8

LT1490A

Supply Voltage

The positive supply pin of the LT1490A should be by-

passed with a small capacitor (about 0.01

µF) within an

inch of the pin. When driving heavy loads an additional

4.7

µF electrolytic capacitor should be used. When using

split supplies, the same is true for the negative supply pin.

The LT1490A is protected against reverse battery voltages

up to 18V. In the event a reverse battery condition occurs,

the supply current is less than 1nA.

condition the input bias current is typically less than 0.5nA,

even if the inputs are 44V above the negative supply.

When operating the LT1490A on total supplies of 20V or

more, the supply must not rise to its final voltage in less

than 1

µs. This is especially true if low ESR bypass capaci-

tors are used. A series RLC circuit is formed from the

supply lead inductance and the bypass capacitor. A resis-

tance of 7.5

Ω in the supply or in the bypass capacitor will

dampen the tuned circuit enough to limit the rise time.

Inputs

The LT1490A has two input stages, NPN and PNP (see the

Simplified Schematic), resulting in three distinct operat-

ing regions as shown in the Input Bias Current vs Common

Mode typical performance curve.

input stage is active and the input bias current is typically

– 1nA. When the input voltage is about 0.5V or less from

current is typically 25nA. Increases in temperature will

cause the voltage at which operation switches from the

offset voltage of the NPN stage is untrimmed and is

typically 600

µV.

A Schottky diode in the collector of each NPN transistor of

the NPN input stage allows the LT1490A to operate with

bias current is typically 3

µA at room temperature. The

input offset voltage is typically 700

µV when operating

The inputs are protected against excursions as much as

each input and a diode from the input to the negative

supply. There is no output phase reversal for inputs up to

inputs and the maximum differential input voltage is 44V.

Output

The output voltage swing of the LT1490A is affected by

input overdrive as shown in the typical performance

curves.

The output of the LT1490A can be pulled up to 18V beyond

is less than 0.5V.

The normally reverse-biased substrate diode from the

limited to 100mA, no damage will occur.

The LT1490A is internally compensated to drive at least

200pF of capacitance under any output loading condi-

tions. A 0.22

µF capacitor in series with a 150Ω resistor

between the output and ground will compensate these

amplifiers for larger capacitive loads, up to 10,000pF, at all

output currents.

Distortion

There are two main contributors of distortion in op amps:

output crossover distortion as the output transitions

from sourcing to sinking current and distortion caused

by nonlinear common mode rejection. Of course, if the op

amp is operating inverting there is no common mode

induced distortion. When the LT1490A switches be-

tween input stages there is significant nonlinearity in the

CMRR. Lower load resistance increases the output cross-

over distortion, but has no effect on the input stage

transition distortion. For lowest distortion the LT1490A

should be operated single supply, with the output always

sourcing current and with the input voltage swing be-

mance Characteristics curves.

APPLICATIO S I FOR ATIO