Checking Op-Amp Slew Rate for the Required Signal Amplitude

An amplifier with ample small-signal bandwidth can still distort a large waveform because its output cannot change voltage quickly enough. Check amplitude and frequency together. A bandwidth number measured with a small test signal is not a guarantee of undistorted full-amplitude operation.

Calculate the requested slope

TI’s slew-rate training material explains the relationship between signal amplitude, frequency and full-power bandwidth. It distinguishes the large-signal slope requirement from the amplifier’s small-signal frequency response.

For a sinewave, maximum slope is two pi times frequency times peak amplitude. An illustrative output of 2 V peak at 100 kHz requires approximately 1.26 V per microsecond. Use peak amplitude in that calculation, not peak-to-peak voltage.

Compare the actual operating conditions

Check the selected amplifier’s slew-rate specification, including whether it is typical or guaranteed. Review supply voltage, gain, load and temperature conditions. Consider positive and negative transitions separately if the device specifies or exhibits different behavior in the two directions.

Leave appropriate margin for the required distortion performance. Merely matching the calculated maximum sinewave slope to a typical slew-rate value is not a complete low-distortion design criterion. Use the manufacturer’s large-signal and distortion information where available.

Include nonsinusoidal signals

Describe the fastest transition and largest excursion the product must reproduce. A pulse, ramp or multiplexed step may demand different behavior from a steady sinewave. Include overloads that can occur during startup or channel switching, while keeping recovery from saturation as a separate check.

Review output-current demand and capacitive loading at the same time. A measured slow edge can result from more than one limitation. Distinguish voltage slew, current-limited charging and bandwidth effects before choosing a replacement amplifier.

Test amplitude dependence

Measure response at the intended frequency while varying amplitude within the circuit’s allowed range. Observe waveform shape and distortion, not only the output’s fundamental amplitude. Compare with a lower-amplitude measurement to identify behavior that appears only under large-signal conditions.

Retain signal amplitude, frequency, gain, supplies and load with the result. Include the most demanding permitted waveform in the verification record. Revisit the check when increasing output range or frequency, even if the small-signal gain remains unchanged. A defensible bandwidth claim states the amplitude and distortion conditions under which the circuit operates, rather than repeating an amplifier’s headline bandwidth as the usable range of every board-level signal. Include the output load on every performance plot and preserve the distinction between typical component behavior and a requirement verified across the intended operating range.