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sn1012 1012afbs
LT1012A/LT1012
The LT1012 may be inserted directly into OP-07, LM11,
108A or 101A sockets with or without removal of external
frequency compensation or nulling components. The
LT1012 can also be used in 741, LF411, LF156 or OP-15
applications provided that the nulling circuitry is
removed.
Although the OP-97 is a copy of the LT1012, the LT1012
directly replaces and upgrades OP-97 applications. The
LT1012C and D have lower offset voltage and drift than the
OP-97F. The LT1012A has lower supply current than the
OP-97A/E. In addition, all LT1012 grades guarantee
operation at ±1.2V supplies.
Achieving Picoampere/Microvolt Performance
In order to realize the picoampere/microvolt level
accuracy of the LT1012, proper care must be exercised.
For example, leakage currents in circuitry external to
the op amp can significantly degrade performance. High
quality insulation should be used (e.g. Teflon, Kel-F);
cleaning of all insulating surfaces to remove fluxes and
other residues will probably be required. Surface coating
may be necessary to provide a moisture barrier in high
humidity environments.
Board leakage can be minimized by encircling the input
circuitry with a guard ring operated at a potential close
to that of the inputs: in inverting configurations the
guard ring should be tied to ground, in non-inverting
connections to the inverting input at Pin 2. Guarding both
sides of the printed circuit board is required. Bulk leakage
reduction depends on the guard ring width. Nanoampere
level leakage into the offset trim terminals can affect offset
voltage and drift with temperature.
Microvolt level error voltages can also be generated in
the external circuitry. Thermocouple effects caused by
temperature gradients across dissimilar metals at the
contacts to the input terminals can exceed the inherent
drift of the amplifier. Air currents over device leads should
be minimized, package leads should be short, and the two
input leads should be as close together as possible and
maintained at the same temperature.
Noise Testing
For application information on noise testing and calcula-
tions, please see the LT1008 data sheet.
Frequency Compensation
The LT1012 can be overcompensated to improve
capacitive load handling capability or to narrow noise
bandwidth. In many applications, the feedback loop around
the amplifier has gain (e.g. Iogarithmic amplifiers);
overcompensation can stabilize these circuits with a single
capacitor.
The availability of the compensation terminal permits
the use of feedforward frequency compensation to
enhance slew rate. The voltage follower feedforward
scheme bypasses the amplifier’s gain stages and slews at
nearly 10V/µs.
The inputs of the LT1012 are protected with back-to-back
diodes. Current limiting resistors are not used, because
the leakage of these resistors would prevent the realization
of picoampere level bias currents at elevated tempera-
tures. In the voltage follower configuration, when the input
is driven by a fast, large signal pulse (>1V), the input
protection diodes effectively short the output to the input
during slewing, and a current, limited only by the output
short-circuit protection will flow through the diodes.
The use of a feedback resistor, as shown in the voltage
follower feedforward diagram, is recommended because
this resistor keeps the current below the short-circuit
limit, resulting in faster recovery and settling of the output.
4
5
6
OUTPUT
OVER COMP
7
V
8
OFFSET TRIM
1
2
3
GUARD
IN
PUTS
V
–
LT1012A * AI01
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APPLICATIO S I FOR ATIO
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