37 - Slew rate verification for wideband amplifiers: The taming of the slew

Abstract

Slew rate defines an amplifier's maximum rate of output excursion. This specification sets limits on undistorted bandwidth, an important capability in A/D driver applications. Slew rate also influences achievable performance in D/A output stages, filters, video amplification, and data acquisition. Because of its importance, amplifier slew rate must be verified by measurement. The delay time is small and is almost entirely due to amplifier propagation delay. During this interval there is no output movement. During slew time the amplifier moves at its highest possible speed toward the final value. Ring time defines the region where the amplifier recovers from slewing and ceases movement within some defined error band. The total elapsed time from input application until the output arrives at and remains within a specified error band around the final value is the settling time. Slew rate, normally measured during the middle 2/3 of output movement at A = +1, is expressed in volts/microsecond. Discounting the initial and final movement intervals ensures that amplifier gain-bandwidth limitations during partial input overdrive do not influence the measurement. Standard laboratory pulse generators easily supplied rise times well beyond amplifier speeds. As slew rates have crossed 1000 V/μs, the pulse generator's finite rise time has become a concern. A recent device, LT1818, has a 2500 V/μs slew rate, or 2.5 V/ns.