A systematic method to approximate capacitance ratios to improve capacitance matching in SC filters

ABSTRACT

A critical issue in the design of switched-capacitor (SC) filters is the capacitance matching, because the filter coefficients depend on capacitance relative values - capacitance ratios. The most successful design method to achieve an accurate capacitance matching employs a parallel arrangement of identical unit capacitors to implement each filter capacitor. This method, combined with a careful layout design, can achieve a capacitance matching accuracy of 0.1%. However, the procedure can be directly applied only if the filter coefficients can be written as rational numbers, since each capacitor is implemented as an integer number of capacitors in parallel. This paper presents a systematic procedure, with low computational effort, to approximate the filter coefficients by integer ratios without causing significant errors in the filter frequency response, whereas keeping the total number of unit capacitors small, in order to save die area. This procedure is applied in a design example of a SC band-pass filter, which has been fabricated in a 0.35 mm CMOS technology.

REFERENCES

• 1: A. C. M. de Queiroz and P. M. Pinheiro. Switched-current ladder band-pass filters. In IEEE International Symposium on Circuits and Systems - ISCAS'94, volume 5, pages 309--312, 30 May-02 June 1994.
• 2: R. Gregorian, K. W. Martin, and G. C. Themes. Switched-capacitor circuit design. In Proc. IEEE, volume 71, pages 941--966, August 1983.
• 3: R. Gregorian and G. C. Temes. Analog MOS integrated circuits for signal processing. John Wiley & Sons, 1986.
• 4: D. Johns and K. Martin. Analog Integrated Circuit Design. John Wiley & Sons, 1997.
• 5: M. J. McNutt, S. LeMarquis, and J. L. Dunkley. Systematic capacitance matching errors and corrective layout procedures. IEEE Journal of Solid-State Circuits, 29(5):611--616, May 1994.
• 6: B. Razavi. Design of analog CMOS integrated circuits. McGraw-Hill, 2001.
• 7: C. Saint and J. Saint. IC Mask Design: Essential Layout Techniques. McGraw-Hill, 2002.
• 8: Y. Tsividis. Mixed analog-digital VLSI devices and technology: an introduction. McGraw-Hill, 1995.
• 9: Y. P. Tsividis. Integrated continuous-time filter design - an overview. IEEE Journal of Solid-State Circuits, 29(3):166--176, March 1994.
• 10: M. E. V. Valkenburg. Introduction to modern network synthesis. John Wiley & Sons, 1960.

INDEX TERMS

Primary Classification:
  B. Hardware
  B.7 INTEGRATED CIRCUITS
      B.7.2 Design Aids

General Terms:
Algorithms, Design

Keywords:
capacitance matching, design method, switched-capacitor filters