Compact lowpass ladder filters using tapped coils

Compact passive LC ladder lowpass filters for pulse shaping are realized using a single inductor with multiple taps. Mutual coupling between different inductors in a ladder results in zeros on the real and imaginary axis and can cause an undershoot in the step response and reduced attenuation in the stopband. Techniques to mitigate these effects are described. A seventh order Bessel filter is realized using the proposed technique in a 0.18 mum CMOS process. This filter exhibits an undershoot of 1.3% and provides a stopband attenuation of 37 dB. It occupies 0.048 mm², which is at least 15% smaller than a realization with separate spirals for each inductor.