What is slew rate? It's a key metric that measures the “speed limit” of voltage changes at the output stage of an operational amplifier. If the required rate of change in the input signal exceeds this limit, the output waveform will become distorted.

Below is a diagram from the LM358DT data sheet. Use this diagram to understand the specific characteristics of the slew rate.

The LM358DT operational amplifier datasheet specifies a typical slew rate of 0.6V/μs and a minimum slew rate of 0.3V/μs. As shown in the diagram, when an input pulse signal is applied at 5 μs, the output signal begins climbing from 0.5V. It takes 10 μs to reach the pulse signal's voltage of 3V. Therefore, the slew rate depicted in the diagram is calculated as (3V - 0.5V) / (10 μs - 5 μs) = 0.5V/μs. The distortion in the output waveform occurs because the required rate of change for the input signal is infinite, exceeding the operational amplifier's “speed limit,” resulting in distortion of the OUTPUT waveform.

Now let's look at another operational amplifier OPA182IDR with a higher slew rate. This amplifier has a typical slew rate of 10 V/μs.

Comparing these two waveforms reveals that operational amplifiers with higher slew rates exhibit lower distortion and more closely resemble the input signal. Therefore, to achieve better signal fidelity, a higher slew rate is preferable.

Correspondingly, oscilloscope and probes with higher bandwidth exhibit greater slew rate in reconstructing pulse signals, resulting in a restoration that more closely approximates the original waveform. Of course, this does not imply an unconditional pursuit of higher bandwidth for oscilloscopes and probes. If the original waveform changes slowly—such as a sine wave signal in the tens of kHz range—and a 200kHz probe adequately meets the requirements for reproducing the measured signal's slew rate, there is no need to incur the higher cost of purchasing a 20MHz or higher bandwidth probe.