7

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 250

Ω 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 150

Ω 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.

Pin Descriptions

PIN NAME

PIN DESCRIPTION

A1, A2, A3

“A” inputs to amplifiers 1, 2 and 3 respectively

B1, B2, B3

“B” inputs to amplifiers 1, 2 and 3 respectively

GND1, GND2, GND3

These are the individual ground pins for each channel.

OUT1, OUT2, OUT3

Amplifier outputs. Note there is no short circuit protection.

Positive power supply. Typically +5V.

Negative power supply, typically -5V.

A/B

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.

PD

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.

EL4331