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LM2903-Q1: LM2903-Q1

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Replies: 3

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Part Number: LM2903-Q1

Hi,

I am using LM2903-Q1 as a comparator need to know the parameter Vicr or Vcm = Common mode input voltage range.

what is its effect on input bias current and how it must be considered.

This is reference taken LM2903 datasheet.

with regards,

SIddharth Sangam

  • Hello SIddharth,

    The common mode specification is on the next line, "Input Common Mode Voltage Range". The recommended input voltage range is 0V to 2V below V+.

    Bias current changes just a few nA over the common mode range since the input stage is fed from a regulated current source. As you start to get above 1V below V+, the input bias current starts to drop into the pA's as the input stage is cut off. Note that the bias current flows OUT of the input pin towards GND due to the PNP input stage.

    Please see the following Appnote, particularly section 2.6, figure 4, which shows the effects of bias current over common mode.

     Application Design Guidelines for LM339/LM393/TL331 Family Comparators

    Regards,

    Paul Grohe

    TI Comparators (CMPS) Applications Group

  • In reply to Paul Grohe:

    Thankyou Paul.

    This helped a lot.

    Just one more thing was there 

    If I have set reference voltage as 3.25VDC at inverting terminal (as common mode input voltage throughout temperature is Vcc-2V , Vcc =5VDC).

    I am providing input voltage as 5VDC at non-inverting terminal.

    What will be the response at output ,will the output voltage be high or any misbehaviour will be there.

    with regards,

    Siddharth Sangam

    8788110527

     

    with regards,

    Siddharth Sangam

  • In reply to Siddharth Sangam:

    Siddharth

    we do not recommend having both inputs outside of the recommended input common mode range.  If you build the circuit as you described with 3.25V on the inverting input and 5V on the non-inverting input while being powered by 5V, you will probably see the appropriate result of logic high at the output.  However, over process and temperature variation, you may all of a sudden find devices where that is not working.  One of the inputs needs to be within the specified common mode input range that you eluded.  Otherwise, you are risking the output of the comparator being undefined.  I would recommend resistor dividing the 3.25V reference down to a value below 3V in order to avoid any potential issues.  But please understand, that typical device will probably show no problem with the setup that you have described.  It is only over process and temperature that you can potentially run into problems. 

    Another option would be to switch to a comparator with a wider input common mode range.

    Chuck

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