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LM139: Can this comparator work fine if Vsupply=(Vin+) - (Vin-)

Oleksandr Makarchuk
Prodigy 30 points
Part Number: LM139

Briefly my question is in topic about comparator LM139AD.

I've read from datasheet: paragraph 9.2.2.1 about input voltage range and still have question.

Comparator supplied with voltage 5V.  To it inputs I dirrectly connect encoder outputs A+, A- and B+, B- (That has also 5V).

Will it work properly or I make mistake because I didn't make lower voltage range?

What can I do to to make right schem? Does I need connect some resistor in parallel to comparator inputs?

Please help me in such simple question for electronic specialists.

  • 0
    Guru 140200 points
    Hi Oleksandr,

    you can use a simple resistive voltage divider to make the input voltage comply with the common mode input voltage range of LM139.

    Or you could use a specified input rail-to-rail comparator.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 30 points

    - When IN– is higher than common mode and IN+ is within common mode, the output is low and the output
    transistor is sinking current
    - When IN+ is higher than common mode and IN– is within common mode, the output is high impedance and
    the output transistor is not conducting.

    I think it should work, isn't it? It will not work when both will be more than 3V.

  • 0 in reply to Oleksandr Makarchuk
    Guru 140200 points

    Hi Oleksandr,

    violating the common mode input voltage range can have an impact on the timing specifications (slow down of comparator switchings and increase of delay times) and can cause other issues. So, it's not recommended to have the input signals running outside the specified common mode input voltage range. It's your decision...

    Kai