Linear Technology LT1490CN8 Linear IC DIP8 Dual R-to-R I/O OA LT1490CN8 Datenbogen

8

LT1490/LT1491

The positive supply pin of the LT1490/LT1491 should be
bypassed with a small capacitor (about 0.01

µ

F) within an

inch of the pin. When driving heavy loads an additional
4.7

µ

F electrolytic capacitor should be used. When using

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

The LT1490/LT1491 are protected against reverse battery
voltages up to 18V. In the event a reverse battery condition
occurs, the supply current is less than 1nA.

The LT1490/LT1491 can be shut down by removing V

+

. In

this condition the input bias current is less than 0.1nA,
even if the inputs are 44V above the negative supply.

When operating the LT1490/LT1491 on total supplies of
30V or more, the supply must not be brought up faster
than 1

µ

s. 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. 5

Ω

 of

resistance in the supply or the bypass capacitor will
dampen the tuned circuit enough to limit the rise time.

The LT1490/LT1491 have two input stages, NPN and PNP
(see the Simplified Schematic), resulting in three distinct
operating regions as shown in the Input Bias Current vs
Common Mode typical performance curve.

For input voltages about 0.8V or more below V

+

, the PNP

input stage is active and the input bias current is typically
– 4nA. When the input voltage is about 0.5V or less from
V

+

, the NPN input stage is operating and the input bias

current is typically 18nA. Increases in temperature will
cause the voltage at which operation switches from the
PNP stage to the NPN stage to move towards V

+

. The input

offset voltage of the NPN stage is untrimmed and is
typically 600

µ

V.

A Schottky diode in the collector of each NPN transistor of
the NPN input stage allows the LT1490/LT1491 to operate
with either or both of its inputs above V

+

. At about 0.3V

above V

+

 the NPN input transistor is fully saturated and the

input bias current is typically 4

µ

A at room temperature.

The input offset voltage is typically 700

µ

V when operating

above V

+

. The LT1490/LT1491 will operate with its inputs

44V above V

–

 regardless of V

+

.

The inputs are protected against excursions as much as
22V below V

–

 by an internal 1k resistor in series with each

input and a diode from the input to the negative supply.
There is no output phase reversal for inputs up to 22V
below V

–

. There are no clamping diodes between the

inputs and the maximum differential input voltage is 44V.

The output voltage swing of the LT1490/LT1491 is af-
fected by input overdrive as shown in the typical perfor-
mance curves. When monitoring voltages within 100mV
of either rail, gain should be taken to keep the output from
clipping.

The output of the LT1490/LT1491 can be pulled up to 18V
beyond V

+

 with less than 1nA of leakage current,  provided

that V

+

 is less than 0.5V.

The normally reverse-biased substrate diode from the
output to V

–

 will cause unlimited currents to flow when the

output is forced below V

–

. If the current is transient and

limited to 100mA, no damage will occur.

The LT1490/LT1491 is internally compensated to drive at
least 200pF of capacitance under any output loading
conditions. A 0.22

µ

F capacitor in series with a 150

Ω

resistor between the output and ground will compensate
these amplifiers for larger capacitive loads, up to 10,000pF,
at all output currents.

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 LT1490 switches between
input stages there is significant nonlinearity in the CMRR.
Lower load resistance increases the output crossover
distortion, but has no effect on the input stage transition
distortion. For lowest distortion the LT1490/LT1491 should
be operated single supply, with the output always sourc-
ing current and with the input voltage swing between
ground and (V

+

 – 0.8V). See the Typical Performance

Characteristics curves.