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LM393: About input impedance

Tsukui Yusuke
Guru 21930 points
Part Number: LM393

Hi,

Could you tell me the input impedance (IN+,IN-) value of LM343?
Customers need specific value information for connection and design with other circuits.

Regards,
Yusuke

  • 0
    Guru 244220 points

    The inputs are the bases of PNP transistors and do not behave like resistors.

    Section 2.1 of Application Design Guidelines for LM339, LM393, TL331 Family Comparators says:

    All PNP emitters in the Darlington input stage have current source connections. These current sources ensure a consistent input bias current that does not vary with the differential input voltage. This consistent current provides a high effective input to input resistance.

    Your customer's circuits must be able to sink the bias currents.

  • 0
    Guru 140200 points

    Hi Tsukui,

    it can be estimated from the following figure of datasheet:

    For a supply voltage of 5V and 25°C die temperature we get

    Zin ~ delta U / delta I = (2.5V + 0.25V) / (2.75nA -2.25nA) = 5.5GOhm,

    superimposed to a DC input bias current of -2.75nA at Uin = 0V. The "-" sign means that the static input bias current is flowing out of the device :-)

    Kai