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LM239: Input impedance consult

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Replies: 3

Views: 122

Part Number: LM239

Hi team,

   I am curious about how to determine the input impedance of the LM239? I cannot find this data in our datasheet. Thanks. 

  • Hi Charles,

    I assume the spec you're more interested in is Input Bias Current and how your resistor divider on the front end could impact the accuracy of the comparator. Input Resistance is generally an older term used for our older parts.

    It's important to know that your supply voltage and input common mode will have an effect on the bias current coming out of the input nodes. As your input common mode increases towards your supply voltage, the input bias will saturate and the "input resistance" will be very high.

    What is the common mode and supply voltage you are running your circuit at?

    The attached app note has a graph on page 7 (Section 2.6) showing the relationship of the input voltage to input bias current at a 5V supply.0083.snoaa35.pdf

    Regards,

    Jonny

  • In reply to Jonathan Nguyen:

    Hi Jonathan,

       Thank you for your response here. The supply voltage is 5V, common voltage is about 3.3V. You mentioned input bias current here, it will have impact in the R105 and R103. It will not have influence in the R119, what is the impact if we remove R119? Thanks.

  • In reply to Lin_Charles:

    Charles,

    The OPA device is characterized in driving a 100pF load, so you should not run into any issues removing R119 in your circuit.

    I noticed though that your voltage divider is violating the device common mode (Vcc - 2V). I would suggest switching to a device such as the TLV7042. It is an open drain dual channel comparator with rail to rail inputs.

    Regards,

    Jonny

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