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Non inverting opamp

Tharanath PP
Intellectual 390 points

Hi,

    I have 2 non inverting op amp (LM 358) having gain of 10. But problem is i cant get the correct gain.

Vin=10.05 mV, GAIN=10, +vcc=3v, -vcc= gnd,

So, I should get 100.5 mv at output. What I am getting is 114.9 mv.   I have another same op amp with same input . That is giving Vout as 77mv. Please help me?

Thanks,

Tharanath

  • 0
    TI__Genius 13935 points
    What is the tolerance on your resistors (Rf and Rg). Did you measure them? Can you attach a schematic and show how the input and output are coupled and the common mode voltages?
  • 0 in reply to Loren Siebert 1
    Intellectual 390 points

    Hi Loren,

         I am attaching the schematic. I am using above resisters in 0603 package. When I measured resistance is almost correct. Let me know importance of common mode voltage in this?

    Regards,

    TharanathDoc2.docxopamp.pdf

  • 0 in reply to Tharanath PP
    TI__Genius 13935 points

    Hello Tharanath,

    I don't see what tolerance resistors you are using.  If they are 10% most of the error could be due to resistor tolerance. 

    Another thing is that your inputs are ground referenced.  This is OK for the inputs, but the amplifier output can only swing to within 5mV of the negative rail (20mV guaranteed).  What is the reason this amplifier uses only a single 3V supply? 

    What is the voltage range of your input signal?  What load are you driving, is it ground referenced?  

    Operating an amplifer very close to the supply rail will often give unsual results. I would recommend using +-5V for this amplifier. 

     

    Regards,

    Loren

     

  • 0 in reply to Loren Siebert 1
    Intellectual 390 points
    Hi Loren,

    I changed resistance to 18k and 2k, Gain is same 10. But still in few op amp 77mv (vin= 10.05mv). If 10 % tolerence for both resistors , at worst case gain could be reduced to 8.36 from 10. Now here 77mv/10.05mv=7.6.

    My input varies from 5 mv to 200mv. Out put of amp is given to msp 430 adc input. that is gnd refernced. I am using 3.3v and gnd for op amp bcz msp 430 is also 3.3v supplied.

    Thank you,
    Tharanath
  • 0 in reply to Tharanath PP
    TI__Genius 13935 points

    Hello Tharanath,

    Are you looking at AC gain, or just the DC output for a given input?  If you are using  DC measurements you also need to factor in the input offset voltage (3mV typical).  At a gain of 10 the 3mV typical offset voltage is 30mV this will range from no offset to a limit of 7mV (x10 = 70mV) .  This will vary from part to part and would look like a gain error if you are only using DC voltages as a test for gain. 

     

    If the AC gain is not as expected it may be a slew rate or bandwidth issue.  I have attached a TINA model so that you can run some experiments. 

     

    Regards,

    Loren

     

     

     

     

     

    6165.LM358.TSC