What is the offset in an Op-Amp?

[eem2am]

Hello,

Please does any reader know what is the offset in an op-amp?

With an inverting op-amp, they put a resistor from the non-inverting input to ground........and i am not sure why.

 

[zning]

national instrument op amp offset

The offset voltage is defined as the voltage that must be applied between the input terminals of an op amp to reduce the dc voltage to zero (or mid-point).
It is usually something caused by the Opamp's internal imbalance/mismatch. and could range from mV to uV.

Usually the offset voltage is considered as a voltage source added to the non-inverting input, and can be either polarity.
So if you grounded the resistor at the inverting end, it will look like an non-inverting opamp with Vos (offset voltage) added as the input, your output voltage should be
Vout = (1 + Rf/R1)*Vos

And that would give you a general idea about the device's offset voltage.

This is my first post in this Forum. Hope this helps answer your question

 

[LvW]

op-amp tricky formula

Hello,

Please does any reader know what is the offset in an op-amp?
With an inverting op-amp, they put a resistor from the non-inverting input to ground........and i am not sure why.

You should not mix offset VOLTAGE with bias/offset CURRENT.

1.) the explanation for opamp offset voltage from eem2am is 100% correct.
2.) But another issue is that you create your own "offset" voltage by the dc voltage developed across the resistor connected at the minus input. This voltage is created by the dc bias CURRENT. However, if you put another resistor od appropriate value between the pos. input and ground this dc offset (which is not an opamp parameter but part of your circuitry) can be reduced (not completely cancelled).
3.) The value of this additional resistor should be app. R1||R2.

 

[Kral]

match opamp offset

eem2am,
Regarding the resistor from the NI input to ground, here's the equation for the output voltage error Voe due to input offset current Ib.
.
Let Rinv be the resistance as "seen" by the inverting input.
.
Let Rni be the resistance "seen" by the Non inverting input.
.
Let Rf be the feedback resistor. Then
.
Voe = IbRf(1-Rni/Rinv)
.
It is apparent from this equation,that if Rni = Rinv, then the error is zero. Note that if the NI input is left open (infinite resistance) the error theoretically goes to infinity. Of course, in the real world, the op-amp would just saturate.
Regards,
Kral

 

[xulfee]

offset non inverter op amp

input offset voltage is tht much voltage applied to input which gives ur output voltage zero,idealy it has to give us zero if both inputs r same but due to mismatch of transistors we didn;t get this,so v need offset voltage,the method u discribe is a trick,it will reduce input offset voltage but it will not give us zero volts at output

 

[Kral]

reducing dc error opamp

eem2am,
I'm not sure if the "trick" xulfee is referring to is the technique of placing a resistor from the NI input to ground or not. In any case, matching the resistances seen by the inverting and non inverting inputs will reduce the error caused by the bias current to exactly zero. However, there may be errors present that are not caused by bias current. These errors may be caused by the following:
. Input offset voltage
. Further explanation is neccessry.
. This is the voltage that must be applied
. between the two inputs to produce zero
. output in the absence of other errors.
. Offset Current
. Finite common mode rejection ratio (CMRR)
. Finite Power supply rejection
Regards,
Kral

 

[diegobb]

op amp offset

Voe = IbRf(1-Rni/Rinv)

You have some kind of problem within the equation, the units doesn't match, [V]=.
Regards, Diego.

 

[Kral]

opamp offset current

diegobb,
The units are correct. Voe has the unit "Volts". IbRf is current times resistance, which also has unit "Volts". Rni/Ri is dimensionless, since the units "Ohms" cancel..
Regards,
Kral

 

[diegobb]

offset in op amp

My mistake ... it seems that I didn't see the "Rf".

Regards, Diego.

 

[electronics_kumar]

offset de op/amp

to make output of op amp as zero when input is nil