Op amp offset voltage

[ericmar]

hi,

May I know what is the Op amp offset voltage in the datasheet referring to?

I am using an Analog Device's AD820 Op amp (with single supply) and I found that when I do not provide any signal to the +Input and have the -Input tied to the output, the output is always 5mV. Is this the offset voltage?

How should I do to get rid of this 5mV as it is affecting the operation of my circuit? BTW, I am using this AD820 op amp as a buffer/voltage follower from the output of a DAC.

Thanks for any help in advance.

Regards,
Eric

 

[pfd001]

This is not the offset voltage. You can connect the input together, the output is the offset voltage

 

[spectrabei]

This is a FET input / Bipolar output device. The rail to rail output swings within 5 mv of the minus supply and 10 mv of the positive supply with no resistive load.
You don't have a resistive load on your output, do you??? Put a pulldown resistor on the output of the op amp. Make sure you chose a resistor that won't allow the op amp to sink more than 12 ma in the positive output state ( 12 ma is typical sink current for this op amp). ALSO read the datasheet, this is all in there.

spectraBEI

Note: Data sheet attached

Note # 2: A good translator site for translating documents from one language to another is at this link: http://babelfish.altavista.com/
Most of these Datasheets (that I've seen) are in english and are PDF docs. What can be done to make them viewable in other languages is open up the PDF doc. click on edit, click on select all, click on copy. This will copy all the verbage in the PDF doc (diagrams will not be copied). Then go to the translator site (link above) select the appropriate languages (example: English to German) Paste the information (from the PDF) into box and click on the TRANSLATE button.

Steve
SpectraBEI

 

[A.Anand Srinivasan]

ya... it is the offset of the opamp...


offset voltage is the difference between the inputs of the opamp independent of the input given,....
since you are operating the opamp in voltage follower configuration it just gives the offset voltage at the output.,...

 

[kwkam]

In the spec, it already stated that typical 0.1mV voltage offset

 

[A.Anand Srinivasan]

this voltage could be a combination of the various parasitic effects... sorry i didnt go through the specs.... the opamp generally suffers from input offset voltage and input bias current and offset in the input bias current... but compared to 0.1mV the 5mV you are obtaining is too high to be explained using this...

is the wire used to provide feedback short enough....

 

[rowokii]

Actually, spectrabei had the answer... you have the op-amp as voltage follower with 0 v as input. The AD820 can only go down to 5mV of the neg. supply rails, so you get 5mV instead of 0.

If you really need full rail to rail, you can either use a supply for the op-amp that is higher and lower then the power and ground of the receiving circuit or change to a op-amp that can do "true" rail-to-rail output.

 

[qqic]

search its spec