Please could someone define slew rate?

WBahn

Joined Mar 31, 2012
29,978
I don't have to prove anything.

All I am saying is slew is already V/s.
Hence slew rate is V/s/s.
And apparently the world is supposed to agree with you simply because YOU are saying it.

I've asserted that slew is NOT a rate and backed up the latter assertion with references to how it is defined and how it has been used in several contexts, including electronics. I have further tried to find even a single reference that agrees with your position and been unable to.
 

WBahn

Joined Mar 31, 2012
29,978
Ok, I can live with that.

Next question, why do people say "baud rate" instead of "baud"?
No one has claimed that baud is not a rate, only that slew is not a rate. Why does it have to be all or nothing?

Baud IS a rate -- though many people confuse baud with bit rate. Using the term baud rate IS technically redundant; that's not going to stop or slow it down, but well written work should make the effort to use it correctly.
 

WBahn

Joined Mar 31, 2012
29,978
For voltage, take V(t) at 10% and 90% of the final value of v(t). (V2(t2)-V1(t1))/(t2-t1)

Often expressed in V/uS or Volts per microsecond
This is not the slew rate. It is merely the average rate of change of voltage between two arbitrarily chosen points on a waveform.

The slew rate is the maximum instantaneous rate of change of the voltage (or whatever the output happens to be) of the output.
 
Ok, suppose I have an audio amplifier. I want to determine it's slew rate. Now what?

There is also a relationship I remember that guesses Bandwidth by doing a measurement.

What's the relationship between slew rate and rise time?

Maybe rise time is the 10/90 meaurement?
 

mvas

Joined Jun 19, 2017
539
A High Slew Rate TL08xx FET-Input Op-Amp

In this datasheet
http://www.ti.com/lit/ds/symlink/tl084.pdf
and all Op-Amp documentation that I have ever read ... states "Slew Rate", never "Slew"
Examples:
... High Slew Rate: 13 V/μs typical ...
... The devices feature high slew rates ...
... SR - Slew rate at unity gain ...
... The devices feature high slew rates ...
... 8.3.2 Slew Rate ...
... The slew rate is the rate at which an operational amplifier can change its output when there is a change on the input ..
... These devices have a 13-V/μs slew rate ...

Op-Amp Slew Rate ...
https://www.electronics-notes.com/a...ts/operational-amplifier-op-amp/slew-rate.php
 

WBahn

Joined Mar 31, 2012
29,978
Ok, suppose I have an audio amplifier. I want to determine it's slew rate. Now what?
You put in a signal that is fast enough and large enough that the output can't keep up. At that point the amplifier's output is being slew-rate limited. Many (most?) amplifiers, particularly if they are intended to have high linearity, will exhibit a linear change in voltage. That rate is the slew rate.

You need to have either no load or a sufficiently light load so that it's time constant does not effect the measurement.
 
Nor would we. Just as we wouldn't say that an aircraft that had a roll rate of 60°/sec was roll rating at 60°/sec, we would say that it was rolling at 60°/sec.

You've done nothing but reinforced my case.



If someone says that the car is speeding at 60 mph they almost certainly do not mean that the speed of the car is 60 mph, but rather that at a speed of 60 mph the car is speeding (i.e., exceeding the speed limit) or some other implied use where the word "speeding" is not directly tied to the 60 mph, such as the car is speeding (down the road) at 60 mph.

And, no, we don't say that the speed rate is 60 mph because speed IS a rate.



Then it should be a trivial matter to come up with all kinds of references from at least some of the hundreds of sites dedicated to proper grammar and used to back up the claim that "slew rate" is improper.
 

MrAl

Joined Jun 17, 2014
11,389
The amplifier's output is slewing at 20V/μs. We don't say the amplifier is slew rating at 20V/μs.

We say a car's speed is 60mph. The car is speeding at 60mph.
We don't say the speed rate is 60mph.

Slew rate and baud rate are examples of poor grammar, regardless of common usage.
Then i guess you have also heard the arguments that "current flow" is bad grammar too because "current" is already a flow of electrons. So it is like we are saying "electron flow flow". ha ha.
To me it is ok though because that's the way we as humans abstract things. We call a "hole" in the ground something physical even though there is nothing there.
 

MrAl

Joined Jun 17, 2014
11,389
Slew rate is in the context of an op amp is a measure of the amplifiers output ability to change from one voltage level to the next voltage level given an input that is faster like a step input.

For example, if the gain was 1 and the output was 0v and the input changed from 0v to 1v and the output took 1us to get to 1v from 0v then the slew rate would be 1 volt per microsecond. If the input stepped from 0v to 2v and the output took 2us to get from 0v to 2v then the slew rate is still 1 volt per microsecond. If the input went from 0v to 1v and the output went from 0v to 1v in 2us then the slew rate would be 0.5 volts per microsecond. If the input went from 0v to 4v and the output went from 0v to 2v in 1us then the slew rate is 2 volts per microsecond. IF the input went from 0v to 10v and the output went from 0v to 2v in 1us then the slew rate is still 2 volts per microsecond.
So it is a measure of how fast the output can get from one level to another level assuming we have a straight line ramping output. The input is ideally a step input and we measure two points at the output v1 and v2 at times t1 and t2 respectively. The slew rate is then:
slew rate = (v2-v1)/(t2-t1)

What this means in the practical sense is how well the amplifier can follow an input signal. If the slew rate is not fast enough then the amplifier may not be able to reproduce a sine wave above a certain frequency, so it is an important measure of the performance of the op amp.
For example, an amplifier that has a slew rate of 2 volts per microsecond can follow a fast signal better than an amplifier that has a slew rate of 1 volt per microsecond, and an amplifier that has a 10 volts per microsecond slew rate can follow an even faster signal than the 2v/us amplifier.

Once you start working with this idea and do a few examples you start to get a good feel for what the slew rate tells you about the particular amplifier. You could try a few examples.
For example, we have an amplifier that has a gain of 1 and the input step goes from 1v to 2v and the slew rate of the amplifier is 5 volts per microsecond. How long does it take the amplifier output to reach steady state?
Hint:
Remember the formula:
slew rate = (v2-v1)/(t2-t1)
 

WBahn

Joined Mar 31, 2012
29,978
If David killed 0ne giant per week. Would he have a slew rate of 52?
Nope, since even a slew rate per your implied definition is not a dimensionless quantity.

He would have a slew rate of 52 giants/year (allowing for there being only approximately 52 weeks in a year). But, equivalently he would have a slew rate of 1 giant/week, or 0.1429 giants/day, or 5.95 milligiants/hour, or 1.65 microgiants/second.

Units matter!
 
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