10
LT2078/LT2079
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APPLICATIONS INFORMATION
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The LT2078/LT2079 devices are fully specified with
V
+
= 5V, V
–
= 0V, V
CM
= 0.1V. This set of operating
conditions appears to be the most representative for
battery powered micropower circuits. Offset voltage is
internally trimmed to a minimum value at these supply
voltages. When 9V or 3V batteries or ±2.5V dual supplies
are used, bias and offset current changes will be minimal.
Offset voltage changes will be just a few microvolts as
given by the PSRR and CMRR specifications. For example,
if PSRR = 114dB (=2µV/V), at 9V the offset voltage change
will be 8µV. Similarly, V
S
= ±2.5V, V
CM
= 0V is equivalent
to a common mode voltage change of 2.4V or a V
OS
change of 7µV if CMRR = 110dB (3µV/V).
A full set of specifications is also provided at ±15V supply
voltages for comparison with other devices and for com-
pleteness.
Single Supply Operation
The LT2078/LT2079 is quite tolerant of power supply
bypassing. In some applications requiring faster settling
time the positive supply pin of the LT2078/LT2079 should
be bypassed with a small capacitor (about 0.1µF). The
same is true for the negative supply pin when using split
supplies.
The LT2078/LT2079 are fully specified for single supply
operation, i.e., when the negative supply is 0V. Input
common mode range goes below ground and the output
swings within a few millivolts of ground while sinking
current. All competing micropower op amps either cannot
swing to within 600mV of ground (OP-20, OP-220, OP-
420) or need a pull-down resistor connected to the output
to swing to ground (OP-90, OP-290, OP-490, HA5141/42/
44). This difference is critical because in many applica-
tions these competing devices cannot be operated as
micropower op amps and swing to ground simultaneously.
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 LT2078 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.
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-
ing devices, however, cannot even sink 6nA without a
pull-down resistor
Since the output of the LT2078/LT2079 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 1, but is not
enough to make the amplifier function properly in the
voltage follower mode.
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):
Figure 1a. Gain 100 Amplifier Figure 1b. Voltage Follower
+
–
5V
1mV
R
99R
100mV
LT2078/79 • F02a
+
–
5V
1mV
OUTPUT
SATURATION
≈ 3.5mV
LT2078/79 • F02b
