11
\$\begingroup\$

For curiosity reasons, I want to know why engineers used the term "slew rate" for this op-amp phenomenon, I mean "slew".

I googled it and found this: "turn or slide violently or uncontrollably".

What is the explanation?

\$\endgroup\$
7
  • 1
    \$\begingroup\$ It's the fastest rate the output can change, and it is not under the control of the input signal (which would be demanding an even faster rate) - so, uncontrolled. \$\endgroup\$ Jul 3 at 18:11
  • 3
    \$\begingroup\$ I think it's related to slew a crane which is to rotate the boom about a vertical axis. There is a maximum angular rate which you can turn the load \$\endgroup\$
    – D Duck
    Jul 3 at 18:48
  • 2
    \$\begingroup\$ @DDuck, you're the only one so far to try looking for the term's origin, rather than kind of guessed at it. You should make this an answer. \$\endgroup\$
    – TonyM
    Jul 3 at 18:55
  • 7
    \$\begingroup\$ Find another dictionary. Slewing doesn't have to be violent or uncontrolled. \$\endgroup\$
    – Simon B
    Jul 3 at 19:50
  • 9
    \$\begingroup\$ According to the online dictionary I found, one of the definitions of slew (when used as an intransitive verb) is "to turn, twist, or swing about : pivot." So I think the slew rate is the rate at which something swings. Seems to be a good word choice to me. merriam-webster.com/dictionary/slew \$\endgroup\$
    – mkeith
    Jul 3 at 20:04

4 Answers 4

27
\$\begingroup\$

If "slew" is sliding violently or uncontrollably¹, the "slew rate" is the speed at which something slides violently or uncontrollably.

In an op amp, the slew rate is the maximum speed at which the output voltage can change; if you instantaneously change the input, the output voltage will "slide violently or uncontrollably" to the new stable point, and the speed at which that happens is called the slew rate. Due to how standard op amps work, the slew rate is approximately constant regardless of input or feedback characteristics, so it's a useful characteristic to put on the datasheet.


¹Since a number of people have said this is wrong, I feel I should note that this is not how I would define the word "slew". But the exact definition doesn't matter--the same argument holds whatever definition you use.

\$\endgroup\$
4
  • 2
    \$\begingroup\$ perhaps "sliding uncontrollably" can be further explained...the normal opamp internal mechanism that sets the linear bandwidth is further constrained by slew. The transition from normal linear behavior to slewing behavior is abrupt. Slewing often shows up when output swings are large & fast. \$\endgroup\$
    – glen_geek
    Jul 3 at 19:17
  • 1
    \$\begingroup\$ And the termination of the slewing can sometimes be inelegant (i.e., cause ringing). Usually the op-amp behavior is tidy around slewing events, but I would not knowingly design a circuit that hits the slew rate limitation without also checking the circuit operation both in simulation and with prototype circuits. \$\endgroup\$
    – TimWescott
    Jul 4 at 0:12
  • 4
    \$\begingroup\$ This answer is quite implausible. I did a Google Books search on the term "slew rate" for the first half of the 20th century, and it looks like the phrase commonly meant "the rate at which something turns to point in a desired new direction." It would make perfect sense to apply the same term to op-amps as a metaphor. On the other hand, when an op-amp is responding to a sudden change in input, the output moves in the desired way at a pre-defined speed, and so it wouldn't make sense at all to describe that using a word that means "slide violently or uncontrollably." \$\endgroup\$ Jul 4 at 15:47
  • \$\begingroup\$ @TannerSwett I'm going based on what the asker said it means. The same argument works either way. \$\endgroup\$
    – Hearth
    Jul 4 at 15:49
16
\$\begingroup\$

I think the term slew rate is derived from servo engineering which in turn acquired this from a nautical term which means to swing a gun or rigging about a vertical axis.

The Oxford English Dictionary uses as a reference for slew: 1962 L. A. Stockdale Servomechanisms vii. 112 The slewing time may form part of the servo specification, i.e. the servo to slew through 90° in the minimum time.

