I setup the following circuit to measure the input offset voltage of an opamp; in this case an 741C opamp. The circuit is the same as in one of the EEVBlog video (https://www.youtube.com/watch?v=YeNpd-sXaHk)
The datasheet (http://www.ti.com/lit/ds/symlink/lm741.pdf) states a typical value of 2mV and an maximum of 7mv. So if all goes correct, i should see an odd 200mV to 700mV in output. However I am reading -160mV there.
Now I have changed the values of R1 and R2 and here is my findings
R1 R2 Voltage out Calculated Vos 100k 1k -166mV -166mV/101 = -1.6mV 10k 1k -16mV -16mV/11 = -1.6mV 10k 100 -166mV -1.6mV
It seams that -1.6mV is getting amplified at the output. Then is this the input offset voltage? Why is it negative and am I doing anything wrong?
I have the circuit in breadboard.
I tried with more than one 741 IC.
I have placed 10uF caps at the opamp supply pins and ground. But the output gone even lower.
- I have tried the first circuit with LM358 opamp as well. The voltage out then is around -0.04mV or -0.44\$\mu V\$ of input offset voltage, which is absurdly low. (Its datasheet says the offset voltage should be around 6mV)