 dB
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
b
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 For a 3 pF
parasitic this requires that the feedback divider
have less than 265X impedance equivalent to
two 510X resistors when a gain of
a
2 is desired
Alternatively a small capacitor across R
F
can be
used to create more of a frequency-compensated
divider The value of the capacitor should match
the parasitic capacitance at the
b
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
Selected
Feedthrough of EL4421 at 3 58 MHz
TD is 0 5in
In1
Channel Select
Input A0
0
1
Selected
b
93 dB
In2
b
88 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
13
TD is 0 5in
ELANTEC [ ELANTEC SEMICONDUCTOR ]
