LT1028AM datasheet(13/20 Pages) LINER

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LT1028/LT1128

Frequency Response

The LT1028’s Gain, Phase vs Frequency plot indicates that

the device is stable in closed-loop gains greater than +2 or

–1 because phase margin is about 50

° at an open-loop

gain of 6dB. In the voltage follower configuration phase

margin seems inadequate. This is indeed true when the

output is shorted to the inverting input and the noninvert-

ing input is driven from a 50

Ω source impedance. How-

ever, when feedback is through a parallel R-C network

(provided CF < 68pF), the LT1028 will be stable because of

interaction between the input resistance and capacitance

and the feedback network. Larger source resistance at the

noninverting input has a similar effect. The following

voltage follower configurations are stable:

Another configuration which requires unity-gain stability

is shown below. When CF is large enough to effectively

short the output to the input at 15MHz, oscillations can

occur. The insertion of RS2 ≥ 500Ω will prevent the

LT1028 from oscillating. When RS1 ≥ 500Ω, the additional

noise contribution due to the presence of RS2 will be

minimal. When RS1 ≤ 100Ω, RS2 is not necessary, be-

cause RS1 represents a heavy load on the output through

the CF short. When 100Ω < RS1 < 500Ω, RS2 should match

RS1 . For example, RS1 = RS2 = 300Ω will be stable. The

noise increase due to RS2 is 40%.

APPLICATI

I FOR ATIO

terminals, can exceed the inherent drift of the amplifier

unless proper care is exercised. Air currents should be

minimized, package leads should be short, the two input

leads should be close together and maintained at the same

temperature.

The circuit shown to measure offset voltage is also used

as the burn-in configuration for the LT1028/LT1128.

1028/1128 AI09

–

33pF

LT1028

Ω

–

LT1028

Ω

500

Ω

1028/1128 AI10

RS1

RS2

LT1028

–

Unity-Gain Buffer Applications (LT1128 Only)

When RF ≤ 100Ω and the input is driven with a fast, large-

signal pulse (>1V), the output waveform will look as

shown in the pulsed operation diagram.

During the fast feedthrough-like portion of the output, the

input protection diodes effectively short the output to the

input and a current, limited only by the output short-circuit

protection, will be drawn by the signal generator. With RF

≥ 500Ω, the output is capable of handling the current

requirements (IL ≤ 20mA at 10V) and the amplifier stays

in its active mode and a smooth transition will occur.

As with all operational amplifiers when RF > 2k, a pole will

be created with RF and the amplifier’s input capacitance,

creating additional phase shift and reducing the phase

margin. A small capacitor (20pF to 50pF) in parallel with RF

will eliminate this problem.

Test Circuit for Offset Voltage

and Offset Voltage Drift with Temperature

–

–15V

10k*

200

Ω*

LT1028

LT1128

1028/1128 AI08

10k*

VO = 100VOS

* RESISTORS MUST HAVE LOW

THERMOELECTRIC POTENTIAL

V O

15V

–

1028/1128 AI07

OUTPUT

6V/

µs

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