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Opamp offset voltage problem

Discussion in 'Electronic Basics' started by [email protected], Feb 2, 2007.

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  1. Guest

    Hi! I want to measure the offset voltage of an opamp. First, I
    configure the opamp in an unity gain buffer and measure the output.
    The offset voltage is 64.16uV. Then, I applied this voltage to the
    inverting input of the opamp. When I simulate it again, instead of
    getting 0V at the output, I get 464.29mV. Is the setup correct? Is the
    offset voltage of 64.16uV correct? Looks a bit small for an opamp.


    * Schematics Netlist *

    I_I1 0 $N_0001 DC 100u
    M_M1 $N_0002 $N_0002 $N_0003 $N_0003 Mbreakp
    + L=1u
    + W=50u
    M_M2 $N_0004 $N_0002 $N_0003 $N_0003 Mbreakp
    + L=1u
    + W=50u
    M_M6 $N_0001 $N_0001 $N_0005 $N_0005 MbreakN-X2
    + L=1u
    + W=10u
    V_V1 $N_0003 0 5V
    V_V2 0 $N_0005 5V
    R_R1 $N_0007 $N_0006 12.245k
    M_M8 $N_0006 $N_0004 $N_0003 $N_0003 MbreakP
    + L=1u
    + W=100u
    M_M14 $N_0003 $N_0006 VOUT VOUT MbreakN-X7
    + L=1u
    + W=10u
    M_M7 $N_0007 $N_0001 $N_0005 $N_0005 MbreakN-X4
    + L=1u
    + W=20u
    M_M5 $N_0008 $N_0001 $N_0005 $N_0005 MbreakN-X3
    + L=1u
    + W=20u
    M_M15 $N_0005 $N_0007 VOUT VOUT MbreakP
    + L=1u
    + W=25u
    M_M4 $N_0004 $N_0009 $N_0008 $N_0008 MbreakN
    + L=1u
    + W=20u
    M_M3 $N_0002 $N_0010 $N_0008 $N_0008 MbreakN
    + L=1u
    + W=20u
    V_V8 $N_0010 0 0V
    V_V9 $N_0009 $N_0010 -64.16u
  2. kell

    kell Guest

    Here's another way that might be more reliable.
    To measure offset:
    ground the non-inverting input; run a pot from the op-amp output to
    ground; connect the pot wiper to the inverting input. Measure the
    output voltage. Calculate offset at the input as output voltage times
    1+(R2/R1), where R1 is the grounded side of the pot and R2 is the side
    connected to the op amp output.
    Is the op amp open loop? I don't think that's how you null them out.
    There are nulling cicuits here:
  3. kell

    kell Guest

    D'oh, or just use a couple of resistors.
  4. Jamie

    Jamie Guest

    you know, back in the old days! i think it is the 741? pins 1 and 8
    gave you nulling inputs you could put a pot on with the wiper to
    - to fix that little problem. :)
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