Design of a sixth order elliptical low pass filter in cascade design with Switched Capacitor second order stages of type biquad

Introduction

I can't publish the Cadence files so the following README shows how to project a sixth order filter with Switched Capacitor in Cadence. For more information please see wiki, the italian language report or the presentation.

Switched capacitor filter

Filters generally consist of resistors, capacitors and inductors. Since the resistive elements are difficult to implement in integrated form and occupy large areas, these are replaced by switched capacitors. You can see that the switched capacitor behaves like a resistor of value R = 1/(f_clk*C). The switched capacitor filter to be designed must meet the following specifications.

Specification	Value
Sampling frequency	2MHz
Pass band gain	0 dB
Bandwidth	0 - 60 kHz
Pass band ripple	< 0.15 dB
Fstop	72 kHz
Attenuation in stopband	40 dB
Power supply voltage	3.3V

Getting started

Prerequisites

• Matlab, Matlab Filter Designer
• Virtuoso Cadence 6

How to use the Matlab code

1. Open Matlab
2. Open filter_design.m
3. Run the parts of the code to calculate the quality factor, the capacitances and the aspect ratio of the MOSFETs that constitute the real op-amp.

Results

Following the steps in wiki it's possible to design a sixth order filter in Cadence with second order sections. The AC responses are shown below.

First stage

Second stage

Third stage

Total filter