8

supply voltage nor output loading, at least down to 150

Ω. For

settling to 0.1%, however, it is best to not load the output

heavily and to run the EL4094 on the lowest practical supply

voltages, so that thermal effects are minimized.

Gain Control Inputs

The gain control inputs are differential and may be biased at

3V above V-. The differential input impedance is 5.5k

Ω, and

the common-mode impedance is more than 500k

Ω. With

zero differential voltage on the gain inputs, both signal inputs

have a 50% gain factor. Nominal calibration sets the 100%

have a completely abrupt transition at the 0% and 100%

points. There is about 10mV of “soft” transfer at the gain

endpoints. To obtain the most accurate 100% gain factor or

best attenuation at 0% gain, it is necessary to overdrive the

gain control input by 30mV or more. This would set the gain

control voltage range as -0.565V to +0.565V, or 30mV

beyond the maximum guaranteed 0% to 100% range. In fact,

the gain control inputs are very complex. Here is a

representation of the terminals:

For gain control inputs between ±0.5V (±90µA), the diode

amount, the bridge becomes a high impedance as some of

from the nominal 5.5K

Ω to about 500KΩ. This is the

condition of gain control overdrive. The actual circuit

produces a much sharper overdrive characteristic than does

the simple diode bridge of this representation.

The gain input has a 20MHz -3dB bandwidth and 17ns

risetime for inputs to ±0.45V. When the gain control voltage

exceeds the 0% or 100% values, a 70ns overdrive recovery

transient will occur when it is brought back to linear range. If

quicker gain overdrive response is required, the Force

control inputs of the EL4095 can be used.

Output Loading

The EL4094 does not work well with heavy capacitive loads.

Like all amplifier outputs, the output impedance becomes

inductive over frequency resonating with a capacitive load.

The effective output inductance of the EL4094 is about

350nH. More than 50pF will cause excessive frequency

response peaking and transient ringing. The problem can be

solved by inserting a low-value resistor in series with the

load, 22

Ω or more. If a series resistance cannot be used,

then adding a 300

Ω or less load resistor to ground or a

“snubber” network may help. A snubber is a resistor in series

with a capacitor, 150

Ω and 100pF being typical values. The

advantage of a snubber is that it does not draw DC load

current.

Unterminated coaxial line loads can also cause resonances,

and they should be terminated either at the far end or a

series back-match resistor installed between the EL4094

and the cable.

The output stage can deliver up to 140mA into a short-circuit

load, but it is only rated for a continuous 35mA. More

continuous current can cause reliability problems with the

on-chip metal interconnect. Video levels and loads cause no

problems at all.

Noise

The EL4094 has a very simple noise characteristic: the

output noise is constant (40nV/

√Hz wideband) for all gain

settings. The input-referred noise is then the output noise

divided by the gain. For instance, at a gain of 50% the input

noise is 40nV/

√Hz/0.5, or 80nV/√Hz.

Bypassing

The EL4094 is fairly tolerant of power-supply bypassing, but

best multiplier performance is obtained with closely

connected 0.1µF ceramic capacitors. The leaded chip

capacitors are good, but neither additional tantalums nor

chip components are necessary. The signal inputs can

oscillate locally when connected to long lines or

unterminated cables.

Power Dissipation

Peak die temperature must not exceed 150°C. At this

temperature, the epoxy begins to soften and becomes

unstable, chemically and mechanically. This allows 75°C

internal temperature rise for a 75°C ambient. The EL4094 in

the 8-pin PDIP package has a thermal resistance of 87°/W,

and can thus dissipate 862mW at a 75°C ambient

temperature. The device draws 17mA maximum supply

current, only 510mW at ±15V supplies, and the circuit has no

dissipation problems in this package.

The SO-8 surface-mount package has a 153°/W thermal

resistance with the EL4094, and only 490mW can be

dissipated at 75°C ambient temperature. The EL4094 thus

cannot be operated with ±15V supplies at 75°C in the

surface-mount package; the supplies should be reduced to

FIGURE 1. REPRESENTATION OF GAIN CONTROL

EL4094