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Question about LMV358

fabricepeden
Intellectual 495 points
Other Parts Discussed in Thread: LMV358, OPA374

Dear All,

I got a problem with a LMV358.

I'm using the DAC of my µc (STM32f205). This output is wired on a voltage follower (realized with a LMV358). The output of this voltage follower is wired on four minus inputs of LMV358 (use as comparator).

The problem that I have is that if I put 0 in my DAC registrer, I got 0V on the ouput of my voltage follower, if I put 500 in my DAC registrer I got about 500mV, if I put 4096, I haven't 3.3V but 2.7V. My DAC is working, because if I disconnect my LMV358, everything isd working fine (4096 = 3.3V). It looks like a saturation ?

So I've tried with another AOP (MCP602), now I got the entire excursion in input (0-> 3.3V) but in output, the maximum voltage that I have is 2.1V.

If somebody got an idea, because I don't know how solve my problem. I've tried to add loads..

Thank you.

  • 0
    TI__Guru 52256 points

    Hello Fabrice,

    The LMV358 is NOT a rail-to-rail input device. It's input voltage range is limited to about 800mV below V+, or 2.5V. 2.7V is about where I would expect it to clamp. The LMV358 is working as expected!

    Look at the "Input Common Mode Voltage Range" line in the datasheet table - this is what is limiting you.

    The MCP602 specified input range is actually worse (1.2V below V+), but is a CMOS input device who tends to be more forgiving, but you are still violating their common mode specifications.

    You need a "rail to rail" INPUT device, that will cover the entire input voltage range. See the LMV931 or OPA374 or similar R-R input devices.

    Regards,

  • 0 in reply to Paul Grohe
    Intellectual 495 points

    Dear Paul,

    Thank you for your explanation. I'll try one of this as soon as I'll be back to the office.

    Best regards,

    Fabrice