Switched-current techniques under pressure: high order and wideband filter design

Abstract

Fully-balanced switched-current Euler integrators featuring S2I cells and common-mode rejection within the circuit have been utilised in the efficient realisation of modified SI ladder filters with simplified input arrangements for savings on component count. Using prototype ladder manipulation, quite remarkable reductions in spreads and total gm were achieved for the 21st order elliptic lowpass filter, while preserving low passband sensitivity. This paves the way for realistic high-order and high frequency SI filters. Wideband switched-current UL-LU and LU-UL filters, implemented using S3I Euler integrators, do not require additional input stages to preserve the exact bilinear mapping; they utilise simple Euler integrators, thus allowing low clock-to-cutoff frequency ratios. Sensitivity simulations reveal that low passband sensitivity is maintained even for large relative bandwidths. This, together with low passband noise, make these structures ideally suited for wideband bandpass designs. (11 pages)