\$\endgroup\$
1
  • \$\begingroup\$ If word origins interest you, I can tell you that we use "slewing" on a daily basis in the flight simulation industry. Positioning the plane in space directly is a very violent operation that can easily crash it due to drastically different pressure vectors. The opposite of that is "trimming", which slowly moves the plane in place, adjusting the forces affecting it as we go. \$\endgroup\$
    – Blindy
    Jul 6 at 15:56
5
\$\begingroup\$

Slew derives from a nautical term, and you can find many 1800s references to slewing a cannon on its trunnions (that would be a vertical movement).

The earliest recorded usage (according to the OED) is from Falconer's Marine dictionary (1769):

To SLUE, is to turn any cylindrical or conical piece of timber about its axis, without removing it. This term is generally expressed of the movement by which a mast or boom is turned about, in its cap or boom-iron.

In the context of an amplifier, the slew rate is what you see when the input is overdriven and the output is changing as fast as possible. It's generally a linear slope (constant dv/dt) for an op-amp. If you have (say) a sine wave that is being distorted by slew rate limitation you may be able to reduce the distortion greatly by reducing the signal level (thus calling for proportionally less dv/dt). Bandwidth limitations will (ideally, anyway) equally affect large and small signals.

\$\endgroup\$
3
  • \$\begingroup\$ I see no justification for saying that "slew" is a nautical term - plenty of reason to think that it was used on dry-land earlier than this. \$\endgroup\$ Jul 5 at 9:42
  • 3
    \$\begingroup\$ @MikeBrockington perhaps, but the OED folks found zero such evidence. \$\endgroup\$ Jul 5 at 10:26
  • 1
    \$\begingroup\$ At risk of being totally off-topic, or exponentially off-topic, @MikeBrockington perhaps you could cite just one of the plentiful reasons to believe that slew was used by landlubbers earlier than 1769. \$\endgroup\$
    – mkeith
    Jul 6 at 13:42
4
\$\begingroup\$

"Slew Rate" is the max. output slope e.g. ΔV/Δt = Io(max)/C(load) due to the internal current limiter.

This is regardless of the feedback at the maximum bandwidth of unity gain. In this sense, this becomes an open-loop like a comparator as it is "the uncontrollable maximum slope" except for the standard test load capacitance such as 20 pF and the internal regulated current limiter used for short circuit protection.

Generally in electronics slew rate is controlled by Io max and C load is not chaotic.

In some cases, low input and/or load resistance is needed to speed up to the specified max. rate as defined in the datasheet. Pay attention to the plots in the datasheet.

ΔV/Δt = Ic(max)/C load=20pF for a specified step input voltage and supply voltage at 25'C

Not all Op Amps have slew rate specs, although they may show a large signal response to a square wave. e.g. LM358 specs. show a large signal square response with a linear ramp of 2.5V/4us.

Op Amp slew rates can range from 400 µV/µs to 18200 V/µs.

For ESL users

  • the use of adjectives with nouns in english vs verb of the same word, can easily change the entire meaning by changing the context of the verb to an adjective of the noun "rate".
\$\endgroup\$
6
  • 4
    \$\begingroup\$ This does not answer the question, which was: why the term 'slew'? This answers: what is a slew rate. Downvoting accordingly, I'm afraid. \$\endgroup\$
    – TonyM
    Jul 3 at 18:53
  • 2
    \$\begingroup\$ No fears @TonyM I stand by my answer , as the electronic question is in the title , whereas the confusion is the use of adjectives with nouns in english vs verb of the same word alone can easily change the entire meaning by context and then that puts your comment and question on the ESL site. \$\endgroup\$ Jul 3 at 19:31
  • 1
    \$\begingroup\$ @TonyM To answer the question "why the term 'slew'" it is useful to know "what is a slew rate" \$\endgroup\$
    – Uwe
    Jul 3 at 20:01
  • 1
    \$\begingroup\$ @Uwe, I don't understand your comment. The answer TS had posted (since had a minor edit) does not address the question asked, as I noted. \$\endgroup\$
    – TonyM
    Jul 3 at 21:13
  • 1
    \$\begingroup\$ @TonyM twix wrote "I would like someone to explain it more technically." And Tony Stewart EE75 did explain it more technically. So I think your comment is wrong. \$\endgroup\$
    – Uwe
    Jul 3 at 21:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.