How to calculate hysteresis of non inverting opamp

Thread Starter

mishra87

Joined Jan 17, 2016
645
Hi All ,

I have design attched circuit.
Could anybody guide me how do i calculate hysteresis of non inverting opamp of attched circuit .

Calculation and concept are really appricated .

Thanks in advance .
 

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Jony130

Joined Feb 17, 2009
5,145
Could anybody guide me how do i calculate hysteresis of non inverting opamp of attched circuit .
To be able to find the threshold voltages (upper and lower) you need to exam two cases.

Case 1
Assumed that the opamp output voltage is at positive saturation (Vsupp) for an ideal case.
Then all you need to do is to find the Vin value for which the voltage at non-inverting is exactly equal to Vred = 2V.

Case 2
The opamp output voltage is at negative saturation (OV for your circuit). Try to find the Vin voltage needed to make the non-inverting voltage equal to Vred (2V).
 

Bordodynov

Joined May 20, 2015
2,497
I'm making it easier. The full hysteresis sweep is equal to the full output voltage sweep multiplied by the resistive divider transfer coefficient (R1 and R2).
The upper and lower switching thresholds are generally not symmetrical with respect to the switching threshold.
 

ericgibbs

Joined Jan 29, 2010
9,537
hi,
This is what I see on your circuit.
What do you make the hysteresis value.?? [note I had to subtract -1v from the Vin so that the plots overlaid]

E
Capture.PNG
 

eetech00

Joined Jun 8, 2013
1,835
Hi All ,

I have design attched circuit.
Could anybody guide me how do i calculate hysteresis of non inverting opamp of attched circuit .

Calculation and concept are really appricated .

Thanks in advance .
Hi

Well...for starters.....how much hysteresis do you want?

eT
 

AnalogKid

Joined Aug 1, 2013
8,302
To calculate the hysteresis, you MUST know the input voltage range, the opamp power supply voltage, and the opamp output voltage range, because these are the voltages at the two ends of a voltage divider that is compared to the voltage at the inverting input.

ak
 

Thread Starter

mishra87

Joined Jan 17, 2016
645
To calculate the hysteresis, you MUST know the input voltage range, the opamp power supply voltage, and the opamp output voltage range, because these are the voltages at the two ends of a voltage divider that is compared to the voltage at the inverting input.

ak
Thanks for your reply

Supply voltage is 12V.

for any input voltage with gain opamp maximum saturate
 

AnalogKid

Joined Aug 1, 2013
8,302
for any input voltage with gain opamp maximum saturate
I have no idea what that means. Let's try this again.

R1 and R2 form a voltage divider. The comparator changes state when the voltage at the R1 - R2 node, which is the non-inverting input, transitions across the the voltage at the inverting input, which you state as 2 V. In order to calculate the voltage at the R1-R2 node, you have to know the voltage at the left side of R1 and the right side of R2. The voltage at the right side of R2 is the comparator output. Its output voltage is determined by the device output stage characteristics. Now that we finally know the magnitude of the power rail, that leaves two questions:

What is the device part number?

What is the device's output voltage range?

ak
 

Thread Starter

mishra87

Joined Jan 17, 2016
645
I have no idea what that means. Let's try this again.

R1 and R2 form a voltage divider. The comparator changes state when the voltage at the R1 - R2 node, which is the non-inverting input, transitions across the the voltage at the inverting input, which you state as 2 V. In order to calculate the voltage at the R1-R2 node, you have to know the voltage at the left side of R1 and the right side of R2. The voltage at the right side of R2 is the comparator output. Its output voltage is determined by the device output stage characteristics. Now that we finally know the magnitude of the power rail, that leaves two questions:

What is the device part number?

Thanks Mr. ak !!!

Device part number is LM358
Pl find attached xls for hysteresis calculation done by me.
Hope this help to other reader.

Regards,


What is the device's output voltage range?

ak
 

Attachments

I'd guess you missed to say what's your input for R1 but that's not really relevant to calculate hysterisis. But anyways, you calculate hysterisis by using this equation VH= VUT-VUL

VH= Hysterisis Voltage
VUT= Upper threshold Voltage
VLT= Lower threshold Voltage

VUT= Vref(1+1/n) - (-Vsat/n) = 0 , since -Vsat is wired to earth.

n is just the relation between R2 and R1, so to find n you simply use this equation n=R2/R1

VLT= Vref(1+1/n) - (+Vsat/n) = 12(1+1/121.43) - (12/121.43) = 1.92 v

VH= VUT-VLT= -1.92 v , and since VH is just the range of noise your circuit is able to handle, VH= 1.92 v

Source: Operational amplifiers and linear integrated circuits Robert f. Coughlin Fourth Edition
 
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Jony130

Joined Feb 17, 2009
5,145
I'd guess you missed to say what's your input for R1 but that's not really relevant to calculate hysterisis. But anyways, you calculate hysterisis by using this equation VH= VUT-VUL

VH= Hysterisis Voltage
VUT= Upper threshold Voltage
VLT= Lower threshold Voltage

VUT= Vref(1+1/n) - (-Vsat/n) = 0 , since -Vsat is wired to earth.

n is just the relation between R2 and R1, so to find n you simply use this equation n=R2/R1

VLT= Vref(1+1/n) - (+Vsat/n) = 12(1+1/121.43) - (12/121.43) = 1.92 v

VH= VUT-VLT= -1.92 v , and since VH is just the range of noise your circuit is able to handle, VH= 1.92 v

Source: Operational amplifiers and linear integrated circuits Robert f. Coughlin Fourth Edition
You made a mistake.

VUT is not 0V but it is equal to:

VUT = 2V*(1 + 5.6/680) = 2.016V or 2(1+1/(680/5.6)) - (-0/(680/5.6)) = 2.016V Using your equation.

And the hysteresis width is equal to:

VH= VUT - VLT = ("+Vsat" - "-Vsat" ) * R1/R2 = 12V * 5.6/680 = 98.823mV
 
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You made a mistake.

VUT is not 0V but it is equal to:

VUT = 2V*(1 + 5.6/680) = 2.016V or 2(1+1/(680/5.6)) - (-0/(680/5.6)) = 2.016V Using your equation.

and VH= VUT - VUL = ("+Vsat" - "-Vsat" ) * R1/R2 = 12V * 5.6/680 = 98.823mV
True, I made a mistake there.

And that actually makes sense, the VUL can't be above the VUT, it wouldn't make any sense.

Thanks!
 

Jony130

Joined Feb 17, 2009
5,145
So, there's a value for VH with those numbers, but if we plot them, it won't make any sense.
What? Which value does not make sense to you?

To be able to find the Upper threshold Voltage all you need is to assume that the opamp output voltage is at positive saturation, (+12V) for an ideal opamp.
Then you need to find the Vin value for which the voltage at non-inverting is exactly equal to Vref = 2V.
And for the circuit under discussion, it is a trivial task (simple voltage divider).

We can use the same tactics and find the Lower Threshold Voltage. But this time we need to find it for a case when opamp output is at negative saturation.
 
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