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EL4452 View Datasheet(PDF) - Intersil

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EL4452 Datasheet PDF : 10 Pages
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Typical Performance Curves (Continued)
Supply Current vs. Die Temperature
EL4452
14-Pin Package Power Dissipation
vs. Ambient Temperature
Applications Information
The EL4452 is a complete two-quadrant multiplier/gain
control with 50MHz bandwidth. It has three sets of inputs; a
differential signal input VIN, a differential gain-controlling
input VGAIN, and another differential input which is used to
complete a feedback loop with the output. Here is a typical
connection:
The gain of the feedback divider is H. The transfer function of
the part is:
VOUT = AO × (((VIN+) - (VIN-)) × ((VGAIN+) - (VGAIN-)) +
(VREF - VFB)).
VFB is connected to VOUT through a feedback network, so
VFB = H × VOUT. AO is the open-loop gain of the amplifier,
and is approximately 3300. The large value of AO drives:
((VIN+) - (VIN-)) × 1/2 ((VGAIN+) - (VGAIN-)) + (VREF -
VFB) 0.
Rearranging and substituting for VFB:
VOUT = (((VIN+) - (VIN-)) × 1/2 ((VGAIN+) - (VGAIN)) +
VREF)/H,
or
VOUT = (VIN × 1/2 VGAIN + VREF)/H
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 1/10, and the divider may be set for higher
output gain, although with the traditional loss of bandwidth.
7
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 130MHz; typical strays of 3pF
thus require a feedback impedance of 400or less.
Alternatively, a small capacitor across RF can be used to
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, a 3pF capacitor across RF and 27pF to ground
will dominate parasitic effects in a 1/10 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
0 VGAIN 2, even though the externally applied voltages
exceed this range. Actually, 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 0.3V 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 -0.5V to
+2.5V will assure the full numerical span of 0 VGAIN 2.
The gain control has a small-signal bandwidth equal to the
VIN channel bandwidth, and overload recovery resolves in
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,
 

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