EL4331CS Просмотр технического описания (PDF) - Intersil
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EL4331CS Datasheet PDF : 10 Pages
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EL4331
Pin Descriptions
PIN NAME
A1, A2, A3
B1, B2, B3
GND1, GND2, GND3
OUT1, OUT2, OUT3
VCC
VEE
A/B
PD
PIN DESCRIPTION
“A” inputs to amplifiers 1, 2 and 3 respectively
“B” inputs to amplifiers 1, 2 and 3 respectively
These are the individual ground pins for each channel.
Amplifier outputs. Note there is no short circuit protection.
Positive power supply. Typically +5V.
Negative power supply, typically -5V.
Common input select pin, a logic high selects the “A” inputs, logic low selects the “B” inputs. If left to float, this pin
will float high and the “A” channels will be selected.
A logic low puts the part into its power-down mode. Note that when this pin is at a logic high (+5V), it will sink typically
1mA. When pulled low, it will source a few µA, typically < 25µA. This pin should not be left floating.
Applications Information
Circuit Operation
Each multiplexing amplifier has two input stages. The
multiplexing amplifiers switch from their “A” inputs to their “B”
inputs under control of the common A/B select pin. The
switching has a make before break action. Each amplifier is
internally connected for unity gain, allowing larger switching
matrixes to be built up. Note however, that each amplifier
likes to see a load of 250Ω or less; load resistances higher
than this, can lead to excessive peaking. Load capacitance
should be kept down below 40pF, and 40pF requires a load
resistance of 150Ω to keep the output from excessive
peaking. Higher capacitive loads can best be driven using a
series resistor to isolate the amplifier from the reactive load.
The ground pins are used as a reference for the logic
controls. Both A/B and PD are referenced to ground. The
supplies do not have to be symmetrical around ground, but
the logic inputs are referred to the ground pins, and the logic
swing must not exceed the +V supply. Due to the fact that all
three channels share common control pins, the three
grounds have to be at the same potential. One third of the
1mA that PD will sink (at 5V) will be seen at each ground pin.
Also, the individual grounds are internally connected to their
channel compensation capacitor in an effort to keep
crosstalk low.
A/B Switching
Referring to the photographs showing the 0V–0V switching
glitches, it will be noted that slower edges on the A/B control
pin result in switching glitches of somewhat less total energy.
The switching action is a make-before-break, so the two
inputs essentially get mixed at the output for a few
nanoseconds. Note that the two inputs are buffered, so there
is no component of one input injected into the other input.
The input impedance does not depend on whether an input
has been selected.
Power-Down
Referring to the photographs of the power-down function and
Figure 4, it will be noted that there is a considerable glitch in
the output as the part powers down. It will also be noted that
the power-down time is considerably longer than power up,
1µs compared to 150ns. In power-down mode, the whole
amplifier, its reference and bias lines are all powered down.
At the same time, the output stage has been configured so
that the powered down output appears as a high impedance.
This allows circuits such as the multiplexer shown in
application #4 to be realized, although the price is the
significant output disturbance as one part turns on before the
other has fully turned off.
Single Supply Operation
Due to the fact that video signals often have negative sync
levels and invariably require ground to be within the signal
swing, running the EL4331 on a single supply rail
compromises many aspects of its performance. It is difficult
to generate a solid, clean, pseudo ground a few volts away
from ground without using more power, and components
than simply providing a negative power rail. A signal ground
has to be capable of handling all the return currents from all
the inputs, as well as the outputs, from DC to frequencies in
excess of 400MHz. While this is by no means impossible, a
negative rail can be generated from a standard +5V rail for a
couple of dollars and a square inch, or less, of board space.
However, a pseudo ground can be derived with for example
an LM336, to give an “AC ground” 2.5V above 0V. The logic
inputs will need some form of level shifting to ensure that the
logic “1” and “0” specifications can be met. The pseudo
ground must be well bypassed to the real ground; note that
the pseudo ground will have to sink/source all the current
that flows in the internal compensation capacitors during
slewing. This can easily be several milliamps in a few
nanoseconds. If the pseudo ground “moves” because one
channel is forcing current into the derived ground, cross-talk
into the other two channels will become very significant.
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