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[SOLVED] Negative threshold comparator

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oy1987

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Hello,

I am designning a circuit that would sample a certain voltage between -3 to 0, and ouput 3.3V if the input is above certain threshold and 0 if it's undr it.
I thought of using a comparator such as LMV339, but it does not supports a negative threshold.
Any suggestions as to what type of comparator should I use?

Thank you all
 

BradtheRad

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Add a certain amount of positive voltage, through a potentiometer, to lift a negative signal into the positive.

Your signal input must be able to sink a few mA.

shift 0 to -3V up to positive via pot 5V supply.png

The 1M resistor is unnecessary. It is there to indicate 0V ground reference.
 
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oy1987

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Hi,
First of all thanks for the solution.
One thing that concerns me about this solution - the 5V DC supply on my board may get interfered with the signal frequency (several kHz).
 

BradtheRad

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Hi,
First of all thanks for the solution.
One thing that concerns me about this solution - the 5V DC supply on my board may get interfered with the signal frequency (several kHz).

It's okay to substitute a higher potentiometer value, to provide greater impedance between all nodes. It's just a matter of adjusting the pot to achieve the proper balance.

However the input resistance of your following device also is a factor. You want the incoming signal to maintain the same amplitude, even after you add the next device.
 
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KlausST

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Hi,

If you have negative voltage signal, then I guess there is negative supply, too.
If so, then there are comparators ready for your application.

Klaus
 
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SunnySkyguy

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If you put 30K from signal to any comparator input and 33K from that input to 3.3V, you will get your signal shifted up and reduced to 0 to 1.57
comparat.jpg
Choose a comparator that runs on a single supply at 0V in that runs off 3.3V and bias the signal, threshold setting and IC in one small circuit. There are many features to choose in an comparator which has more speed than an Op Amp, but can work fine for many apps. If using a op amp. choose one with Rail-to-Rail Output and sense at ground input or Rail to rail input.

Or use the comparator you chose.

https://www.ti.com/lsds/ti/amplifiers-linear/comparator-overview.page

-depending on your calibration, you can choose fixed R's instead of a 10K Pot or scale to larger values.
You may want hysteresis or noise filtering too.

- - - Updated - - -

You can also reverse +- inputs.
 
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oy1987

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Tnaks
At the moment it seems that a voltage monitor such as MC34161 should do it.
 

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