LT1055MH datasheet(10/16 Pages) LINER

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LT1055/LT1056

10556fc

APPLICATIONS INFORMATION

The voltage noise spectrum is characterized by a low 1/f

corner in the 20Hz to 30Hz range, significantly lower than

on other competitive JFET input op amps. Of particular

interest is the fact that with any JFET IC amplifier, the

frequency location of the 1/f corner is proportional to the

square root of the internal gate leakage currents and,

therefore, noise doubles every 20

°C. Furthermore, as

illustrated in the noise versus chip temperature curves,

the 0.1Hz to 10Hz peak-to-peak noise is a strong function

of temperature, while wideband noise (f0 = 1kHz) is

practically unaffected by temperature.

Consequently, for optimum low frequency noise, chip

temperature should be minimized. For example, operating

an LT1056 at

±5V supplies or with a 20°C/W case-to-

ambient heat sink reduces 0.1Hz to 10Hz noise from

typically 2.5

µVP-P (±15V, free-air) to 1.5µVP-P. Similiarly,

the noise of an LT1055 will be 1.8

µVP-P typically because

of its lower power dissipation and chip temperature.

High Speed Operation

Settling time is measured in the test circuit shown. This

test configuration has two features which eliminate prob-

lems common to settling time measurments: (1) probe

capacitance is isolated from the “false summing” node,

and (2) it does not require a “flat top” input pulse since the

input pulse is merely used to steer current through the

diode bridges. For more details, please see Application

Note 10.

As with most high speed amplifiers, care should be

taken with supply decoupling, lead dress and component

placement.

When the feedback around the op amp is resistive (RF), a

pole will be created with RF, the source resistance and

capacitance (RS, CS), and the amplifier input capacitance

(CIN ≈ 4pF). In low closed-loop gain configurations and

with RS and RF in the kilohm range, this pole can create

excess phase shift and even oscillation. A small capacitor

(CF) in parallel with RF eliminates this problem. With RS

(CS + CIN) = RFCF, the effect of the feedback pole is

completely removed.

0.01 DISC

–

15V

–15V

15k

10k

4.7k

µF

SOLID

TANTALUM

µF

SOLID

TANTALUM

µF

SOLID

TANTALUM

µF

SOLID TANTALUM

10pF (TYPICAL)

10k

PULSE GEN

INPUT

(5V MIN STEP)

0.01 DISC

Ω

LT1055

LT1056

AMPLIFIER

UNDER

TEST

AUT OUTPUT

HP5082-8210

HEWLETT

PACKARD

15V

–15V

= 1N4148

–15V

1/2

U440

1/2

U440

Ω

100

Ω

DC ZERO

2N160

2N5160

2N3866

LT1055/56 AI04

OUTPUT

TO SCOPE

Settling Time Test Circuit

–

CIN

OUTPUT

LT1055/56 AI03

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