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LMH7220: Using Hysteresis

Guru 16770 points
Part Number: LMH7220

Hi

I have question for using hysteresis.

Please see followings (① and ②).

We want to make hysteresis and LVDS output.

Then I have following questions.

1. As for the image ①, can it be offered hysteresis and LVDS out? 

2. As for the image ②, can it be offered hysteresis and LVDS out?

BestRegards

  • Hi,

    Thanks for your question. You can find the responses below.

    1. Yes, this circuit will have hysteresis and LVDS outputs.

    2. What is the purpose of resistor Rf on the inverting input? Without this resistor, the circuit will already have hysteresis and LVDS outputs.

    Regards,

    Jaskaran

  • Hi Jaskaran

    Thank you for your reply.

    >2. What is the purpose of resistor Rf on the inverting input? Without this resistor, the circuit will already have hysteresis >and LVDS outputs.

    I referred a schematic from Figure 1 of SNOA478A user guide.
    That shows feedback resister on inverting input.

    Is it impossible to have hysteresis using feedback on inverting input like Figure 1 of SNOA478A?

    BestRegards
  • Hi,

    I see that figure 1 of SNOA478A shows a feedback resistor on both the inverting and non- inverting inputs, but for hysteresis you need to place the resistor on one of the inputs and not both. This was also confirmed through simulation of the circuit.

    Regards,
    Jaskaran
  • Hi Jaskaran

    Thank you for your reply.

    > for hysteresis you need to place the resistor on one of the inputs and not both.

    That to say, can not LMH7220 accept differential input with hysteresis?

    For example, if the feedback path is placed on one of the inputs like following, input impedance would not be match for each input.

    Or, is it possible to handle differential input with using hysteresis?

    BestRegards

  • In addition to previous question, I want to confirm feedback path.

    >for hysteresis you need to place the resistor on one of the inputs and not both.

    In the following two cases, which one can the hysteresis be brought? Both?

    BestRegards

  • Hi,

    For your first question: The LMH7220 can accept differential inputs and you will need to place a termination resistor between the inputs.

    For your second question: There can be hysteresis on both the cases that you presented ("Positive out to positive in" and "Negative out to negative in").

    Regards,
    Jaskaran
  • Hi Jaskaran

    Thank you for your answer.  It is very helpful for me.

    As a result, we will choose the following configuration.

    Could you please tell us the way to calculate hysteresis?

    Now I suppose following procedure.  (I don’t know if it is correct or not)

    A single output voltage can be considered Voh=1.4v and Vol=1.0v by each.

    If Rf = 2Kohm, Rp = 1Kohm and Vref = 1.2v are assumed,  the hysteresis

    may be obtained with the following equation.

    - Vth_HtoL (Output High to Low)

    (Vout - Vref) * Rp/(Rp+Rf) + Vref = (1.4 – 1.2)*1K/(1K+2K) + 1.2 = 1.27v

    - Vth_LtoH (Output Low to High)

    (Vref - Vout) * Rf/(Rp+Rf) + Vout = (1.2-1) * 2K/(1K+2K) + 1 = 1.13v

    ⊿Vth = 0.14v

    I attached signal image.

    Is my understanding correct?

    If not correct, can you teach us the correct calculation?

    BestRegards

  • Hi,

    Glad to hear the previous answer was helpful. I looked over your procedure for calculating hysteresis and your calculation is correct based on the assumptions you made. You can find more information on comparator hysteresis in Section 2.1 of the following document:

    http://www.ti.com/lit/ug/tidu020a/tidu020a.pdf

    Regards,

    Jaskaran

  • Hi Jaskaran

    Thank you for your reply.
    We will continue consideration.

    BestRegards