7

The gain of the feedback divider is:

The transfer function of the part is:

or

by the feedback divider ratio. The EL4451 is stable for a

may be used for higher output gain, although with the

traditional loss of bandwidth.

It is important to keep the feedback divider’s impedance at

the FB terminal low so that stray capacitance does not

diminish the loop’s phase margin. The pole caused by the

parallel impedance of the feedback resistors and stray

capacitance should be at least 150MHz; typical strays of 3pF

thus require a feedback impedance of 360

Ω or less.

create more of a frequency-compensated divider. The value

of the capacitor should scale with the parasitic capacitance

at the FB input. It is also practical to place small capacitors

across both the feedback and the gain resistors (whose

values maintain the desired gain) to swamp out parasitics.

For instance, two 10pF capacitors across equal divider

resistors for a maximum gain of 4 will dominate parasitic

effects and allow a higher divider resistance.

The REF pin can be used as the output’s ground reference,

for DC offsetting of the output, or it can be used to sum in

another signal.

Gain-Control Characteristics

0

≤ V

exceed this range. Actually, the gain transfer function around

0 and 2V is “soft”, that is, the gain does not clip abruptly

levels cannot be precisely determined, they are extrapolated

from two points measured inside the slope of the gain

+2.5V will assure the full numerical span of 0

The gain control has a small-signal bandwidth equal to the

about 20nsec.

Input Connections

The input transistors can be driven from resistive and

capacitive sources, but are capable of oscillation when

presented with an inductive input. It takes about 80nH of

series inductance to make the inputs actually oscillate,

equivalent to four inches of unshielded wiring or 6 of

unterminated input transmission line. The oscillation has a

characteristic frequency of 500MHz. Often placing one’s

finger (via a metal probe) or an oscilloscope probe on the

input will kill the oscillation. Normal high-frequency

construction obviates any such problems, where the input

source is reasonably close to the input. If this is not possible,

one can insert series resistors of around 51

Ω to de-Q the

inputs.

Signal Amplitudes

Signal input common-mode voltage must be between

(V-)+3V and (V+)-3V to ensure linearity. Additionally, the

differential voltage on any input stage must be limited to ±6V

to prevent damage. The differential signal range is ±2V in the

EL4451. The input range is substantially constant with

temperature.

The Ground Pin

The ground pin draws only 6µA maximum DC current, and

may be biased anywhere between(V-)+2.5V and (V+)-3.5V.

The ground pin is connected to the IC’s substrate and

frequency compensation components. It serves as a shield

within the IC and enhances input stage CMRR and

feedthrough over frequency, and if connected to a potential

other than ground, it must be bypassed.

Power Supplies

The EL4451 works with any supplies from ±3V to ±15V. The

supplies may be of different voltages as long as the

requirements of the ground pin are observed (see the

Ground Pin section). The supplies should be bypassed close

to the device with short leads. 4.7µF tantalum capacitors are

very good, and no smaller bypasses need be placed in

parallel. Capacitors as small as 0.01µF can be used if small

load currents flow.

Single-polarity supplies, such as +12V with +5V can be

used, where the ground pin is connected to +5V and V- to

ground. The inputs and outputs will have to have their levels

shifted above ground to accommodate the lack of negative

supply.

The power dissipation of the EL4451 increases with power

supply voltage, and this must be compatible with the

EL4451