LINEAR Application Note 94 Documentation User Guide

Application Note 94

May 2003

Slew Rate Verification for Wideband Amplifiers The Taming of the Slew Jim Williams

INTRODUCTION Slew rate defines an amplifier’s maximum rate of output total elapsed time from input application until the output excursion. This specification sets limits on undistorted arrives at and remains within a specified error band bandwidth, an important capability in A/D driver applica- around the final value is the settling time.1 tions. Slew rate also influences achievable performance in

Slew rate, normally measured during the middle 2/3

D/A output stages, filters, video amplification and data

of output movement at A␣ =␣ +1, is expressed in volts/

acquisition. Because of its importance, amplifier slew rate

microsecond. Discounting the initial and final move-

must be verified by measurement. Deriving a measure-

ment intervals ensures that amplifier gain-bandwidth

ment approach requires understanding slew rate’s rela-

limitations during partial input overdrive do not influ-

tionship to amplifier dynamics.

ence the measurement.

Amplifier Dynamic Response Historically, slew rate measurement has been relatively

simple.2 Early amplifiers had slew rates of typically 1V/µs,

Figure 1 shows that amplifier dynamic response compo-

with later versions sometimes reaching hundreds of

nents include delay, slew and ring times. The delay time is

volts/µs. Standard laboratory pulse generators easily sup-

small and is almost entirely due to amplifier propagation

plied rise times well beyond amplifier speeds. As slew rates

delay. During this interval there is no output movement.

have crossed 1000V/µs, the pulse generator’s finite rise

During slew time the amplifier moves at its highest

time has become a concern. A recent device, the LT1818

possible speed towards the final value. Ring time defines

(See Box Section, “A 2500V/µs Slew Rate Amplifier with

the region where the amplifier recovers from slewing and ceases movement within some defined error band. The

A = 1V/DIV (UNCALIB)

INPUT

RING TIME

SLEW RATE MEASUREMENT

B = 1V/DIV

REGION IS MIDDLE

(INVERTED)

2/3 OF TRANSITION

OUTPUT

TIME 1ns/DIV AN94 F02

DELAY TIME

Figure 2. LT1818 Slew Rate (Upper Trace) is Comparable to

AN94 F01

Schottky TTL Transition Time (Lower Trace)

Figure 1. Amplifier Response Components Include Delay, Slew and Ring Times. Slew Rate is Typically Measured During Middle , LTC and LT are registered trademarks of Linear Technology Corporation. 2/3 of Slew Time Note 2. The term “slew rate” has a clouded origin. Although used for

many years in amplifier literature, there is no mention of it on the Philbrick K2-W (the first standard product op amp, introduced in

Note 1. Although not considered here, settling time determination is a

January 1953) data sheet, dated 1964. Rather, the somewhat more

high order measurement challenge. It is treated in considerable detail

dignified “maximum rate of output swing” is specified.

in References 2, 3, and 4.

AN94-1