[Solved] How to Lower Threshold of OTS Schmitt Trigger?

Thread Starter

johnyradio

Joined Oct 26, 2012
254
Is it possible to change the threshold of an off-the-shelf (OTS) non-inverting Schmitt?
(Schmitt chip, not made with op amp or discreet transistors).

I'm guessing i need to offset the input signal with a resistor divider, correct?
Or maybe offset the supply-voltage to the Schmitt?
 
Last edited:

danadak

Joined Mar 10, 2018
4,057
Normally the Schmidt window not real wide, and its speced poorly, eg. min/max.

So simple offset circuit a tad challenging.

Without knowing the overall design requirements difficult to give you further advice.


Regards, Dana.
 

crutschow

Joined Mar 14, 2008
24,949
You could add a series input resistor and a feedback resistor from the non-inverting output to add hysteresis.
For example, a 10kΩ input resistor and a 100kΩ feedback resistor will give a hysteresis about 1/10th of the output voltage swing.
 

Thread Starter

johnyradio

Joined Oct 26, 2012
254
You could add a series input resistor and a feedback resistor from the non-inverting output to add hysteresis.
Thx, but unclear. It's a one input Schmitt. So there's only a non-inverting input. It's an OTS Schmitt, so it already has hysteresis.

Would your suggestion work in that case?

Apologies, I changed the question to zero volts.
 
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Thread Starter

johnyradio

Joined Oct 26, 2012
254
I was trying to find out why you are constrained to using a simple low accuracy Schmidt logic gate
versus an single OpAmp and a couple of Rs....
I attempted this circuit, and my output was identical to my input, continuously varying voltage, instead of 1 and 0.
i connected ref to Ground, since i want output to go high when input goes above 0V.

Tried switching the inputs, cuz I want non-inverting logic. Neither worked.

Prolly missing some R's
1583544901157.png
 
Last edited:

crutschow

Joined Mar 14, 2008
24,949
Would your suggestion work in that case?
Below is the LTspice simulation of two circuits using CD4093 Schmitt-trigger NAND gates (two connected in series to get a non-inverting output.

As you can see, the circuit output with the feedback (red trace) has more hysteresis than the circuit without (yellow trace).

1583548033675.png
 

crutschow

Joined Mar 14, 2008
24,949
That's ok. So is it possible to lower both thresholds? Or only to increase total hysteresis?
You can lower them to a few mV with a comparator or op amp with essentially no hysteresis.

I misunderstood.
I thought you wanted to increase the hysteresis, not lower it.
If you don't want hysteresis than you don't want to use a Schmitt-trigger which, by definition, has hysteresis.
 

Thread Starter

johnyradio

Joined Oct 26, 2012
254
I would like a small amount of hysteresis, preferably just on the downswing, not the upswing.

The question here is less about hysteresis, and more about lowering both thresholds.
 

Sensacell

Joined Jun 19, 2012
2,567
You have not explained much about the application, but if the signal is repetitive, you might be able to AC couple the input, as long as the amplitude of the signal is greater than the total hysteresis.

Then the absolute voltage level doesn't matter, just the change-in-voltage.
 
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