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LM358: Output voltage 0.03V

saurabh kshirsagar
Prodigy 30 points
Part Number: LM358
Other Parts Discussed in Thread: LM7705

Even at zero input voltage the LM358 gives 0.03V output.

Problem is that this signal corresponds to frequency. It shows 18RPM with zero input voltage. Can I reduce the output offset to 0.1mv ??

  • 0
    Guru 140200 points

    Hi Saurabh,

    the output voltage of OPAmp cannot fully reach the negative supply rail (0V):

    One remedy is to use a small negative supply voltage instead of 0V. The LM7705 could be used, e.g..

    Kai

  • 0
    TI__Guru*** 139956 points

    Hello Saurabh,

    Output voltage could be the result of offset voltage or it may a result of load current. In some cases, it is the result of different ground potentials. 

    What is the (non inverting) gain of the the circuit and the load current magnitude and direction? Remember, that the feedback network is part of the output load.

  • 0 in reply to Ron Michallick
    TI__Expert 6625 points

    Hello Saurabh, 

    Like Kai mentioned, this is due to output swing limitation. As you can see from the datasheet for a 5V supply, the negative rail can swing as close as 5mV (typical value) to the negative rail. One solution is to use a dual-supply, if you need the output to be 0V.

    I will go ahead and close this thread for now. Please let us know if you've any further questions. 

    Regards,

    Bala Ravi

  • 0 in reply to Bala Ravi
    Prodigy 30 points

    Thanks everybody to provide some useful insight!!

    It is clear that providing a negative voltage could solve the issue. 

    But in this case I am limited to +15V and GND. I need options within this supply range.

    As of now I have come a conclusion of replacing the IC namely LM358 (2mV offset) by other low offset voltage, precision amplifier OP-AMP IC.

    Any study material or video link to showcase, the reduction in offset voltage of OP-AMPS ,from any of the esteemed members is highly appreciated.

    Thanks.

  • 0 in reply to saurabh kshirsagar
    Guru 140200 points

    Hi Saurabh,

    it's not so much a question of input offset voltage but of output voltage swing. So, choosing an ultra low offset voltage OPAmp will hardly solve the issue.

    Can you show a schematic of your circuit?

    Kai 

  • 0 in reply to kai klaas69
    Prodigy 30 points

    Hi Kai ,

    Following is the schematic with values and a DAC at its input.

  • 0 in reply to saurabh kshirsagar
    TI__Guru*** 139956 points

    Saurabh,

    Can you measure the voltage at op amp pins 1,2,3?  Where does "Analog_OUT2" go?  Does any current flow here?

  • 0 in reply to saurabh kshirsagar
    Guru 140200 points

    Hi Saurabh,

    I would again recommend the LM7705. It generates a negative supply voltage of -0.232V, simply out of the positive +5V supply. Have a look into the datasheet of this very nice "trouble shooter"! Also, choose a better OPAmp than the LM358. Take an OPAmp with input-output-rail-to-rail ability which can also work with a +5V (and -0.232V) supply voltage. TI has lots of them.

    Or you might also want to think about the use of one of TI's charge pumps. These produce a negative supply voltage entirely out of the positive supply voltage:

    Kai