Op-amp part replacement comparison

Hi,

The current schematic is for analog output. Thus they use an amplifier / OPAMP.

If you want a digital output ... you first have to modify the schematic ... then you may use a comparator.

Generally:
comparators and OPAMPS are different devices, made for different operating modes.
* a comparator never has negative feedback. But it may have positive feedback (to generate a hysteresis).
* an OPAMP never uses positive feedback. (It would cause the circuit to act as a comparator with hysteresis. But then use a comparator IC, since many OPAMPS are not designed to operate as a comparator). A OPAMP amplifier circuit always uses negative feedback.

Klaus

* an OPAMP never uses positive feedback. (It would cause the circuit to act as a comparator with hysteresis. But then use a comparator IC, since many OPAMPS are not designed to operate as a comparator). A OPAMP amplifier circuit always uses negative feedback.

Click to expand...

Its more complicated than the above :


Using OpAmps as comparators (sometimes advantageous) - https://www.analog.com/media/en/training-seminars/tutorials/MT-084.pdf

danadakk said:

Its more complicated than the above :

Click to expand...

What exactly does the video show ... that is "more complicated" than what I have written?

danadakk said:

Using OpAmps as comparators (sometimes advantageous)

Click to expand...

Can you please point out where exactly the document refers to something "advantageous"?

Klaus

More alluding to one could envision considerations. Maybe to help more
completely this discusses both advantages and disadvantages (like DC and
Vos inaccuracies in most comparators) -


MT084 ap note did allude to using leftover OpAMP as a comparator for costs savings.

One can also think in terms of low noise OpAmps having an advantage over comparators.
And of course one can get power OpAmps which would make a good power comparator.
I googled power comparators to see if there are any of significance, biggest I 'found was
360 mA output current. OpAmps I found > 10A.

The video establishes one can use +fdbk as long as net is negative, linear operation (excluding
some osc topologies which utilize + fdbk).

Hi,

The OP wants do discuss the use of a COMPARATOR instead of an OPAMP in a cricuit with 100% negative feedback.
So the use of an OPAMP with positive feedback is not the OP´s topic at all.

Sadly it was me who brought in the phrase

KlausST said:

an OPAMP never uses positive feedback

Click to expand...

... in the section with the headline "Generally" .. where I just wanted to give a brief information about the differences between OPAMP and COMPARATOR circuits.

*****
OFF TOPIC from here
==================

MT-084:

I neither can find the words "advantage", nor "benefit", nor "leftover", nor "cost", nor "offset", nor "VOS" ... what do I need to search for?
Maybe my PDF search function does not work.

The document starts with:

Even though op amps and comparators may seem interchangeable at first glance there are some
important differences. Comparators are designed to work open-loop, they are designed to drive
logic from their outputs, and they are designed to work at high speed with minimal instability.
Op amps are not designed for use as comparator...

Click to expand...

and ends with:

SUMMARY
Operational amplifiers are not designed to be used as comparators, so this tutorial has been,
intentionally, a little discouraging.

Click to expand...

and in the following:

nevertheless there are some applications where the use of an
op amp as a comparator is a correct engineering decision—what is important is to make it a
considered decision, and ensure that the op amp chosen will perform as expected. To do this it is
necessary to read the data sheet carefully, to consider the effects of non-ideal op amp
performance, and to calculate the effects of op amp parameters on the application. Since the op
amp is being used in a non-standard manner some experiment may also be necessary–since the
amplifier used for the experiment will not necessarily be typical the results of experiments
should always be interpreted somewhat pessimistically.

Click to expand...

So is the document meant for newbies ... or rather for experienced designers?

danadakk said:

One can also think in terms of low noise OpAmps having an advantage over comparators.

Click to expand...

The output of a comparator .. as well as of an OPAMP with positive feedback is digital HIGH/LOW ... how much impact has the input noise to the output?
How can one measure it?
I do understand the noise measurement of an OPAMP ... in it´s "usual" operation mode as an amplifier. But have no idea how it can be measured on a comparator with hysteresis.
Comparator datasheets don´t talk much about noise ... but due to their lower open loop gain (compared to OPAMPs) .. doesn´t the input referred noise has less impact on the output than at an OPAMP?

danadakk said:

And of course one can get power OpAmps which would make a good power comparator.
I googled power comparators to see if there are any of significance, biggest I 'found was
360 mA output current. OpAmps I found > 10A.

Click to expand...

I don´t understand how is this related to
* the OP´s question
* the documents you refer to

Klaus

MT-084:

I neither can find the words "advantage", nor "benefit", nor "leftover", nor "cost", nor "offset", nor "VOS" ... what do I need to search for?
Maybe my PDF search function does not work.

Click to expand...

Yes it appears they did not use your words, or as you pointed out your self limiting search for a term,
and not reading the document, well meaning I am sure, but crippling. Did you do the same for the
second doc -

The output of a comparator .. as well as of an OPAMP with positive feedback is digital HIGH/LOW ... how much impact has the input noise to the output? How can one measure it?

Click to expand...

Histogram over time. Use Vref with low noise, compare with noise generator with offset, and create histogram.
Or bias comparator into linear region to look at self generated noise spectrum....

Can you please point out where exactly the document refers to something "advantageous"?

Click to expand...

Above not OP related, just adding to your “advantageous” quest.



Knight

danadakk said:

Yes it appears they did not use your words, or as you pointed out your self limiting search for a term,
and not reading the document, well meaning I am sure, but crippling. Did you do the same for the
second doc -

Click to expand...

