Systematic capacitance matching errors and corrective layout procedures

Precise capacitor ratios are employed in a variety of analog and mixed signal integrated circuits. The use of identical unit capacitors to form larger capacitances can easily produce 1% accuracy, but, in many cases, 0.1% accuracy can provide important performance advantages. Unfortunately, the ultimate matching precision of the ratio is limited by a number of systematic and random error sources. We have analyzed the source and significance of the systematic error sources on actual integrated circuit layouts and isolated five key contributors. Based on this analysis, we have developed a list of generic layout rules and a layout scheme that predict matching accuracies better than 0.1% for the individual systematic error sources using capacitor sizes in the range of 20-40 /spl mu/m.>