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.
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.
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.?
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.
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.
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.
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.
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.?
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.?
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.
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?
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?
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..