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threshold detector

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spatan

New Member
Hi!!! I need help in designing a threshold detector. It should be able to detect a threshold of 1.6V and give me a logic high when the input voltage in exactly 1.6V and for anything above 1.6V it should be able to give me a logic low. What kind of a circuit do you think I need for this application. I was thinking of maybe a differential amplifier followed by an inverter. What amount of gain do I need to set for the diff amp incase I select that....thank you...
 

spatan

New Member
help needed again

Hi there!! Thanks a lot for your quick reply :) . I had a doubt about comparators though I don't know if it might sound stupid. Does the opamp work in open loop configuration when it works as a comparator? And how much of an open loop gain do I need to set while designing the circuit 'coz i need a quci logic high when it detects my threhsold voltage.


thanks again
 

ivancho

New Member
The comparator is a dedicated chip.... yes you can make a comparator out of op-amps..... but why do that if you have a chip that is already made to do that?

THe comparator works as following.... It compares the reference voltage you set (with a voltage divider for example) and when voltage is detected then the output is turned on. The output is open collector... which simply means that when it is on you have a low signal.... for that reason you are going to need a pullup resistor to have signal high when no "voltage is detected".

Check out the datasheet for the LM339. Look at the Typical applications, Basic comparator on page 6.

Good Luck

Ivancho
 

crust

Member
The LM339 mentioned can have better than 1us transition time so it is well suited to relatively high speed signals. At lower frequencies (perhaps 10kHz or so), you can use an opamp or comparator. At higher frequencies the comparator has better performance.
 

spatan

New Member
Sorry...misunderstanding

I am so sorry for not having made myself clear :( . I actually cannot use a chip. I need to design it using CMOS transistors. This circuit is actually a part of my dual slope analog to digital converter and that is why I need to design an analog CMOS comparator. Sorry for not being clear about this.Can you help me out here about how I need to go about designing the comparator then?
 

Roff

Well-Known Member
Re: Sorry...misunderstanding

spatan said:
I am so sorry for not having made myself clear :( . I actually cannot use a chip. I need to design it using CMOS transistors. This circuit is actually a part of my dual slope analog to digital converter and that is why I need to design an analog CMOS comparator. Sorry for not being clear about this.Can you help me out here about how I need to go about designing the comparator then?
Do you mean you need to design a comparator using discrete MOSFETs, or you need to design a comparator that will be incorporated on a chip?
 

spatan

New Member
Comparator

Hi!! I am not sure I actually understand your question :( . My project is to design a dual slope A/D converter and I need to design all the building blocks using the CMOS technology i.e using mosfets. In this design one of the blocks is a threshold comparator.

Thank you.
 

Roff

Well-Known Member
Re: Comparator

spatan said:
Hi!! I am not sure I actually understand your question :( . My project is to design a dual slope A/D converter and I need to design all the building blocks using the CMOS technology i.e using mosfets. In this design one of the blocks is a threshold comparator.

Thank you.
What I mean is, are you designing an A/D on a chip, i.e., are you designing an integrated circuit? I'm guessing you're not. In any case, have you done a Google search for "CMOS comparator"? Try it - you'll like it.
 

spatan

New Member
Thank you and one more doubt

Thanks a lot for your suggestion. I did find a circuit and i am going to try simulating it :D . But I had another question. Isn't a comparator supposed to give a high if one input is higher than the other and low for the vice versa case? If so then how will i get it to give a logic high when both the inputs are exactly equal?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Re: Thank you and one more doubt

spatan said:
Thanks a lot for your suggestion. I did find a circuit and i am going to try simulating it :D . But I had another question. Isn't a comparator supposed to give a high if one input is higher than the other and low for the vice versa case? If so then how will i get it to give a logic high when both the inputs are exactly equal?
Set the threshold slightly lower - then when the value been tested equals what you want (or higher) it will go high. To avoid 'hunting' it's common to apply positive feedback (yes POSITIVE feedback) around a comparator, this makes it much faster and cleaner in operation, it really snaps from one level to the other.
 

spatan

New Member
sense amp??

Hi!!! I had another thought. Is it possible to ue a sense amplifier for my application. If so is there any gud website where I can read about its working.

Thank you.
 
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