Any new Matching techniques???????

[mahendra]

Hi all

I want to match Differential pair with offset <100uV any matching techniques u aware other than Common centroid, plz help me.Thanx in advance.

Regards
Mahendra

 

[Humungus]

No black magic!!!

MAtching is proportional to a constant parameter deending of the kind of device (let's call it Ao) and the device's size.

For an OpAmp, if very properly designed, the offset can be mainly due to the Vth mismatch of the input pair.

That is sigma(Voff) ~ sigma(Vth) = Ao/sqrt(W.L), where W and L represent the size of the input transistors.

Reaching 100uV of offset just by design is nearly impossible as besides the offset contributed by the output transistors you will see also the one introduced by the other parts of the OpAmp (mirrors and second stage) as well as a contribution of the systematic offset.

You could tune your OpAmp, but tuning is only useful in some cases and for a given temperature only. Also, you'll see aging effects and you'll have to re-tune your system periodically.

If a so low offset is absolutely needed, I recommend you to use an auto-zero scheme.