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  1. #1
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    Level Shifter trip points problem

    Hi guys,

    I have a simple Schmitt Trigger, very well known by everyone.

    The problem that I am trying to understand is: the positive trip point (if we can called it that way), looking at the the hysteresis plot, is at something around 790mV. The negative trip point is around 700mV.

    I wanted to test this by mounting a simple transient testbench using a piece wise waveform form like 3 triangles where 2of them passes through the trip points and one of them doesn't reach one of the trip point to confirm if it works.

    The problem is that, the negative trip point is working correctly but the positive one is not. The input triangles when reach around 700mV the output of the Schmitt Trigger works, but when it comes to the positive one, the Schmitt Trigger only triggers around 900mV.

    Anyone can suggest something?

    •   Alt4th April 2016, 11:24

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    re: Level Shifter trip points problem

    I presume your hysteresis plot is recorded as static transfer characteristic. The transient test signals may be simply too short to trigger. The comparator behavior can be characterized by a response time versus overdrive curve. Fast response needs some overdrive beyond the 790 mV point.



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    re: Level Shifter trip points problem

    Hi FvM, thanks for the reply.

    It is not a comparator, it's a Schmitt trigger.

    Regarding the response time that you mentioned, maybe if I put a very slow varying input voltage, like 5s to 10s I should be able to see the trip points?

    Can you give me a tip on how to characterize the response time versus overdrive curve? Is it easy?

    Thanks in advance.



    •   Alt4th April 2016, 12:06

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    re: Level Shifter trip points problem

    Hi,

    Please show your simulation. Circuit and plots.

    Klaus



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    re: Level Shifter trip points problem

    Hi Klaus, I don't have the plots here with me. Can you tell me why you want to see the circuit and the plots?

    The circuit is a simple ST, with cross-coupled pmos pair, two not's, one at one input of the nmos (bellow cross-coupled pair) and the other at the output.

    The waveforms is constructed using the pwlv source. (0s, 0V), (0.5ms, 1.5V), (0.75ms, 0.75V), (1ms, 1.5V), (1.25ms, 0.6V), (1.5ms, 1.5V), (2ms, 0V).



    •   Alt4th April 2016, 13:31

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    re: Level Shifter trip points problem

    Hi,

    Can you tell me why you want to see the circuit and the plots?
    Because this are reliable informations about your circuit that easily can be validated by the users.
    A picture (schematic/scope...) sometimes says more than a hundred words.

    ***
    Example:
    where 2of them passes through the trip points and one of them doesn't reach one of the trip point
    This is not very informative
    * because we don´t see a voltage nor a timing.. but the results surely depend on voltage and timing..
    * And it doesn´t tell what is the difference between the two triangles that pass the trip points.
    * And why doesn´t the third signal pass any trip point? Because it is above both trip boints, or below both trip points or exactely between both trip points?
    --> the picture could give us the answers..

    Especially for the schematic there are lot of experts who will find out simple improvements...

    Klaus



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    re: Level Shifter trip points problem

    Hi Klaus and everybody. I must apologize because I might have induced you guys in error because I messed up with the topic name. What I meant was LEVEL SHIFTER. However, everything I said before remains valid, because I was thinking on that circuit, however gave the wrong name to the topic.

    Regards.
    Last edited by erikl; 5th April 2016 at 18:09. Reason: title changed by moderator



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