# calculate slew rate

Discussion in 'Homework Help' started by Rail Ranger, Sep 1, 2006.

1. ### Rail Ranger Thread Starter Member

Aug 22, 2006
15
0
i just took a practice exam of which a question took me at least one third the timed test to try to answer, causing me to miss one third of all 30 questions, it was question 16 of 30, and attached is the exam question. based on the given variables, i am at a complete loss on how to calculate by any kind of formula the answer desired. the question deals with calculating the minimum slew rate of an op-amp that can be used to avoid distortion. this practice test is from the electronic's technician program, if anyone is familiar with it. looking at the givens, one would see where i'm coming from at my distate for this program to learn electronics from as required text and computer program in my apprenticeship. the exam was for 90 minutes with just 30 problems.

i really don't know how to find the slew rate. what variables are needed to calculate slew rate to be able to answer the question?

anybody with the patience to elaborate, please do.

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2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Hi,

The relationship of frequency to time is T = 1/f. That tells you how fast the amp's output has to swing in order to keep up with the waveform. The magnitude of the signal gives the clue as to how many volts per unit of time the op amp has to be able to swing in order to make the slew rate (or at least not distort the waveform).

Just to have fun, figure the amswer in KV/fortnights.

3. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
You said the answer was 479V/ns? How come the page you posted says 436V/ns?

The formula for a sine wave is v=A*sin(wt), where A is the peak amplitude. The maximum slew rate of a sine wave occurs during zero crossings (zero radians, pi radians). To find the maximum slew rate, we need to find the derivative of A*sin(wt) at zero radians.

slewrate(max)=d(A*sin(wt))/dt=Aw*cos(wt), evaluated at wt=0.
A=v(rms)*sqrt(2) (this is the peak amplitude)
A=135*1.414=190.92V
cos(0)=1
w=2*pi*f=2*pi*624e6=3.921e9
slewrate(max)=190.92*3.921e9*1
slewrate(max)=748.5e9=748.5V/ns

This assumes that you only need unity gain. If you need gain, the slew rate must be multiplied by the gain.

I think both the answers you posted are wrong.

BTW, this is an impossible task for an op amp.

4. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
Good practice is to ignore any question that takes more than two minutes to answer. Go back to the unanswered questions once all others are done, time permitting.

5. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Ron H has hit the nail on the head here. I think the key aspects that this question was testing was that you understand that you need to convert the RMS voltage to a peak voltage to calculate the Slew Rate, and that you can do some basic rearrangement of the Slew Rate formula. Also you need to recognise 748.5e+9 equates to 748.5 V/ns.

Dave

6. ### Rail Ranger Thread Starter Member

Aug 22, 2006
15
0

appreciate your input, normally, i do just that, i bypass the hard ones, get the easiest done first, go back and tackle them again. all the while, using the print screen button and pasting into word to document my tests. found it useful too, buggy software and all, i have concrete evidence to present to the official who i need to see, to document where the program falls short.

anyhow, i got caught up in the question, determined to try to answer it, and failed it miserably. the beauty of the practice test? i can take it over again, until i'm ready for the final which requires a floppy, no turning back so to speak. still, even during the floppy, for real final exam, i can still use alt+tab and print screen buttons, to document my questions, to refer to, when i need to make corrections to my wrong answers. which also answers other related question from the other forum member, that being, why i had a wrong answer and the other answer that was generated at end of test. i only displayed the facts in my query.

Aug 22, 2006
15
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8. ### Rail Ranger Thread Starter Member

Aug 22, 2006
15
0
kv i can figure to be kilovolts, but what's fortnights? i have no clue unless it's an adverb meaning the night before? now, that would be for fun.

9. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Google can find almost anything - even the definition of "fortnight".

10. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
2 weeks, 14 days, 336 hours, 20160 minutes, 1209600 seconds.