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Opamp offset voltage problem

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 non- 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.

Thanks. Amy

  • 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
Reply to
amy_burton2007
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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:

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Reply to
kell

offset voltage of an opamp. First, I

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

Reply to
kell

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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Reply to
Jamie

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