Electronic Systems Design Group

Department of Electronicsand Computer Science

University of Southampton, UK

Analogue Filter IP Cores for Design

Reuse

Bashir Al-Hashimibmah@ecs.soton.ac.ukrw01r@ecs.soton.ac.uk

Reuben Wilcock

Reuben Wilcock ASP 2002

University of Southampton, UK

Slide 2

Outline

• Introduction and Motivations

• Analogue Filter IP Core Design

• Example Results

• Concluding remarks

Reuben Wilcock ASP 2002

University of Southampton, UK

Slide 3

Introduction

• System on Chips (SoC) employ IP cores

• Analogue IP core design is demanding– Difficult to trade as high level description– Correct operation depends on many factors

Important to redesign for particular specifications and process to ensure

functionality

Reuben Wilcock ASP 2002

University of Southampton, UK

Slide 4

Analogue Filter IP Core Design

• What are the considerations?

Circuit design technique

Filter Methodology

Automation

Reuben Wilcock ASP 2002

University of Southampton, UK

Slide 5

Solution ?

Circuit Design Technique

• Requirements– Simple, easily designed blocks – No high quality passive components– Compatible with present and future processes– High performance

• Solution: Switched Current– Designs based on current mirrors

– No linear passive components

– Current mode allows low Vdd

– High performance cells availableCgs1 Cgs2

M2M1

Iin Iout

S1

Reuben Wilcock ASP 2002

University of Southampton, UK

Slide 6

Solution ?

Filter Methodology

• Requirements– Suitable for Switched Current– Regular structures and simple design procedure– Based on LC ladder to inherit low sensitivity– Bilinear transform so Nyquist limit can be approached

• Solution: Wave Filters– Ideal for Switched Current

– Easily designed blocks

– Based on LC ladders

– Bilinear transformA22=0

2220R00

R10

R20 =

10

00 R01

R11

R21 =

11

2101 R02

R12

R22

12

02

z-1 z-1

B22A00

B00

-z-1

Vi

Ri L1

C1 C2 Ro

+v-

Reuben Wilcock ASP 2002

University of Southampton, UK

Slide 7

Automation

• Requirements:– Increase productivity– Transistor level

simulations– Optimisation

Step 1

Design referenceLC filter

Interactiveiteration

LC values fromtables

Idealsimulations

Step 2

Derive wave filter& coefficients

Automaticoptimisation[Yufera ‘94]

Samplingfrequency ratio

Idealsimulations

Step 3

Design memorycell [Hughes ‘00]

Interactiveoptimisation

Fs, Bias (Power)Mod, Vgt

BSimsimulations

Step 4

Put filter togetherand verify

Revisit 1-3 asnecessary

Results fromsteps 1-3

Filter response(BSim)

Solution ?

• Solution: SKILL tool– Automate hand-

calculations

– Integrated in Cadence

– Manual/Automatic optimisation loops

Step 5

Layout(using pCells)

Reuben Wilcock ASP 2002

University of Southampton, UK

Slide 8

Step 1: Passive Filter Design

• De-normalise values• Frequency response

• Choose filter type/function/order• Normalised component values

Reuben Wilcock ASP 2002

University of Southampton, UK

Slide 9

Step 2: Wave Filter Design

• Optimise [Yufera ’94]

• Frequency response from behavioural models

• Decide cutoff/sample ratio• Coefficients are calculated

Reuben Wilcock ASP 2002

University of Southampton, UK

Slide 10

Step 3: Memory Cell Design

• S2I memory cell [Hughes ’00]• Trade off design parameters• First cut design calculated• DC, transient simulations• Optimise for gm, Cgs, Ctot,

Switch Ron and settling

Reuben Wilcock ASP 2002

University of Southampton, UK

Slide 11

Step 4: Complete Filter Design

• Save all design variables, dimensions and coefficients to a single file

• Schematic representing entire transistor level design is opened

• Spectre RF used to give an AC response in minutes

• Revisit steps 1 – 3 as necessary

Reuben Wilcock ASP 2002

University of Southampton, UK

Slide 12

Schematic Hierarchy

• Parameter passing with pPar(“”)

• Hierarchy from top to transistor level

Reuben Wilcock ASP 2002

University of Southampton, UK

Slide 13

Conclusions

• Switched Current has good potential for IP cores

• Wave is suitable as a filter methodology

• Automation tools should be integrated into powerful CAD packages

• Our tool allows a designer to rapidly develop Switched Current analogue filter cores.

• Future work will involve – Extending the filter library

– Including Class AB cell as alternative to S2I

– May include layout (step 5)

Reuben Wilcock ASP 2002

University of Southampton, UK

Slide 14

Contact

Reuben Wilcock

Electronic Systems Design

Department of Electronics and Computer Science

University of Southampton

United Kingdom

rw01r@ecs.soton.ac.uk