# Opamp offset voltage problem

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

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
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

2. ### kellGuest

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. ### kellGuest

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

4. ### JamieGuest

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.