EL4452CS datasheet(8/12 Pages) ELANTEC

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EL4452C

Wideband Variable-Gain Amplifier with Gain of 10

Applications Information

Contd

Thus the output is equal to the difference of the

VIN’s times the difference of VGAIN’S and offset

by VREF all gained up by the feedback divider

ratio The EL4452 is stable for a divider ratio of

and the divider may be set for higher output

gain although with the traditional loss of band-

width

It is important to keep the feedback divider’s im-

pedance at the FB terminal low so that stray ca-

pacitance does not diminish the loop’s phase

margin The pole caused by the parallel imped-

ance of the feedback resistors and stray capaci-

tance should be at least 130 MHz typical strays

of 3 pF thus require a feedback impedance of

400

X or less Alternatively a small capacitor

across RF can be used to create more of a fre-

quency-compensated divider The value of the ca-

pacitor should scale with the parasitic capaci-

tance 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 a

3 pF capacitor across RF and 27 pF to ground

will dominate parasitic effects in a

divider

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

The quantity VGAIN in the above equations is

bounded as 0sVGAINs2 even though the exter-

nally applied voltages exceed this range Actual-

ly the gain transfer function around 0 and 2V is

‘‘soft’’ that is the gain does not clip abruptly

below the 0%-VGAIN voltage nor above the

100%-VGAIN level An overdrive of 03V must be

applied to VGAIN to obtain truly 0% or 100%

Because the 0%- or 100%- VGAIN levels cannot

be precisely determined they are extrapolated

from two points measured inside the slope of the

gain transfer curve Generally an applied VGAIN

range of b05V to a25V will assure the full nu-

merical span of 0sVGAINs2

The gain control has a small-signal bandwidth

equal to the VIN channel bandwidth and over-

load recovery resolves in about 20 nsec

Input Connections

The input transistors can be driven from resistive

and capacitive sources but are capable of oscilla-

tion 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 unterminat-

ed input transmission line The oscillation has a

characteristic frequency of 500 MHz Often plac-

ing one’s finger (via a metal probe) or an oscillo-

scope probe on the input will kill the oscillation

Normal high-frequency construction obviates

any such problems where the input source is rea-

sonably close to the input If this is not possible

one can insert series resistors of around 51

X to

de-Q the inputs

Signal Amplitudes

Signal input common-mode voltage must be be-

tween (Vb) a 25V and (Va) b 25V to ensure

linearity Additionally the differential voltage on

any input stage must be limited to g6V to pre-

vent damage

The differential signal range is

05V in the EL4452 The input range is sub-

stantially constant with temperature

The Ground Pin

The ground pin draws only 6

mA maximum DC

current and may be biased anywhere between

(Vb)a25V and (Va)b35V 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 by-

passed

Power Supplies

The EL4452 operates with power supplies from

3V to g15V The supplies may be of different

voltages as long as the requirements of the

ground pin are observed (see the Ground Pin sec-

tion) The supplies should be bypassed close to

the device with short leads 47

mF tantalum ca-

pacitors are very good and no smaller bypasses

need be placed in parallel Capacitors as small as

001

mF can be used if small load currents flow

Single-polarity supplies

such as a12V with

5V can be used where the ground pin is con-

nected to a5V and Vb to ground The inputs

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