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dcmatch results

mate0409

 Hi all,

 i'm doing a dcmatch analysis and can't understand the results. I'm analyzing a current mirror and at the output node i place a voltage source to measure the current. For example i get the following result:

 probe: 10.02uA +/- 301.6nA (3 sigma).

Now, dcmatch displays the variation sigmaOut and sigmaIds:

sigmaOut: 202nA, sigmaIds: 4.07% (first device)

sigmaOut: 202nA, sigmaIds: 4.07% (second device) 

...

sigmaout: 4.23nA, sigmaIds: 11.6% (fifth device) <-- why do i get an sigmaIds contribution of 11.6% for a current much smaller than 202nA which correspondens to a sigmaIds of 4.07%?

..

sigmaOut: 512pA, sigmaIds: 11.5% ??

 

(3sigma variation)

301.6nA/10.02uA -> 3.01%

The total sigmaIds = sqrt( sigmaids1^2 + sigmaIds2^2 + ... ). But if I sum all contributions together i get an deviation of 21%! Whats wrong here and where is my mistake?

 Thank you!

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