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LM393: IN+ Pin Impedance Consult

Part Number: LM393

Hi Team,

Customer is consulting IN+ pin(3) impedance value here. Pls help us here.



  • The inputs are the bases of PNP transistors, so they always source current. The bias current is approximately constant; see section 2.1 of the Application Design Guidelines for LM339, LM393, TL331 Family Comparators. You could divide the input voltage by the bias current to get a resistance, but that value is less useful than the current itself.

    If you want to estimate the dynamic resistance: figures 6-21…6-23 show the bias current vs. input voltage of the LM393B.

  • Hi Stanley,

    under ideal conditions and at room temperature the dynamic input resistance of LM393 is beyond 100MOhm. You can easily derive this value from the input bias current versus input voltage plots. But keep in mind that the input bias current change is dominated by a high static input bias current which does not only show high manufacturing tolerances but also a high temperature drift. So when it comes to the LM393 it should not be talked about the input impedance.

    Another approach makes more sense: Due to the high input bias current of LM393 all source resistances should be limited to sufficiently low values. Otherwise the input bias current causes a voltage drop across the source resistance which appears as additional offset voltage and which can easily exceed the input offset voltage of LM393. A good rule is to keep the source resistance below

    R < 5mV / 250nA = 20k

    where 5mV is the maximum input offset voltage and 250nA is the maximum input bias current of LM393 at room temperature.

    So, when you have source resistances in your application which are higher than 20k, you should better go for a CMOS comparator providing much smaller input bias currents compared to the LM393.


  • Hi Stanley,

    As Clemens and Kai have mentioned, "Impedance" is an antiquated term.

    It is better to know that there is a constant 25nA flowing from V+, then OUT the input towards ground.

    Please see the LM393 Application note for more information (section 2.1):

     Application Design Guidelines for LM339, LM393, TL331 Family Comparators

    For less bias current, see the LM393B, which has almost 1/10th the bias current (higher "impedance").

    And if the supply is less than 5V, the LM393LV has CMOS (no bias current) R-R inputs, so the "impedance" is 10's of Gohms.