Op Amp Input Offset Voltage vs Common Mode Voltage

Thread Starter

himmels

Joined Mar 25, 2018
13
I would like to use an op amp AD8618 for a design, and need the input offset voltage to be low. For this op amp it seems fine with maximum of 65uV, but looking at the charts on page 6 etc, it seems to indicate that the input offset voltage in fact changes depending on the common mode voltage (at the inputs?) to much higher than that. (https://www.analog.com/media/en/technical-documentation/data-sheets/AD8615_8616_8618.pdf)

I intend to bias the amp at 2.5V, which could mean very poor results!? Is this a characteristic of all op amps, could someone explain to me or provide me a resource on this phenomenon? (and perhaps recommend me a better op amp with low Vos and a decent current output capability). Cheers
 

ericgibbs

Joined Jan 29, 2010
12,931
hi h,
The figures in page #3, are typical figures, there is always some production spread.
Is this page #6 image that you are referring too.?
E
AA1 27-Jan-19 12.06.gif
 

Thread Starter

himmels

Joined Mar 25, 2018
13
Hi Eric,

Yes exactly, seems odd that they advertise maximum 65uV, but in reality I've got to design for up to 120uV (still not bad, but you know!)
 

ericgibbs

Joined Jan 29, 2010
12,931
hi,
Analog are stating that graph shows the spread over 200 samples.
If it is critical that you have a <65uVoffset at a CMV of 2.5v, I would suggest you contact Analog, using their web Contact, ask for clarification.
If you get a response, please share it.
E
 

ericgibbs

Joined Jan 29, 2010
12,931
hi h,
Interestingly I see this graph on that datasheet.
The typical value they state includes the Note 3. 'Guaranteed by Design' whatever that means.???
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AA1 27-Jan-19 12.27.gif
 

OBW0549

Joined Mar 2, 2015
3,566

Thread Starter

himmels

Joined Mar 25, 2018
13
Hi Eric,

Yes many thanks, I think I'm going to go with the MAX44250 for this situation where offset voltage is very important (and 96mA drive capacity not bad). Also the settling time of 400ns for 2V step and can drive up to 500pF loads etc. Lots of positives
 
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