Detailed explanation and analysis of op amp parameters - part 18, slew rate (SR)

Jacktang

Detailed explanation and analysis of op amp parameters - part 18, slew rate (SR) [Copy link]

I always think that the slew rate (SR) of an op amp is as important as its gain-bandwidth product GBW. But it is often overlooked. The reason why it is important is that the gain-bandwidth product GBW is tested under small signal conditions. The signals processed by the op amp are often very large amplitude signals, which requires more attention to the slew rate of the op amp.

The slew rate can be understood as the maximum change speed of the op amp output signal when a step signal is input to the op amp, as shown in the figure below.

Its mathematical expression is:

Therefore, the unit of slew rate found in the op amp data sheet is V/us. The following table is the slew rate of the op amp marked in the op amp datasheet.

I have measured the waveform of OPA333's response to a step signal in the lab, as shown below. I hope it will be more intuitive for everyone:

After discussing the definition and phenomenon, let's look at the source of the slew rate SR. First, let's look at the internal structure of the op amp:

This diagram looks familiar. Yes, the SR of the op amp is mainly limited by the internal second-stage Cc capacitor. This capacitor also determines the bandwidth of the op amp. The slew rate of the op amp is mainly determined by the speed of charging the second-stage Miller capacitor. If you look deeper, the size of this capacitor will affect the slew rate of the op amp, and the size of the charging current will also affect the charging speed. This also explains why the slew rate of general ultra-low power op amps is not too high. It's like a slow water flow and a large pool. It will take longer to fill the pool.

The following table shows the slew rate and quiescent current of some commonly used TI op amps:

The above briefly talked about a factor that affects the slew rate SR. Now let's talk about the impact of SR on the amplifier circuit. Its direct impact is to make the rise time or fall time of the output signal too slow, thus causing distortion. The figure below is the waveform of the OPA333 when the gain G=10 is tested. Since the gain-bandwidth product of OPA333 is 350kHz, the bandwidth is theoretically 35kHz when the gain is 10. But the figure below is the result of the test at 24kHz. Obviously, the output waveform has been distorted because the slew rate is not enough. The bandwidth has also become about 27kHz.

powerlove

The information is very good. It would be even better if there was a detailed PPT explanation. Thank you.