Hi,

How can help me have a perfect understanding of slew rate?

Why some of manufacturers don't write that parameter in their data sheets?

Thanks

 

Slew rate is affected by capacitance and is just the maximum speed that the output on something like an op-amp can move at to follow an even faster input signal.

Like if the slew rate is 10V/us and you input a signal that is changing slower than that, the output will be just fine. But the input signal has an area where it is changing at 10V/us or higher, the output will be 10V/us since the device's output can't react that fast. So a really vertical input would appear as a steady slope equal to the slew rate.

Some manufacturers just don't put it on their sheet just because.

 

I look at "large signal frequency response" or "max peak output voltage vs frequency" instead of slew rate and see that an old LM324 or LM358 opamp does poorly above only 5kHz, an old 741 opamp does poorly above only 9kHz and a TL07x opamp does well up to 100kHz with sine-waves.

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Uncle $crooge

 

Sometimes, as in process control, you want that oh so limited slew rate because nothing can or should change faster than that.

 

...the machines stay intact, no broken hydraulics, bent actuators, etc, and you get to keep your sanity as well as your job.

 

A circuit that needs a slow slew rate should use an integrating opamp circuit with a capacitor in its negative feedback loop. Then any opamp will work.

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Uncle $crooge

 

Nothing happens (or doesn't happen) - just because.
One good reason for not putting slew rate in datasheets is to save the customer (that's you) money. If they do not specify it, they do not need to test it == less $$$.

Especially since they usually provide enough other information for one to derive what the maximum slew rate should be -as it relates to bandwidth.

 

Thanks,

but the biggest problem for me is "V/us" and calculating it, can you explain more about it?

 

Slew rate is just the rate of rise (or fall) of the signal. See some examples below. Note that you would not normally see multiple slew rates from the same device, although a circuit could easily be designed to do this.
EDIT: If you have a device that slews extremely rapidly, like 1000v/us, this is equivalent to 1v/ns. Slew rates are traditionally specified in v/us, but the units can be scaled otherwise, like v/s, v/ms, etc.

Attached Images

slew rate.PNG (18.4 KB, 11 views)

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Ron

 

At the risk of dating oneself, and anyone else who cares to hold up their hand, LOL, remember the National Semiconductor LH0033 and LH0063 ?

They were called the "Fast" and "Damn Fast" buffers, at 1500V/uS and 6000V/uS. This was written as the tilte, right in the databooks. I think Bob Pease had something to do with the naming. Speaking of Mr Pease, there was once a member here that used a picture of Mr. Pease from one of the trade magazines.

 

I once designed an LH0033 into a product. You can buy op amps today that will slew faster than 6000v/usec.

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Ron