Slew Rate and Slew Rate Limiting

[Puppet123]

Hello,

I have a question about slew rates and slew rate limiting.

Is I measure the slew rate for an opamp - say positive and negative, and I also measure the maximum rate of change at the output of the opamp.

If the value of the maximum change of the output of the opamp is greater than the slew rates positive and negative (single ended output) - then I dont get slew rate limiting OR is it less than the positive and negative slew rates ?

Where is this explain, what text ?

Thank you.

[KlausST]

Hi,

Is I measure the slew rate for an opamp - say positive and negative, and I also measure the maximum rate of change at the output of the opamp.

Please explain:
Are you talking about a
* differential output Opamp
* or falling and rising signal

What's the idea behind your question?
Usually one does not like slew rate limiting at Opamp output at all. ...because this will cause a not correctly feedbacked regulation loop. In this situation the loop is not closed anymore. This will cause distortion in best case...but one can exoect a lot of ringing maybe oscillations. In case of slew rate limitation at least one stage of the Opamp becomes saturated ... and it may take some "regeneration time" until it comes back to regulated state.

Klaus

[LvW]

If the value of the maximum change of the output of the opamp is greater than the slew rates positive and negative (single ended output) - then I dont get slew rate limiting OR is it less than the positive and negative slew rates ?

Your question is not quite clear to me.
The slew rate is identical to the maximum possible large-signal rise time of the opamp (with feedback !!). It is given in Volts per µsec.
Example: If you want to use an opamp for amplifying sinus signals, the maximum slope of the sinus (zero crossing) must not be greater than the slew rate of the opamp.
Otherwise, the ouput will look like a triangle wave.

[dick_freebird]

Every amplifier has a maximum slew rate.

Internally compensated op amps have lower slew rates,
dominated by the comp cap and internal bias currents.
This is one reason you see "10X gain stable" op amps,
not everybody likes one specific tradeoff.

You can even find "programmable" op amps with a bias
pin which will afford you some flexibility in setting slew
rate (along with fellow travelers) by external component
values.

[asdf44]

Is the OP saying that they observed an opamp output changing faster than the published slew rate limits?

If so I'd say that slew rates are usually characterized as a single number that has to apply across a wide operating range and so I'd expect to see peak slew rates exceeding the published spec in many instances.

[dick_freebird]

Slew rate depends on things like input overdrive, and this is often
made substantial to put the slew rate in the best light (fastest).
At low overdrive you will see much less as the front end current
is only partially steered and the gain stage "fights itself" as far
as throwing slew current out the back.