Sorry, it was not "my words" ... it was the words you gave. And since I did not find the information .. I even expanded the search with similar words.

Again you show an aggressive attitude when I ask simple questions about informations you gave. Why?
I took your informations seriously, did read through the documents and your posts, asked for details, asked for help.
I asked for "exact locations" in the document where the "advantageous" use of an OPAMP is written. (A simple answer could be: "Page#6, bottom")
I took effort in finding the location on my own. I did not find them ... so I asked for help.
What´s wrong with asking for help?

Still I can´t find the part where it gives the "several reasons to use OPAMPs as comparators".
Call me dumb, call the PDF search dumb ... which page is it in? Is this asking too much? Without getting back a disrespectful attitude...

I don´t think that my posts were disrespectful.
Mainly I asked for your assitance to clarify things you wrote and how they fit to the informations in the linked documents.

Which "second doc" are you referring to?
--> You twice gave a link to the same MT-084 doc. And the other link guides me to a 50 minutes long video
In which post is the link? Or do I have to follow links within the linked pages? Is it asking too much.. for a smiple clear answer? ... without attitude?

****

danadakk said:

Histogram over time. Use Vref with low noise, compare with noise generator with offset, and create histogram.
Or bias comparator into linear region to look at self generated noise spectrum....

Click to expand...

I understad this setup... (hopefully)
But how can the use of an external noise source .. find out the internal noise of the OPAMP_comparator?

And... as far as I understand .. you can´t get an OPAMP_with_positive_feedback .... into linear region. Please explain how this can be done. (without being upset and unfair)
This is as serious meant question ... for me and others to learn new things.
I understand that for you - who know how these things work - it may be an annoying and stupid question.

Klaus

My error, second doc attached.

And... as far as I understand .. you can´t get an OPAMP_with_positive_feedback .... into linear region. Please explain how this can be done. (without being upset and unfair)

Click to expand...

I posted MIT video with the following comment :



I had no idea this would cause you to conclude I am aggressive, that I am adverse to a forum discussing
engineering and focused on delivering ACCURATE info. Occasionally I get corrected, I consider that a
best in class process. But all these psychological conclusions of my intent and interpretation not really
useful here. I will go out on a limb and post you are not qualified in that department. Correct me if I am
wrong about this.....

The MIT video seems to me to state that NET negative feedback is desired, eg. that one can have both
positive and negative feedback and have a linear result. What "triggered" me was your statement :

* an OPAMP never uses positive feedback. (It would cause the circuit to act as a comparator with hysteresis. But then use a comparator IC, since many OPAMPS are not designed to operate as a comparator). A OPAMP amplifier circuit always uses negative feedback.

Click to expand...

Highlight an OPAMP never uses positive feedback.

I understand that for you - who know how these things work - it may be an annoying and stupid question.

Click to expand...

More psychological

The rest of your non OP or topic discourse I will leave for .......

Your attitude is outstanding!

You miss to post the link .. and then you accuse people for not reading this document!

danadakk said:

Highlight an OPAMP never uses positive feedback.

Click to expand...

... and you intentionally ignored what I wrote thereafter.
"It would cause the circuit to act as a comparator with hysteresis...."
And that´s exactly what the MIT video shows.
In the first quarter of the video they said "it makes bang bang to either supply rail ... later we will try to make some useful funtion"
And the whole last thrid they disccussed a "a comparator with hysteresis...." (Compare this with what I have written)

Klaus

KlausST said:

Your attitude is outstanding!

You miss to post the link .. and then you accuse people for not reading this document!


... and you intentionally ignored what I wrote thereafter.
"It would cause the circuit to act as a comparator with hysteresis...."
And that´s exactly what the MIT video shows.
In the first quarter of the video they said "it makes bang bang to either supply rail ... later we will try to make some useful funtion"
And the whole last thrid they disccussed a "a comparator with hysteresis...." (Compare this with what I have written)

Klaus

Click to expand...

The video shows that NET negative feedback, eg (POSfdbk - NEGfdbk) results in stable final value if that
difference is <=0.

Appeal to others on forum group, watch MIT video and confirm/deny my observation is in error.

danadakk said:

The video shows that NET negative feedback, eg (POSfdbk - NEGfdbk) results in stable final value if that
difference is <=0.

Click to expand...

if:
(POSfdbk - NEGfdbk) <=0

simply means .. the negative feedback dominates....

--> I did never disagree. (You are free to show the people WHERE I disagreed to this. I bet you can´t ... because I did not)

******
And:

results in stable final value

Click to expand...

Is an OPAMP with positive feedback ... I.e. a comparator with hysteresis considered "unstable"?

--> I did never disagree to that. (You are free to show the people WHERE I disagreed to this. I bet you can´t ... because I did not)

Klaus

I see no need for all these filters, an Op Amp or a comparator unless you state some useful output specifications like PW50 = 1 ms max, centred on transition, active low. My design may be preferred for glitch filtering, resistor stress reduction, and simplicity. However, you will get better advice with design specs like purpose and PW.

You can reduce PW50 to 1us if it was necessary with an XOR gate with RC delay as a 1-shot comparator on a sine wave.



I intended the 5.6 uF cap to be 1 uF ceramic and the design here works from 0.1 uF to 1mF+

There is no need to choose Line, Neutral with this differential design and will work on 120Vrms 60Hz..