LT1880 LINER [Linear Technology], LT1880 Datasheet - Page 8

• Manufacturer Part Number: LT1880
• Description: SOT-23, Rail-to-Rail Output, Picoamp Input Current Precision Op Amp
• Manufacturer: LINER [Linear Technology]

LT1880
APPLICATIO S I FOR ATIO
8
The LT1880 single op amp features exceptional input
precision with rail-to-rail output swing. Slew rate and
small signal bandwidth are superior to other amplifiers
with comparable input precision. These characteristics
make the LT1880 a convenient choice for precision low
voltage systems and for improved AC performance in
higher voltage precision systems. Obtaining beneficial
advantage of the precision inherent in the amplifier de-
pends upon proper applications circuit design and board
layout.
Preserving Input Precision
Preserving the input voltage accuracy of the LT1880
requires that the applications circuit and PC board layout
do not introduce errors comparable to or greater than the
40 V offset. Temperature differentials across the input
connections can generate thermocouple voltages of 10’s
of microvolts. PC board layouts should keep connections
to the amplifier’s input pins close together and away from
heat dissipating components. Air currents across the
board can also generate temperature differentials.
The extremely low input bias currents, 150pA, allow high
accuracy to be maintained with high impedance sources
and feedback networks. The LT1880’s low input bias
currents are obtained by using a cancellation circuit on-
chip. This causes the resulting I
uncorrelated, as implied by the l
comparable to I
input resistances in each input lead, as is commonly
recommended with most amplifiers. The impedance at
either input should be kept as small as possible to mini-
mize total circuit error.
PC board layout is important to insure that leakage cur-
rents do not corrupt the low I
precision, high impedance circuits, the input pins should
be surrounded by a guard ring of PC board interconnect,
with the guard driven to the same common mode voltage
as the amplifier inputs.
Input Common Mode Range
The LT1880 output is able to swing nearly to each power
supply rail, but the input stage is limited to operating
between V
–
+ 1V and V
BIAS
. The user should not try to balance the
U
+
– 1.2V. Exceeding this common
U
BIAS
of the amplifier. In high
BIAS
OS
W
specification being
+
and I
BIAS
U
–
to be
mode range will cause the gain to drop to zero, however no
gain reversal will occur.
Input Protection
The inverting and noninverting input pins of the LT1880
have limited on-chip protection. ESD protection is pro-
vided to prevent damage during handling. The input tran-
sistors have voltage clamping and limiting resistors to
protect against input differentials up to 10V. Short tran-
sients above this level will also be tolerated. If the input
pins can see a sustained differential voltage above 10V,
external limiting resistors should be used to prevent
damage to the amplifier. A 1k resistor in each input lead
will provide protection against a 30V differential voltage.
Capacitive Loads
The LT1880 can drive capacitive loads up to 600pF in unity
gain. The capacitive load driving capability increases as
the amplifier is used in higher gain configurations, see the
graph labled Capacitive Load Response. Capacitive load
driving may be increased by decoupling the capacitance
from the output with a small resistance.
Getting Rail-to-Rail Operation without Rail-to-Rail
Inputs
The LT1880 does not have rail-to-rail inputs, but for most
inverting applications and noninverting gain applications,
this is largely inconsequential. Figure 1 shows the basic op
amp configurations, what happens to the op amp inputs,
and whether or not the op amp must have rail-to-rail
inputs.
30
25
20
15
10
5
0
10
V
T
A
S
Capacitance Load Response
= 15V
= 25 C
CAPACITIVE LOAD (pF)
100
A
V
= 10
A
V
= 1
1000
1880 G25
10000