EL4430CN 데이터 시트보기 (PDF) - Intersil
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EL4430CN Datasheet PDF : 10 Pages
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EL4430, EL4431
Applications Information
The EL4430 and EL4431 are designed to convert a fully
differential input to a single-ended output. It has two sets of
inputs; one which is connected to the signal and does not
respond to its common-mode level, and another 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-) + (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 about 600 for the EL4430 and EL4431. The large
value of AO drives:
(VIN+) - (VIN-) + (VREF - VFB)→0.
Rearranging and substituting for VFB:
VOUT = ((VIN+) - (VIN-) + VREF)/H.
Thus, the output is equal to the difference of the VINs and
offset by VREF, all gained up by the feedback divider ratio.
The input impedance of the FB terminal (equal to RIN of the
input terminals) is in parallel with an RG, and raises circuit
gain slightly.
The EL4430 is stable for a gain of 1 (a direct connection
between VOUT and FB) or more and the EL4431 for gains of
2 or more. 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 of resistors RF and RG and stray capacitance
should be at least 200MHz; typical strays of 3pF thus require
a feedback impedance of 270Ω or less. Two 510Ω resistors
are acceptable for a gain of 2; 300Ω and 2700Ω make a
good gain-of-10 divider. 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 terminal input.
It is also practical to place small capacitors across both the
feedback resistors (whose values maintain the desired gain)
to swamp out parasitics. For instance, two 10pF capacitors
(for a gain of 2) across equal divider resistors will dominate
parasitic effects and allow a higher divider resistance.
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 4 of unshielded wiring or about 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 approximately 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
EL4430 and EL4431. 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 CMRR over frequency, and if
connected to a potential other than ground, it must be
bypassed.
Power Supplies
The instrumentation amplifiers work well on any supplies
from ±3V to ±15. The supplies may be of different voltages
as long as the requirements of the Gnd pin are observed
(see the Ground Pin section for a discussion). 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 low 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 dissipation of the amplifiers increases with power supply
voltage, and this must be compatible with the package
chosen. This is a close estimate for the dissipation of a
circuit:
PD= 2 × VS × IS, max + (VS-VO) × VO/RPAR
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