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LM311: Threshold voltage

Part Number: LM311

Hello team,

I received a question from the customer as follows,


When the frequency of the input signal exceeds 180 kHz, the comparator operates even though it is below the threshold(Only Hi side).

[Condition] See attached

Yellow line: Input voltage, Sin wave, 7.5Vpk, 180kHz  or more

Red line: Hi side threshold voltage, 9V

Blue line: Low side threshold voltage. -9V

Green line: Error




  • Hi Masa,

    this can mean that the LM311 is damaged. Does a fresh LM311 show the same behaviour?

    Another cause can be noise on signal ground or on the supply voltage rails. Do you have any circuitry in the same circuit with the LM311 which generates pulses or has to handle pulses? Is the supply voltage provided by switchers (DC/DC converters).

    Another cause can be stray coupling effects caused by an unsuited layout.

    We need to see a schematic and get further information from you about your application.


  • Hi Masa, 

    Thanks for reaching out. Can you show a complete schematic? What is the output connected to? What supply voltage are you operating at? 

  • Hello Kai, Chi, 

    Thanks for the support. See attached schematic. If it need around the this schematic. I will request to the customer.

    And supply voltage value is +/- 12V. 



  • Hi Masa,

    Is R96 actually 2.2ohms? That would be extremely small and cause excessive output power dissipation and increase your overall system supply currents.

  • Hi Masakazu,

    I see several issues. At first, I would replace both LM311 for the case that one or both LM311 are damaged.

    The input(s) of LM311 could be damaged due to an overvoltage. A current limitng resistor in front of both inputs could provide some protection. You can also add a small capacitance from both inputs to signal ground (behind the current limiting resistor(s)) to form a low pass filter.

    R965 and especially R982 are rather low ohmic and will cause a high current spike into the outputs of LM311 and into signal ground. This can cause ground noise. So, if you have further similar LM311 circuits on your board, the switchings from these LM311 could add so much noise on signal ground that IC70 is erroneously toggling, not because of the input signal itself but because of the noise superimposed to signal ground. This would explain why IC70 is already toggling when the input signal comes near the threshold voltage (set by the voltage divider at the +input).

    A good remedy against ground noise related issues is to add low pass filtering and to use a solid ground plane. So, I would add low pass filtering at the inputs of LM311 as already discussed above and I would add capacitors in parallel to R961 and R962.

    I would also think about increasing R982 and eventually R965 and R983.

    A layout mistake can also cause mistriggering. Keep in mind that any current flowing into the outputs of LM311 (pin 7) must flow back to C459. Imagine how the current is flowing when one of the outputs of LM311 is toggling low: The current is flowing out of C459, through R965, into pin 7, out of pin1 and back to the ground terminal of C459. To keep the ground noise minimal, it's essential that the distance between both pin 1 and the ground terminal of C459 is as short as possible. The same is true for pin 1 of IC71 and the ground terminal of C470.

    As a last tipp, sometimes adding small series resistances in the supply voltage lines can help to suppress supply voltage noise. Mount them in front of the supply voltage decoupling caps, in order to form low pass filter.


  • Thank you Kai for your great inputs. 

    Masa, please refer to Kai's suggestions. I hope they will help you with your issues.

  • Hello Kai, Chi, 

    Thanks for the strong support and investigations. I will describe it to the customer.