Using OP-Amp to Schmitt Trigger

[Lyallbay04]

Hey guys,

I'm converting a analogue to digital signal using an op-amp to make a Schmitt trigger. It needs to be non-inverting, so i just change Vin to the positive feedback, right? But also how do i find the Upper and lower threshold and what dictates how far a part these thresholds have to be apart? Does it depend on the amount of noise that the circuit has.

Thanks

 

[ericgibbs]

Hey guys,

I'm converting a analogue to digital signal using an op-amp to make a Schmitt trigger. It needs to be non-inverting, so i just change Vin to the positive feedback, right? But also how do i find the Upper and lower threshold and what dictates how far a part these thresholds have to be apart? Does it depend on the amount of noise that the circuit has.

Thanks

hi,
If you get the datasheet for the LM393 dual comparator from www.datasheetarchive.com it shows the method of positive feedback that will give the circuit 'hysteresis'
Do you follow.?

 

[Lyallbay04]

Im using an LM324. set up like this

 

[ericgibbs]

Im using an LM324. set up like this

hi,
That circuit is connected as NI amplifier,
ie with negative feedback [output to INV input].

For a schmitt, you require positive feedback [output to NI input], the value of the feedback resistor will set the 'hysteresis' of the circuit.

The circuitt you require, is a Comparator, OPA's dont make the best comparators.

Did you look at the LM393 data, it does explain hysteresis..

 

[Lyallbay04]

Ok yea i see what i did wrong. But im only given a LM124/224/324/2902 to use. i understand now that the feedback resistor controls the hystersis. But isn't that circuit a comparator?? Sorry getting a bit confused.

Thanks

 

[ericgibbs]

Ok yea i see what i did wrong. But im only given a LM124/224/324/2902 to use. i understand now that the feedback resistor controls the hystersis. But isn't that circuit a comparator?? Sorry getting a bit confused.

Thanks

hi,
Yes its a Comparator, but isnt that what you need to convert the analog signal to digital.?

Post some details, ie: whats the range of input voltages, what output voltage, whats the application/assignment..????

 

[Lyallbay04]

Yea, sorry jst got confused with reply. analogue input signals with amplitudes between 0.2 and 4V. Where processing analogue voltage signal from a reciever and filtering out unwanted noise signal and then converting to a digital signal. Im doing the latter part.

 

[ericgibbs]

Yea, sorry jst got confused with reply. analogue input signals with amplitudes between 0.2 and 4V. Where processing analogue voltage signal from a reciever and filtering out unwanted noise signal and then converting to a digital signal. Im doing the latter part.

What output voltage range do you require from the Comparator.

Another question, what is the frequency range of the signal to the comparator.?

 

[Lyallbay04]

16 to 66Hz does that seem right and i think the output voltage range is 0-5V.
Sorry if im getting this wrong im a bit new to this stuff.

 

[ericgibbs]

16 to 66Hz does that seem right and i think the output voltage range is 0-5V.
Sorry if im getting this wrong im a bit new to this stuff.

hi,
No need to be sorry, its not a problem.

The reason for asking the frequency is because the LM324 has a limited specification.

You say you have an input voltage range of +0.2V and +4V, now you have to decide at which voltage you want the comparator to switch over.

Example: if the input voltage is less than +2V then the output is 0V and if the input voltage is greater than +2V, the output is +5V.

A point to note, if the LM324 is powered from a single +5V power supply, the LM324 will NOT give a 0V or a +5V output..
more like +1V and +3.5V, is that acceptable.?

 

[Lyallbay04]

yea thats fine. But isnt there suppose to be two threshold levels for a schmitt trigger?

 

[ericgibbs]

yea thats fine. But isnt there suppose to be two threshold levels for a schmitt trigger?

hi,
Yes there are two levels, I was working my way to that point slowly, so that I did not confuse you...

If you can say what the 'centre' switching level is, then we add some positive feedback, Output to NI input...OK.

A point to note, if the LM324 is powered from a single +5V power supply, the LM324 will NOT give a 0V or a +5V output..
more like +1V and +3.5V, is that acceptable.?

 

[Lyallbay04]

So that will make it non-inverting right. I think the switching level is 2.5 V. But not sure why we need a cente switching level??

 

[ericgibbs]

So that will make it non-inverting right. I think the switching level is 2.5 V. But not sure why we need a cente switching level??

hi,
Look at these actual comparator circuits, the NI and INV options.
[attached drawing]

To design most circuits a decision has to be made regarding its operating points.

Example: say that you decide a centre voltage point is +2.5V, when you add hysteresis you will have to select the Lower and Upper voltage switching limits.
You set this centre level by applying a fixed voltage reference to one of the Comp inputs.

Say its +2V and +3V limits you require , you set these by the positive feedback level, choosing the correct feedback resistor. OK.

You will find that when you add the feedback it will move the centre voltage point, so you have to make a small adjustment in the reference voltage.

Do you follow OK.?

 

[Lyallbay04]

Ohk yea i understand.

 

[Lyallbay04]

I think i was over complicating it.
So i use a non-inverting comparator which will give me a digital ouptut signal with the same polarity as the input signal. I then choose the correct feedback resistor to give which sets the limits.

I change the vref by changing the value of reisitor?

Thanks for your help

 

[ericgibbs]

I think i was over complicating it.
So i use a non-inverting comparator which will give me a digital ouptut signal with the same polarity as the input signal. I then choose the correct feedback resistor to give which sets the limits.

I change the vref by changing the value of reisitor?

Thanks for your help

hi,
The blue text is correct.

The Vref is usually set by two resistors forming a potential divider from the +Vsupply to the 0V.
Its a good idea while you are testing, to add a variable resistor in series with two fixed resistors.

Something like this drawing.

REMEMBER: the LM324 OPA does not make a good comparator..

 

[Lyallbay04]

Does this look better i had a bit of problem using my first set-up. Does this look better to you?

 

[ericgibbs]

Does this look better i had a bit of problem using my first set-up. Does this look better to you?

hi,
I have changed the value of the resistors, they were too low.
Also the 10K from Vref is not required, but I usually use a 470R.

Why have you deleted the Rh hysteresis set pot,??

Otherwise looks OK.

 

[Lyallbay04]

Is the hysteresis set pot just a varaiable resistor to help set it??