LT1880

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

µV 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-

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-

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

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.

APPLICATIO S I FOR ATIO

CAPACITIVE LOAD (pF)

30

25

20

15

10

5

0

10

100

1000

10000

1880 G25

Capacitance Load Response

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.