Suitable Opamp for this Low Pass Filter

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Discussion Overview

The discussion revolves around selecting suitable operational amplifiers (op-amps) for use in a low pass filter, focusing on the criteria of slew rate and input offset voltage. Participants analyze specific op-amps from a provided table and assess their appropriateness based on the filter's requirements.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant identifies the TLV2721 as unsuitable due to its slow slew rate of 0.18 V/µs.
  • Another participant calculates the cutoff frequency to be approximately 115 kHz and seeks guidance on evaluating other op-amps.
  • Discussion on the importance of comparing the slew rate at the steepest part of the waveform, particularly at the first 0.1 µs.
  • Participants debate the correct interpretation of slew rate calculations, with some suggesting a value of 2 V/0.1 µs, leading to a slew rate of 20 V/µs.
  • There is a suggestion that the TLC071 is the best option available, although it may still not meet the required specifications.
  • One participant expresses confusion about the calculations and seeks clarification on the slew rate derived from the graph.
  • Another participant notes that being faster than required should not disqualify an op-amp, although cost and power consumption may be considerations.
  • Discussion on the relevance of input offset voltage, with some participants suggesting that it may not be critical depending on the circuit configuration.
  • Participants acknowledge the limitations of the information provided, primarily relying on the step response graph without additional circuit parameters.

Areas of Agreement / Disagreement

Participants express differing views on which op-amps are suitable, with some suggesting that only one op-amp is appropriate while others argue that additional options may still be valid despite being above spec. The discussion remains unresolved regarding the implications of input offset voltage and the overall suitability of the op-amps.

Contextual Notes

Participants note the lack of additional circuit parameters, which limits the ability to make definitive conclusions about the suitability of the op-amps beyond the provided step response graph.

Whiley

Homework Statement


[/B]
Select (with justification) from the op-amps listed in TABLE A those which are NOT suitable for use as the active component in the filter of (a) above.

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Homework Equations

The Attempt at a Solution


Currently The only Op amp I see that is not suitable is TLV2721 due to slow slew rate of 0.18 V per micro second.
 

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I have found that the cutoff frequency is approximatly 115kHz.

If anyone could point me in the right direction with what I might be looking for with respect to the other aspects to each of the op amps please.
 
Last edited by a moderator:
Welcome to the PF. :smile:

I would look at that slew rate for the first 0.1us, and make my choices based on that...
 
Thank you and thanks for your reply,

Do you mean that I should find the slew rate that we have based on the first 0.1us and compare to the op amps from the table?
 
Whiley said:
Thank you and thanks for your reply,

Do you mean that I should find the slew rate that we have based on the first 0.1us and compare to the op amps from the table?
Yes, that is what I would do. Look at the very start of the waveform where the slope is the steepest -- the opamp needs to be able to keep up with that slew rate to match the specs. I only see one opamp that comes close...
 
So do we have a slew rate of 2V/us at 0.1us?
 
Whiley said:
So do we have a slew rate of 2V/us at 0.1us?
That's about what I got as well. So what is the ballpark slew rate needed per us?
 
So would this make the 2.5V/us op amp the correct choice as the others are either too slow or above spec?
 
Whiley said:
So do we have a slew rate of 2V/us at 0.1us?
Whiley said:
So would this make the 2.5V/us op amp the correct choice as the others are either too slow or above spec?
No. Sorry, but I guess I misread your reply... "slew rate of 2V/us at 0.1us" is not right. It is 2V/0.1us, which equals what slew rate per us?

EDIT -- please see later posts
 
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  • #10
20V/us?
 
  • #11
Yes, better. So only one opamp in the table comes close to that, right?

EDIT -- please see later posts
 
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  • #12
Yes, the TLC071 although even though its closest its still not suitable?
 
  • #13
It's the best you can do with those selections. And the problem just asks you to eliminate the ones that are not suitable anyway. And 16V/us is reasonably close to 20V/us...

EDIT -- please see later posts
 
Last edited:
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  • #14
Ok great! Thanks very much I appreciate the help :smile:
 
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  • #15
berkeman said:
No. Sorry, but I guess I misread your reply... "slew rate of 2V/us at 0.1us" is not right. It is 2V/0.1us, which equals what slew rate per us?
Hi, sorry but when going back through this I got a little confused as to how you came up with 2V/0.1us. On the graph at 0.1us the voltage is 0.2V so therefore should the slew rate be 2V/us? Am I missing somthing.
 
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  • #16
Whiley said:
Hi, sorry but when going back through this I got a little confused as to how you came up with 2V/0.1us. On the graph at 0.1us the voltage is 0.2V so therefore should the slew rate be 2V/us? Am I missing somthing.

Then I am too. I also see 0.2V at ~ 0.1 uSec, and that would be 2V/uSec, eliminating only one op-amp.

Another op-amp might be marginal, considering we are estimating from a graph here, and it would be steeper for the first 0.05 uSec, but we are hitting diminishing returns for a practical matter.
 
  • #17
NTL2009 said:
Then I am too. I also see 0.2V at ~ 0.1 uSec, and that would be 2V/uSec, eliminating only one op-amp.
Oops, I think you are right. Thanks for catching my error.
 
  • #18
No problem, so with this then is it safe to say that the TLC2201a is suitable and the other 3 are not. 2 of them are above spec is it reasonable to state that they are not suitable due to being above spec or do you think they will expect more?

Also out of curiosity is there any other drawback to going above spec in terms of slew rate other than cost and power consumption?
 
  • #19
No, IMO being faster than needed is fine, and should not disqualify them.

Being faster, they may be a bit more expensive, so you might eliminate them from your search for that. But on technical grounds, I think they should be fine. If there were other criteria given in the problem (like current consumption or input offset voltage, etc.), then you might be able to eliminate more of the parts on technical grounds.
 
  • #20
Thanks so 1.5+ mV input offset is too large where as 0.6 mV is negligible and therefor this is still the only suitable op amp, is this a fair statement?
 
  • #21
Depends on the circuit. In some circuits, the input offset voltage doesn't matter much. Are you given more of the circuit or additional constraints, or just the time domain step response graph that you've posted so far?
 
  • #22
I have not been given any circuit paramaters other than the step response of the output and I am to determine which op amp is not suitable. With no information with regards to offset correction etc. I am assuming the circuit is simply RC filter with a voltage follower op amp configuration.
 
  • #23
Whiley said:
NOT suitable for use as the active component in the filter of (a) above
If there is no more information than the step response graph, I don't think you can go much farther. If you have a typical filter circuit that you have been studying as part of this unit, then you might be able to say more. If you are using a DC follower after the filter, then the input offset voltage (or input offset currents) may come into play.
 
  • #24
Updated this comment as there isn't a circuit I got mixed up sorry.
 

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  • #25
Thanks for your help again, I think I have everything I need thanks :smile:
 
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