EL4451 View Datasheet(PDF) - Intersil
Part Name
Description
Manufacturer
EL4451 Datasheet PDF : 10 Pages
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EL4451
The gain of the feedback divider is:
H = RG/(RG + RF)
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 600. The large value of AO drives:
((VIN+)-(VIN-))×((VGAIN+)-(VGAIN-))+(VREF - VFB) → 0
Rearranging and substituting for VFB:
VOUT = (((VIN+) - (VIN-)) × ((VGAIN+) - (VGAIN)) +
VREF)/H
or
VOUT = (VIN × 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 EL4451 is stable for a
direct connection between VOUT and FB, and the divider
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 360Ω or 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, 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
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,
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 51Ω 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
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