EL4421 Intersil Corporation, EL4421 Datasheet - Page 10

• Manufacturer Part Number: EL4421
• Description: Multiplexer/Amplifier, Video Mux 2:1, 80MHz, Gain=1, Muxd Dual Input
• Manufacturer: Intersil Corporation

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 -input will add phase lag to the feedback
path and increase frequency response peaking or even
cause oscillation. One solution is to choose feedback
resistors whose parallel value is low. The pole frequency of
the feedback network should be maintained above at least
200MHz. For a 3pF parasitic, this requires that the feedback
divider have less than 265Ω impedance, equivalent to two
510Ω resistors when a gain of +2 is desired. Alternatively, a
small capacitor across R
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 desired gain)
to swamp out parasitics. For instance, two 10pF capacitors
across equal divider resistors will dominate parasitic effects
and allow a higher divider resistance.
The other major concern about the divider concerns
unselected-channel crosstalk. The differential input
impedance of each input stage is around 200kΩ. The
unselected input's signal sources thus drive current through
that input impedance into the feedback divider, inducing an
unwanted output. The gain from unselected input to output,
the crosstalk attenuation, if R
the feedback resistor is 0Ω and very little crosstalk is
induced. For a gain of +2, the crosstalk is about -60dB.
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 3.58MHz:
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SELECT INPUTS,
FEEDTHROUGH OF EL4441 AND EL4443 AT 3.58MHz
A1 A0
00
01
10
11
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Selected
-101dB
Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality
-80dB
-96dB
IN1
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F
can be used to create more of a
10
EL4421, EL4422, EL4441, EL4442, EL4443, EL4444
F
Selected
/R
-77dB
-76dB
-84dB
IN2
IN
. In unity-gain connection
Selected
-90dB
-77dB
-66dB
IN3
Selected
-92dB
-90dB
-66dB
IN4
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 90mV occurs, and the transient settles out in 20ns.
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 6ns
apart. This can be accomplished by inserting one gate delay
in one of the two logic inputs when they are truly
synchronous.
CHANNEL SELECT INPUT,
FEEDTHROUGH OF EL4421 AT 3.58MHz
A0
0
1
Selected
-93dB
IN1
Selected
-88dB
IN2