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I'm using an opamp to amplify a 3.5 Vpp (peak to peak) 10 kHz sinewave which seems fine apart from a tiny noise mostly visible at its' peaks which does not particularly concern me much. The blue sinewave is the input and the output is the red sinewave amplified at 6.7 Vpp. There is a distortion and that is at both no-load and loaded conditions. This opamp is meant to be used as a resolver primary coil driver which is the case of my application. The distortion is something that worries me because it could affect the accuracy of the resolver-to-digital converter. My question is what can I do to minimize the distortion as I do not see any reason for it. The datasheet refers to zero crossover distortion and secondly the opamp is "obliged", to a certain extend, to provide an output signal based on the gain factor which is not so great that could drop the BW so low to affect amplification ratio. The BW is 600 kHz and the signal being amplified is 10 kHz.

Can anybody make any suggestions?

Regards

Images:

Schematic

Input & output signals

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  • \$\begingroup\$ Welcome to EE.SE! Which opamp are you using? How close to the rails are you? \$\endgroup\$
    – winny
    Commented Nov 16, 2018 at 8:17
  • \$\begingroup\$ Its a typical slew-induced distortion (triangle form). \$\endgroup\$
    – LvW
    Commented Nov 16, 2018 at 8:37

3 Answers 3

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I haven't gone through the whole circuit to figure out if there are other problems but you appear to be slew rate limited.

Slew Rate = 2πfV = 2*3.14*10k*6.7 = 0.42V/us

Datasheet shows 0.17V/us (typical)

Redo the test with 1kHz input signal and see if the output cleans up. If it does - that was your problem.

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enter image description here

That looks like slew rate limiting to me. The op-amp you are using is specified as having a slew rate of 0.17 volts per microsecond and this is somewhat "slow" for an op-amp. Even the 741 dinosaur op-amp has a slew rate that is 0.5 volts per micro second.

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  • \$\begingroup\$ Beat me to the punch while I was reading the datasheet :) \$\endgroup\$
    – EasyOhm
    Commented Nov 16, 2018 at 8:14
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I would like to thank all of you that took the time to post your thoughts regarding the issue i am facing. I am attaching a few snapshots of the output waveform at various frequencies & amplitudes.

It is obvious that slew rate of the opamp i am using is the culprit. Despite the fact that i have been in the electronics business for 10+ years i took one thing for granted and that was the one and only thing to mess everything up. This opamp was designed to do the exact same thing that i am doing but obviously there are some conditions where issues arise.

So, 1KHz with the amplitude very close to the pos rail and i got a perfect output signal. Now when it comes to 10KHz it is not a problem if i keep the output low enough as the second snapshot implies! Above that amplitude, rising & falling edges of the original signal become steeper and distortion, limited by the slew rate, starts to be visible until i reach an amplitude where rising & falling edges are stuck with the slew rate. I really did not see that comming! I got my lesson.

Many thanks to all of you.

*The yellow dotted horizontal ruler is the positive rail @ 10V

Output is 1KHz @ 9.6Vpp Output is 10KHz @ 5.6Vpp Output is 10KHz @ 7.1Vpp Output is 10KHz @ 8Vpp

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  • \$\begingroup\$ This doesn't appear to be an answer. I suspect it should be added to the question as an update and this post deleted. (This isn't a forum and answers float up and down with votes and user sorting preferences.) Can you copy the markup from this into your question and delete this to save the mods a job? By the way, you could make your images a lot more legible with a white background. \$\endgroup\$
    – Transistor
    Commented Nov 17, 2018 at 14:04
  • \$\begingroup\$ I am sorry but i am confused. I clicked on the button "add answer" and i answered my own question. Why is the button there if i am not supposed to do that? It is certainly an answer as it describes findings and solution to the issues. If i am making anything wrong here please let me know. \$\endgroup\$
    – user204536
    Commented Nov 17, 2018 at 14:51
  • \$\begingroup\$ Ah, OK. It wasn't clear to me on scanning through it. Your approach is correct. If you wish you can even mark your own answer as correct by 'accepting' it to indicate that the question is solved. \$\endgroup\$
    – Transistor
    Commented Nov 17, 2018 at 14:53
  • \$\begingroup\$ I did try to click on the V but i got a message saying "You can accept your own answer is XX hours" . So i guess i will do that later on. Thanks for the response. \$\endgroup\$
    – user204536
    Commented Nov 17, 2018 at 15:01
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    \$\begingroup\$ You' re absolute right. I did not bother learning the basics of working with the site i guess and that is why i did what i did. Appologies. Your comment makes perfect sense. \$\endgroup\$
    – user204536
    Commented Nov 18, 2018 at 4:31

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