Offset voltage - Operational amplifier?

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igorbog

Junior Member level 3
How to calculate Voff??
In the theory we coonect V+ to V- to make Vepsilon=0. then we check Voutput and devide by Av. or Voffset=Vout? thanks

LvW

Advanced Member level 5
How to calculate Voff??
In the theory we coonect V+ to V- to make Vepsilon=0. then we check Voutput and devide by Av. or Voffset=Vout? thanks

What do you mean with "in the theory"? Do you refer to the definition? In this case, you are wrong.
Voff is defined as a voltage that has to be across the input terminals in order to produce Vout=0 !

---------- Post added at 13:50 ---------- Previous post was at 13:36 ----------

Because Voff is a parasitic rather than a design parameter it cannot be calculated - only measured.

pavel_adameyko

Full Member level 4
How to calculate Voff??
In the theory we coonect V+ to V- to make Vepsilon=0. then we check Voutput and devide by Av. or Voffset=Vout? thanks
Hello
The simple way to calculate offset voltage of operational amplifier is to put it into unity gain configuration - i.e. connect it output and negative input terminal. The difference between voltages at positive and negative terminals (or output) would be offset voltage.

LvW

Advanced Member level 5
Pavel, for practical purposes this procedure may be OK - however, it is not in accordance with the definition of Voff.
Voff is defined for Vout=0 (as I have mentioned before)!

pavel_adameyko

Full Member level 4
Pavel, for practical purposes this procedure may be OK - however, it is not in accordance with the definition of Voff.
Voff is defined for Vout=0 (as I have mentioned before)!
Hello, Lvw
I agree with your definition - but you should adjust it for the case when your amplifier has single power supply - in this case Vout = Vcc/2 or smth like that but not 0.

LvW

Advanced Member level 5
Hi Pavel,

I know what you mean. However,
* the original question did not mention this specific situation with a singke supply, and
* the parameter Voff is specified as mentioned before only for double supply.
* In case of single supply I think, you cannot "adjust" the specification.
The reason is as follows: The dc output will be never exactly at Vcc/2 and it is by far not correct to allocate this
deviation to any unsymmetry of the opamp input (that - in fact - is the cause of Voff). In contrast, the output deviation
with respect to Vcc/2 will mostly be caused by tolerances of the supply voltage and of the resistive voltage divider.

connect0715207

Newbie level 4
can anyone tell me what would be the voffset for a given vout of opamp

LvW

Advanced Member level 5
can anyone tell me what would be the voffset for a given vout of opamp

I am not sure if you will be satisfied with my answer, however, I am afraid that's the only one:

If the output voltage contains a dc component that does not originate from amplification of an input signal (double supply assumed), this dc component is produced by the input offset.

FvM

Super Moderator
Staff member
can anyone tell me what would be the voffset for a given vout of opamp
You can refer to the large signal DC gain to calculate an expectable Vout dependence of the offset voltage. But in my opinion, it won't bring you far. For a real OP, the DC transfer characteristic will be considerably nonlinear. Furthermore, you'll notice, that particularly for rail-to-rail amps (which usually involve a double input stage), the offset voltage will much more depend on the input common mode voltage than on Vout. So you would need to specify the measurement conditions exactly.

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