How to measure slew rate of Op-Amp Practically ?

Thread Starter

Aswinth

Joined Nov 1, 2016
31
Hi, I am under a internship and my supervisor wants me to practically measure the slew of AD8602ARMZ op-amp.

I am aware that this value is available in the datasheet along with a well plotted graph, but my task here is to recreate the graph on the oscilloscope. So far I have read the basics of slew rate and learnt how to calculate it if I get a graph. But I am not sure if I am in the right path and if this will be possible.

My knowledge and idea on Op-amp slew rate and how to measure it come from this article here: https://components101.com/article/u...in-op-amp-and-how-to-measure-and-calculate-it

Here is what I have done so far,

1. As discussed in article, I have created a voltage follower circuit using AD8602ARMZ op-amp.
2. I have verified that the op-amp circuit is working my measuring the input and output voltage
4. Used an Arduino to generate PWM signals for the input signal of voltage follower
5. Mounted one channel of my scope to input and other channel to the output of my voltage follower circuit

With that I am stuck. The input wave from arduino and output wave from op-amp both look identical I am not able to measure any change in time or change in voltage.

I have tried changing the freqency of input pulse but it did not help. would appreciate if anyone can suggest a way forward.
 

Ian0

Joined Aug 7, 2020
10,001
What setting do you have for the timebase of your oscilloscope?

By the way, there is a mistake in your instructions:
Screenshot from 2024-03-20 07-44-30.png"100%" should read "10%"
 

WBahn

Joined Mar 31, 2012
30,236
Hi, I am under a internship and my supervisor wants me to practically measure the slew of AD8602ARMZ op-amp.

I am aware that this value is available in the datasheet along with a well plotted graph, but my task here is to recreate the graph on the oscilloscope. So far I have read the basics of slew rate and learnt how to calculate it if I get a graph. But I am not sure if I am in the right path and if this will be possible.

My knowledge and idea on Op-amp slew rate and how to measure it come from this article here: https://components101.com/article/u...in-op-amp-and-how-to-measure-and-calculate-it

Here is what I have done so far,

1. As discussed in article, I have created a voltage follower circuit using AD8602ARMZ op-amp.
2. I have verified that the op-amp circuit is working my measuring the input and output voltage
4. Used an Arduino to generate PWM signals for the input signal of voltage follower
5. Mounted one channel of my scope to input and other channel to the output of my voltage follower circuit

With that I am stuck. The input wave from arduino and output wave from op-amp both look identical I am not able to measure any change in time or change in voltage.

I have tried changing the freqency of input pulse but it did not help. would appreciate if anyone can suggest a way forward.
What is the slew rate of your input signal?

A screenshot or picture of your scope traces would go a long way to make sure that everyone is on the same page.
 

LvW

Joined Jun 13, 2013
1,765
Question: Did you use a REAL opamp (model) with frequency-dependent gain (identical with a certain delay within the opamp)?
Note: It is the delayed feedback signal which is responsible for the slew rate.
 

BobTPH

Joined Jun 5, 2013
9,124
but my task here is to recreate the graph on the oscilloscope.
What graph? If it is slew rate vs temperature for instance, that would be damn near impossible. Can you clip and post the graph, or at least link to the datasheet and tell us what figure you are talking about?
 
Last edited:

crutschow

Joined Mar 14, 2008
34,679
It is the delayed feedback signal which is responsible for the slew rate.
Not sure what that means.
The slew rate of an op amp is usually determined by how fast the internal compensation capacitor can be charged and discharged, for a step change in input, and is only indirectly related to the op amp's gain-bandwidth product.
That's why comparators, which are similar to op amps but without the compensation capacitor, have a much faster output slew rate.

Below is the sim of the op amp, showing its output slew rate for a non-inverting gain of 1.:
Note that the input is a fast rise and fall time, 1MHz square-wave.

