# Homework Help: Suitable Opamp for this Low Pass Filter

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1. Aug 13, 2017

### Whiley

1. The problem statement, all variables and given/known data

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.

2. Relevant equations

3. 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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2. Aug 13, 2017

### Whiley

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: Aug 13, 2017
3. Aug 13, 2017

### Staff: Mentor

Welcome to the PF.

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

4. Aug 13, 2017

### Whiley

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?

5. Aug 13, 2017

### Staff: Mentor

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...

6. Aug 13, 2017

### Whiley

So do we have a slew rate of 2V/us at 0.1us?

7. Aug 13, 2017

### Staff: Mentor

That's about what I got as well. So what is the ballpark slew rate needed per us?

8. Aug 13, 2017

### Whiley

So would this make the 2.5V/us op amp the correct choice as the others are either too slow or above spec?

9. Aug 13, 2017

### Staff: Mentor

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: Aug 14, 2017
10. Aug 13, 2017

### Whiley

20V/us?

11. Aug 13, 2017

### Staff: Mentor

Yes, better. So only one opamp in the table comes close to that, right?

EDIT -- please see later posts

Last edited: Aug 14, 2017
12. Aug 13, 2017

### Whiley

Yes, the TLC071 although even though its closest its still not suitable?

13. Aug 13, 2017

### Staff: Mentor

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: Aug 14, 2017
14. Aug 13, 2017

### Whiley

Ok great! Thanks very much I appreciate the help

15. Aug 14, 2017

### Whiley

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.

16. Aug 14, 2017

### NTL2009

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. Aug 14, 2017

### Staff: Mentor

Oops, I think you are right. Thanks for catching my error.

18. Aug 14, 2017

### Whiley

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. Aug 14, 2017

### Staff: Mentor

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. Aug 14, 2017

### Whiley

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. Aug 14, 2017

### Staff: Mentor

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. Aug 14, 2017

### Whiley

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. Aug 14, 2017

### Staff: Mentor

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. Aug 14, 2017

### Whiley

Updated this comment as there isnt a circuit I got mixed up sorry.

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25. Aug 14, 2017

### Whiley

Thanks for your help again, I think I have everything I need thanks