LT1078/LT1079
U
W U U
APPLICATIONS INFORMATION
difference is critical because in many applications these
competing devices cannot be operated as micropower op
amps and swing to ground simultaneously.
enough to make the amplifier function properly in the
voltage follower mode, Figure 1b.
5V
5V
As an example, consider the instrumentation amplifier
shown on the front page. When the common mode signal
is low and the output is high, amplifier A has to sink
current. When the common mode signal is high and the
output low, amplifier B has to sink current. The competing
devices require a 12k pull-down resistor at the output of
amplifier A and a 15k at the output of B to handle the
specified signals. (The LT1078 does not need pull-down
resistors.) When the common mode input is high and the
output is high these pull-down resistors draw 300µA (150µA
each), which is excessive for micropower applications.
R
99R
100mV
–
+
–
+
OUTPUT
SATURATED
≈ 3.5mV
1mV
1mV
LT1078/79 • F01a
LT1078/79 • F01b
Figure 1b. Voltage Follower
Figure 1a. Gain 100 Amplifier
Single supply operation can also create difficulties at the
input. The driving signal can fall below 0V — inadvertently
or on a transient basis. If the input is more than a few
hundred millivolts below ground, two distinct problems
can occur on previous single supply designs, such as the
LM124, LM158, OP-20, OP-21, OP-220, OP-221, OP-420
(1 and 2), OP-90/290/490 (2 only):
The instrumentation amplifier is by no means the only
application requiring current sinking capability. In seven
of the nine single supply applications shown in this data
sheet the op amps have to be able to sink current. In two
of the applications the first amplifier has to sink only the
6nA input bias current of the second op amp. The compet-
ingdevices, however, cannotevensink6nAwithoutapull-
down resistor
1. Whentheinputismorethanadiodedropbelowground,
unlimited current will flow from the substrate (V–
terminal) to the input. This can destroy the unit. On the
LT1078/LT1079, resistors in series with the input protect
the devices even when the input is 5V below ground.
Since the output of the LT1078/LT1079 cannot go exactly
to ground, but can only approach ground to within a few
millivolts, care should be exercised to ensure that the
output is not saturated. For example, a 1mV input signal
will cause the amplifier to set up in its linear region in the
gain 100 configuration shown in Figure 1a, but is not
2. When the input is more than 400mV below ground (at
25°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 LT1078/LT1079 output does not reverse, as
illustrated in Figure 2, even when the inputs are at –1V.
4V
2V
4V
2V
4V
2V
0V
0V
0V
6VP-P INPUT
–1V TO 5V
1ms/DIV
1ms/DIV
OP-90 EXHIBITS OUTPUT PHASE REVERSAL
1ms/DIV
LT1078/LT1079 NO PHASE REVERSAL
LT1078/79 • F02a
LT1078/79 • F02b
LT1078/79 • F02C
Figure 2. Voltage Follower with Input Exceeding the Negative Common Mode Range (VS = 5V, 0V)
11
Linear [ Linear ]
