Suitable Opamp for this Low Pass Filter

Click For Summary
SUMMARY

The discussion focuses on selecting suitable op-amps for a low pass filter, specifically identifying the TLV2721 as unsuitable due to its slow slew rate of 0.18 V/µs. Participants emphasize the importance of comparing slew rates, particularly at the initial 0.1 µs of the waveform, to determine suitability. The TLC2201A is identified as a suitable option, while others are eliminated based on their specifications exceeding the required slew rate. The conversation highlights the significance of input offset voltage and other circuit parameters in making final decisions.

PREREQUISITES
  • Understanding of op-amp specifications, particularly slew rate
  • Familiarity with low pass filter design principles
  • Knowledge of input offset voltage and its impact on circuit performance
  • Ability to interpret step response graphs for op-amps
NEXT STEPS
  • Research the slew rate requirements for various op-amps in filter applications
  • Learn about the impact of input offset voltage on op-amp performance
  • Explore different op-amp configurations for low pass filters
  • Investigate the trade-offs between op-amp speed, cost, and power consumption
USEFUL FOR

Electrical engineers, students studying circuit design, and anyone involved in selecting op-amps for filter applications will benefit from this discussion.

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.

upload_2017-8-13_16-29-24.png

upload_2017-8-13_16-29-47.png


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.
 

Attachments

  • Fig.2.png
    Fig.2.png
    14.2 KB · Views: 587
  • TableA.png
    TableA.png
    7.3 KB · Views: 595
Physics news on Phys.org
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
 
Last edited:
  • #10
20V/us?
 
  • #11
Yes, better. So only one opamp in the table comes close to that, right?

EDIT -- please see later posts
 
Last edited:
  • #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:
  • Like
Likes   Reactions: Whiley
  • #14
Ok great! Thanks very much I appreciate the help :smile:
 
  • Like
Likes   Reactions: berkeman
  • #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.
 
  • Like
Likes   Reactions: NTL2009
  • #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.
 

Attachments

  • upload_2017-8-14_17-39-51.png
    upload_2017-8-14_17-39-51.png
    2.5 KB · Views: 604
  • #25
Thanks for your help again, I think I have everything I need thanks :smile:
 
  • Like
Likes   Reactions: berkeman

Similar threads

Comparing Op-Amp Parameters: Ideal vs A & B
  • · Replies 3 ·
Replies
3
Views
2K
Second order low pass filter with unity gain
  • · Replies 6 ·
Replies
6
Views
3K
Engineering Practical design of an inverting Schmitt trigger with noise tolerance
  • · Replies 21 ·
Replies
21
Views
3K
OpAmp general rules-of-thumb (educated guess) on filter type
  • · Replies 7 ·
Replies
7
Views
2K
Cascaded low-pass filter followed by a buffer amplifier
  • · Replies 11 ·
Replies
11
Views
4K
Op amp transfer function determination ()
  • · Replies 1 ·
Replies
1
Views
2K
Output voltage and output current of an Opamp
  • · Replies 13 ·
Replies
13
Views
3K
Engineering How to tell if the circuit is a High or low pass filter?
  • · Replies 9 ·
Replies
9
Views
35K
How Do You Design an Op-Amp Bandpass Audio Filter with LED Indicators?
  • · Replies 3 ·
Replies
3
Views
4K
Confusion on: High Pass Filter at Op Amp Input
  • · Replies 6 ·
Replies
6
Views
3K