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Regarding Change of state at comparator output while probing its input voltages with Multimeter

TI_Electronics
Intellectual 345 points
Other Parts Discussed in Thread: LM111

While testing following circuit, I observed comparator (LM111) changing its output whenever I tried to find out input voltage value at comparator input pin using Digital Multimeter.  What is the reason behind change of output voltages while probing comparator input pins using Digital Multimeter?

  • 0
    TI__Guru* 93105 points

    Hello Kamlesh,

    When you place the DMM lead to the comparator input you are coupling noise directly into the input. The DMM lead being exposed to the surrounding environment is subjected to everything from 50/60 Hz power line fields, to RF energy. Since it is a high impedance circuit the noise voltages developed can be high enough to cause the comparator output to flip between states. Comparators have very high gain so a small voltage change at one input in relation to the voltage level at the other input results in the output changing state.

    If you aren't using a shielded cable with the DMM, then give that a try. If you don't have a shielded cable, you might try reducing the value of the resistor connected between the input signal pin and ground. That will load the external noise sources and reduce their amplitude.

    Do note that DMMs sometimes output sampling noise through their inputs and that can be injected into a circuit. That too could be a source of noise. If that is the case, you may want to try a different DMM.

    Regards, Thomas

    PA - Linear Applications Engineering