1710943688427.png
 

ronsimpson

Joined Oct 7, 2019
3,131
Your scope also has slew rate limits. I think you need a 100mhz scope or faster. You probably need to learn how to connect the ground on the scope without using the long ground clip. The PCB needs to have good layout with bypass capacitors. The input signal needs to get to the amp without distortions from long wires. Your signal is only 5V/uS but you are on the edge of needing to be careful how you connect everything together.
1710944313961.png
 

Ian0

Joined Aug 7, 2020
10,001
Not sure what that means.
The slew rate of an op amp is usually determined by how fast the internal compensation capacitor can be charged and discharged, for a step change in input, and is only indirectly related to the op amp's gain-bandwidth product.
That's why comparators, which are similar to op amps but without the compensation capacitor, have a much faster output slew rate.

Below is the sim of the op amp, showing its output slew rate for a non-inverting gain of 1.:
Note that the input is a fast rise and fall time, 1MHz square-wave.
Usually a delay in the feedback loop is a good way of making it oscillate.
 

LvW

Joined Jun 13, 2013
1,765
Not sure what that means.
The slew rate of an op amp is usually determined by how fast the internal compensation capacitor can be charged and discharged, for a step change in input, and is only indirectly related to the op amp's gain-bandwidth product.
Let me explain:
At the very first moment the input step will bring the 1st stage of the opamp into saturation because the feedback is delayed (not yet active).
As a consequence, the 1st stage works like a current source and will linearly charge the compensation capacitor of the 2nd stage. This effects results in a quasilinear slope of the step response which we call "slew rate".
As soon as the delayed feedback signal arrives at the opamp input the 1st stage - and with it the whole device - will come back to linear operation and the output voltage is the amplified version of the input signal.
 

LvW

Joined Jun 13, 2013
1,765
Usually a delay in the feedback loop is a good way of making it oscillate.
Yes - however, this applies to (a) an opamp which is not unity-gain compensated or (b) to an external delay unit within the feedback path. The slew rate is caused by the unavoidable internal signal delay.
 

crutschow

Joined Mar 14, 2008
34,679
Your scope also has slew rate limits. I think you need a 100mhz scope or faster.
That's way overkill.
The op amp slew rate has a rise-time of a several 100ns, while a 100MHz scope has a risetime on the order of a few ns.
Even a 10MHz scope should be adequate.
(Sim below showing the effect of a 10MHz, 1-pole LP filter on the slew-rate signal, which basically just delays the signal slightly).

1710950738499.png
 

Thread Starter

Aswinth

Joined Nov 1, 2016
31
What is the slew rate of your input signal?

A screenshot or picture of your scope traces would go a long way to make sure that everyone is on the same page.
I am not allowed to take pictures inside office. But still there is an option in the scope itself to take screenshot will share that
 

Thread Starter

Aswinth

Joined Nov 1, 2016
31
Not sure what that means.
The slew rate of an op amp is usually determined by how fast the internal compensation capacitor can be charged and discharged, for a step change in input, and is only indirectly related to the op amp's gain-bandwidth product.
That's why comparators, which are similar to op amps but without the compensation capacitor, have a much faster output slew rate.

Below is the sim of the op amp, showing its output slew rate for a non-inverting gain of 1.:
Note that the input is a fast rise and fall time, 1MHz square-wave.

View attachment 317986
Thank you I will look more into this.

As per my understanding an ideal op-amp should have a slew rate of zero. Meaning slew rates cannot be avoided in a practical op-amp because of the internal compensation capacitor and the lower the slew rate is better the op-amp.

I dint know comparators did not have internal compensation capacitors. Its something new i have to look into after i am done with this

Again thank you for the simulation and graph
 

WBahn

Joined Mar 31, 2012
30,236
Thank you I will look more into this.

As per my understanding an ideal op-amp should have a slew rate of zero. Meaning slew rates cannot be avoided in a practical op-amp because of the internal compensation capacitor and the lower the slew rate is better the op-amp.

I dint know comparators did not have internal compensation capacitors. Its something new i have to look into after i am done with this

Again thank you for the simulation and graph
You have the slew rate backwards. Slew rate is in volts/time (such as V/µs). An ideal opamp has infinite slew rate, while real opamps have finite slew rates.

If the slew rate were zero, the output would never change.

Having a higher slew rate does not always equate to "better". It allows you to work with higher frequency signals, but it also adds high frequency noise to the signal as well. In general, all else being equal, you would choose an opamp that has a comfortably higher slew rate than what you actually need.
 
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