LINEAR High-Speedsettling Time Measurement for Precise Wideband Amplifier AN79 Data Manual User Guide

Application Note 79 September 1999

30 Nanosecond Settling Time Measurement for a Precision Wideband Amplifier Quantifying Prompt Certainty Jim Williams

Introduction SETTLING TIME

INPUT

Instrumentation, waveform generation, data acquisition,

RING TIME

feedback control systems and other application areas utilize wideband amplifiers. New components (see page 2

“A Precision Wideband Dual Amplifier with 30ns Settling ALLOWABLE OUTPUT

Time”) have introduced precision while maintaining high ERROR

speed operation. The amplifier’s DC and AC specifications SLEW

OUTPUT

approach or equal previous devices at significantly lower

DELAY TIME AN79 F01

cost while saving power.

Figure 1. Settling Time Components Include Delay, Slew and

Settling Time Defined

Ring Times. Fast Amplifiers Reduce Slew Time, Although Longer Ring Time Usually Results. Delay Time is Normally a

Amplifier DC specifications are relatively easy to verify.

Small Term

Measurement techniques are well understood, albeit often tedious. AC specifications require more sophisticated approaches to produce reliable information. In particular, pensation. Additionally, the architecture of very fast ampli- amplifier settling time is extraordinarily difficult to deter- fiers usually dictates trade-offs which degrade DC error mine. Settling time is the elapsed time from input applica- terms.1 tion until the output arrives at and remains within a

Measuring anything at any speed requires care. Dynamic

specified error band around the final value. It is usually

measurement is particularly challenging. Reliable nano-

specified for a full-scale transition. Figure 1 shows that

second region settling time measurement constitutes a

settling time has three distinct components. The delay

high order difficulty problem requiring exceptional care

time is small and is almost entirely due to amplifier

in approach and experimental technique.2

propagation delay. During this interval there is no output movement. During slew time the amplifier moves at its

highest possible speed towards the final value. Ring time , LTC and LT are registered trademarks of Linear Technology Corporation. defines the region where the amplifier recovers from Note 1: This issue is treated in detail in latter portions of the text. Also see Appendix D “Practical Considerations for Amplifier

slewing and ceases movement within some defined error

Compensation.

band. There is normally a trade-off between slew and ring

Note 2: The approach used for settling time measurement and its

time. Fast slewing amplifiers generally have extended ring description borrows heavily from a previous publication. See

times, complicating amplifier choice and frequency com- Reference 1.

AN79-1