10
LT1112/LT1114
111214fb
Voltage Follower with Input Exceeding the Common Mode Range (V
S
= ±5V)
INPUT: ±5.2V Sine Wave
LT1112 Output
OP-297 Output
The LT1112 dual and LT1114 quad in the plastic and
ceramic DIP packages are pin compatible to and directly
replace such precision op amps as the OP-200, OP-297,
AD706 duals and OP-400, OP-497, AD704 quads with
improved price/performance.
The LT1112 in the S8 surface mount package has the
standard pin configuration, i.e., the same configuration as
the plastic and ceramic DIP packages.
The LT1114 quad is offered in the narrow 16-pin surface
mount package. All competitors are in the wide 16-pin
package which occupies 1.8 times the area of the narrow
package. The wide package is also 1.8 times thicker than
the narrow package.
The inputs of the LT1112/1114 are protected with back-to-
back diodes. 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 is recommended because
this resistor keeps the current below the short-circuit
limit, resulting in faster recovery and settling of the output.
The input voltage of the LT1112/1114 should never ex-
ceed the supply voltages by more than a diode drop.
However, the example below shows that as the input
voltage exceeds the common mode range, the LT1112’s
output clips cleanly, without any glitches or phase rever-
sal. The OP-297 exhibits phase reversal. The photos also
illustrate that both the input and output ranges of the
LT1112 are within
800mV of the supplies. The effect of
input and output overdrive on the other amplifiers in the
LT1112 or LT1114 packages is negligible, as each
amplifier is biased independently.
Advantages of Matched Dual and Quad Op Amps
In many applications the performance of a system de-
pends on the matching between two operational amplifiers
rather than the individual characteristics of the two op
amps. Two or three op amp instrumentation amplifiers,
tracking voltage references and low drift active filters are
some of the circuits requiring matching between two op
amps.
The well-known triple op amp configuration illustrates
these concepts. Output offset is a function of the difference
between the offsets of the two halves of the LT1112. This
error cancellation principle holds for a considerable num-
ber of input referred parameters in addition to offset
voltage and its drift with temperature. Input bias current
will be the average of the two noninverting input currents
(I
B
+
). The difference between these two currents (∆I
B
+
) is
the offset current of the instrumentation amplifier. Com-
mon mode and power supply rejections will be dependent
only on the match between the two amplifiers (assuming
perfect resistor matching).
APPLICATIO S I FOR ATIO
WUUU
