Switched Capacitor Filter Design for Mixed Signal Applications by - - PowerPoint PPT Presentation

Switched Capacitor Filter

Design for Mixed Signal Applications

by Klaus Jørgensen Napier No. 04007824 Supervisor

• Dr. Mohammad Y Sharif

Contents

• Aims of the Project
• Switch Capacitor Filter Basic Theory
• Low-pass Switch Capacitor Filter
• State Variable Filter
• Switch Capacitor Filter Final
• Advantage & Disadvantage
• Questions

Aims of the Project

• Design a State Variable Filter with Low-

pass, High-pass & Band-pass outputs.

• Understand the basic theory of Switch

Capacitors Filter.

• Replace the resistors in the State Variable

Filter with Switch Capacitors.

• Simulate and implement the Switch

Capacitor Filter suitable for audio applications.

Switch Capacitor Filter Basic Theory

• A simplified version on how a

capacitor can work as a resistor by charting and recharges the capacitor by using a switch.

• In practice the switch is replaced

with two MOSFET, the gates of the MOSFET is controlled by two clock pulses which do not overlap each outer.

• A

switch capacitor resistor requires a very small silicon area to make a very large resistor value, if implement a resistant of 10MΩ, a good value of the capacitor will be in the range of 1pF to 10pF and a sample frequency around 100kHz.

• Capacitor of 1pF requires a silicon

area around 0.01mm2

10MΩ 100k 1p 1 f C 1 R

S R

= × ⇒ × =

Low-pass Switch Capacitor Filter

R 1.592k C 10n V1 1Vac 0Vdc

VDB

J1 BC264A J2 BC264A C1 628p C2 10n V1 1Vac 0Vdc V2 TD = 0 TF = 0 PW = 500n PER = 1u V1 = 0 TR = 0 V2 = 5 V3 TD = 500n TF = 0 PW = 500n PER = 1u V1 = 0 TR = 0 V2 = 5

VDB

There are six ways to make a switch capacitor resistor

Cs 628p Q2 Q1

In Out

fs Cs 1 R × = Parallel.

Cs 628p Q2 Q1

Out In

fs Cs 1 R × =

Series.

Out In

Cs1 528p Cs2 100p Q1 Q2

( ) fs

Cs2 Cs1 1 R × + = Series-Parallel.

Q1 Q2 Cs 157p Q1 Q2

Out In

fs Cs 4 1 R × × =

Bilinar.

Cs 628p Q2 Q1 Q2 Q1

In Out

Negative Transresistance Realization.

fs Cs 1 R × =

C12 628p Q1 Q1 Q2 Q2

Out In

Positive Transresistance Realization.

fs Cs 1 R × =

State Variable Filter

• One advantages of using

SVF is that by changing the value of only a few components the parameters for the whole circuit is changed.

• U1A and U1D performs as

summing function, and U1B and U1C performs as integrators.

• R1 determine the gain for

the hole circuit.

• The ratio between R4 and

R5 determined where the Centre frequency

• The ratio between R6

and R7 specifics the value of Q.

• 3dB

• 85
• 80
• 75
• 70
• 65
• 60
• 55
• 50
• 45
• 40
• 35
• 30
• 25
• 20
• 15
• 10
• 5

Switch Capacitor Filter Final

• 2

• 6

• 9

• 13

• 12Vcc
• 12Vcc

• 12Vcc

• 12Vcc

• 12Vcc

Switch Capacitor Filter output

State Variable Filter with Switch Capacitor Final

High-pass Band-pass Low-pass Band-reject

• 3 dB
• 40 dB

1000 10000 100000 1000000 10000000 1E8

• 85
• 80
• 75
• 70
• 65
• 60
• 55
• 50
• 45
• 40
• 35
• 30
• 25
• 20
• 15
• 10
• 5

5

frequency dB

Advantage & Disadvantage

• f Switch Capacitor Filter
• Take up very little

space of the chip area

• The accuracy of the

capacitor it typically within 0,1%

• Is very reliable for

temperature changes

• Low supply use
• Because the fS has to

be 50 to 200 times bigger then the fC the SC filter technology can’t be used at very high frequencies

• Has to uses two clock

frequencies Advantage Disadvantage

Questions.