LT1014_17 Datasheet, PDF (18/35 Pages) Texas Instruments – QUAD PRECISION OPERATIONAL AMPLIFIERS

LT1014, LT1014A, LT1014D
QUAD PRECISION OPERATIONAL AMPLIFIERS
SLOS039D − JULY 1989 − REVISED AUGUST 2009
APPLICATION INFORMATION
single-supply operation
The LT1014 is fully specified for single-supply operation (V_CC− = 0). The common-mode input voltage range
includes ground, and the output swings within a few millivolts of ground.
Furthermore, the LT1014 has specific circuitry that addresses the difficulties of single-supply operation, both
at the input and at the output. At the input, the driving signal can fall below 0 V, either inadvertently or on a
transient basis. If the input is more than a few hundred millivolts below ground, the LT1014 is designed to deal
with the following two problems that can occur:
1. On many other operational amplifiers, when the input is more than a diode drop below ground, unlimited
current flows from the substrate (V_CC− terminal) to the input, which can destroy the unit. On the LT1014,
the 400-Ω resistors in series with the input (see schematic) protect the device even when the input is 5 V
below ground.
2. When the input is more than 400 mV below ground (at T_A = 25°C), the input stage of similar type operational
amplifiers saturates, and phase reversal occurs at the output. This can cause lockup in servo systems.
Because of unique phase-reversal protection circuitry (Q21, Q22, Q27, and Q28), the LT1014 outputs do
not reverse, even when the inputs are at −1.5 V (see Figure 24).
However, this phase-reversal protection circuitry does not function when the other operational amplifier on the
LT1014 is driven hard into negative saturation at the output. Phase-reversal protection does not work on an
amplifier:
D When 4’s output is in negative saturation (the outputs of 2 and 3 have no effect)
D When 3’s output is in negative saturation (the outputs of 1 and 4 have no effect)
D When 2’s output is in negative saturation (the outputs of 1 and 4 have no effect)
D When 1’s output is in negative saturation (the outputs of 2 and 3 have no effect)
At the output, other single-supply designs either cannot swing to within 600 mV of ground or cannot sink more
than a few microproamperes while swinging to ground. The all-npn output stage of the LT1014 maintains its low
output resistance and high gain characteristics until the output is saturated. In dual-supply operations, the output
stage is free of crossover distortion.
5
5
5
4
4
4
3
3
3
2
1
2
2
0
1
1
−1
0
0
−2
(a) V_I(PP) = −1.5 V to 4.5 V
−1
(b) Output Phase Reversal
Exhibited by LM358
−1
(c) No Phase Reversal
Exhibited by LT1014
Figure 24. Voltage-Follower Response
With Input Exceeding the Negative Common-Mode Input Voltage Range
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