11
LT 2078/LT 2079
20789fa
APPLICATIONS INFORMATION
WU
U
1. When the input is more than a diode drop below ground,
unlimited current will flow from the substrate (V 鈥�
terminal) to the input. This can destroy the unit. On the
LT2078/LT2079, resistors in series with the input protect
the devices even when the input is 5V below ground.
2. When the input is more than 400mV below ground (at
25
掳C), the input stage saturates and phase reversal
occurs at the output. This can cause lockup in servo
systems. Due to a unique phase reversal protection cir-
cuitry, the LT2078/LT2079 output does not reverse, as
illustrated in Figure 2, even when the inputs are at 鈥�1V.
Distortion
There are two main contributors of distortion in op amps:
distortion caused by nonlinear common mode rejection
and output crossover distortion as the output transitions
from sourcing to sinking current. The common mode
rejection of the LT2078/LT2079 is very good, typically
108dB. Therefore, as long as the input operates in the
normal common mode range, there will be very little
common mode induced distortion. If the op amp is oper-
ating inverting there is no common mode induced distor-
tion. Crossover distortion will increase as the output load
resistance decreases. For the lowest distortion the LT2078/
LT2079 should be operated with the output always sourc-
ing current, this is usually accomplished by putting a
resistor from the output to V 鈥�. In an inverting configura-
tion with no load, the output will source and sink current
through the feedback resistor. High value feedback resis-
tors will reduce crossover distortion and maintain
micropower operation.
Matching Specifications
In many applications the performance of a system de-
pends on the matching between two op amps, rather than
Figure 2. Voltage Follower with Input Exceeding the Negative Common Mode Range (VS = 5V, 0V)
2V
0V
6VP-P INPUT
1ms/DIV
鈥�1V TO 5V
LT2078/79 F01a
1ms/DIV
LT2078/LT2079 NO PHASE REVERSAL
LT2078/79 F01C
2V
0V
4V
1ms/DIV
OP-90 EXHIBITS OUTPUT PHASE REVERSAL
LT2078/79 F01b
2V
0V
4V
Table 1
LT2078AC/LT2079AC/LT2078AI/LT2079AI
LT2078C/LT2079C/LT2078I/LT2079I
PARAMETER
50% YIELD
98% YIELD
50% YIELD
98% YIELD
UNITS
VOS Match, VOS
LT2078
30
110
50
190
V
LT2079
40
150
50
250
V
Temperature Coefficient
VOS
0.5
1.2
0.6
1.8
V/掳C
Average Noninverting IB
68
6
10
nA
Match of Noninverting IB
0.12
0.4
0.15
0.5
nA
CMRR Match
120
100
117
97
dB
PSRR Match
117
105
117
102
dB
