EL4421C/22C/41C/42C/43C/44C
Multiplexed-Input Video Amplifiers
Applications Information Ð 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. 12X to 51X should suffice. A 22X series
resistor will limit peaking to 2.5 dB with even a
220 pF load.
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
Input Connections
R
IN
. In unity-gain connection the feedback resis-
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.
tor is 0X and very little crosstalk is induced. For
a
b
a gain of 2, 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 3.58 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 of EL4441 and EL4443 at 3.58 MHz
In1
In2
In3
In4
b
b
b
b
b
b
00
01
10
11
Selected
77 dB
90 dB
77 dB
92 dB
90 dB
66 dB
Select
Inputs,
A1A0
b
80 dB
Selected
b
b
101 dB
76 dB
Selected
b
b
b
96 dB
84 dB
66 dB
Selected
Feedback Connections
A feedback divider is used to increase circuit
Feedthrough of EL4421 at 3.58 MHz
gain, and some precautions should be observed.
b
In1
In2
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. For a 3 pF
parasitic, this requires that the feedback divider
have less than 265X impedance, equivalent to
b
Channel Select
Input, A0
0
1
Selected
88 dB
Selected
b
93 dB
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.
a
two 510X resistors when a gain of 2 is desired.
Alternatively, a small capacitor across R can be
F
used to create more of a frequency-compensated
divider. The value of the capacitor should match
b
the parasitic capacitance at the input. It is also
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.
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.
13
ELANTEC [ ELANTEC SEMICONDUCTOR ]
