Op-Amp Input Offset

[chrisabird]

If someone could point me in the right direction to read more on this I'd be very grateful.

I have a LM324 configured as a simple voltage follower, single rail, output connecting back to inverting, and an input to the non-inverting. The output it is fed into an analog input of a ATMega328.

What i am noticing is that there is a -200mv discrepancy between the input and the output, i was expecting it follow withing the tolerance of the input offset (2mv).

Have i totally misunderstood this, and am i looking in the wrong place for this problem?

[Ian.M]

200mV seems excessive.  Are you sure your +in is within the acceptable common mode range, which does *NOT* extend to the OPAMP's positive supply?

[chrisabird]

Thanks Ian.M. 

Yep, that was the problem, my input signal is just outside the common mode range. I forgot to check that  when switching parts

[danadak]

Useful amplifier calculator -


http://masteringelectronicsdesign.com/summing-amplifier-calculator-java/


Regards, Dana.

[Brutte]

Why buffer? Does the source of the signal has high output impedance and ADC requires a low impedance source?
Well, LM324 does not have a bias current compensation, just use similar impedances on both inverting and non-inverting nodes.
BTW, this ADC is impossibly slow (15ksps?), unless your source has an impedance of megaohm range, just use a capacitor instead of a buffer. Guaranteed 0.0V offset.