7

LT1122

S

APPLICATI

I FOR ATIO

excess phase shift and even oscillation. A small capacitor

completely removed.

Settling Time Measurements

Settling time test circuits shown on some competitive

devices’ data sheets require:

1. A “flat top” pulse generator. Unfortunately, flat top

pulse generators are not commercially available.

2. A variable feedback capacitor around the device under

test. This capacitor varies over a four to one range.

Presumably, as each op amp is measured for settling

time, the capacitor is fine tuned to optimize settling

time for that particular device.

3. A small inductor load to optimize settling.

The LT1122’s settling time is 100% tested in the test

circuit shown. No “flat top” pulse generator is required.

The test circuit can be readily constructed, using commer-

cially available ICs. Of course, standard high frequency

board construction techniques should be followed. All

LT1122s are measured with a constant feedback capaci-

tor. No fine tuning is required.

Speed Boost/Overcompensation Terminal

Pin 8 of the LT1122 can be used to change the input stage

operating current of the device. Shorting pin 8 to the

positive supply (Pin 7) increases slew rate and bandwidth

by about 25%, but at the expense of a reduction in phase

margin by approximately 18 degrees. Unity gain capaci-

tive load handling decreases from typically 500pF to

100pF.

Conversely, connecting a 15k resistor from pin 8 to

reduces slew rate and bandwidth by 25%. Phase margin

and capacitive load handling improve; the latter typically

increasing to 800pF.

High Speed Operation

As with most high speed amplifiers, care should be taken

with supply decoupling, lead dress and component

placement.

The power supply connections to the LT1122 must main-

tain a low impedance to ground over a bandwidth of

20MHz. This is especially important when driving a signifi-

cant resistive or capacitive load, since all current delivered

to the load comes from the power supplies. Multiple high

quality bypass capacitors are recommended for each

power supply line in any critical application. A 0.1

µF

ceramic and a 1

µF electrolytic capacitor, as shown, placed

as close as possible to the amplifier (with short lead

lengths to power supply common) will assure adequate

high frequency bypassing, in most applications.

OUTPUT

+

–

LT1122•TA04

µ

V+

7

2

6

3

4

1 F

µ

0.1 F

µ

1 F

µ

0.1 F

+

V–

LT1122

LT1122•TA03

+

+

–

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of circuits as described herein will not infringe on existing patent rights.