Switched Capacitor Filter

Transcription

1 Switched Capacitor Filter Design for Mixed Signal Applications by Klaus Jørgensen Napier No Supervisor Dr. Mohammad Y Sharif

2 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

3 Aims of the Project Design a State Variable Filter with Lowpass, 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.

4 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. 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 MΩ, a good value of the capacitor will be in the range of pf to pf and a sample frequency around khz. Capacitor of pf requires a silicon area around.mm2 R = C R f S p k = MΩ

5 Low-pass Switch Capacitor Filter Vac Vdc V R.592k C n V = V2 = 5 TD = TR = TF = PW = 5n PER = u V2 V = V3 V2 = 5 TD = 5n TR = TF = PW = 5n PER = u J J2 Vac Vdc V C 628p C2 n

6 There are six ways to make a switch capacitor resistor Parallel. Bilinar. Q Q Cs 628p R = Cs fs Cs 57p Q R= 4 Cs fs Q Series. Positive Transresistance Realization. Q Q Cs R = Cs fs C2 628p R = Cs fs 628p Q Series-Parallel. Negative Transresistance Realization. Q Cs 528p Cs2 p R = ( Cs+ Cs2) fs Cs 628p Q R = Cs fs

7 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. UA and UD performs as summing function, and UB and UC performs as integrators. R 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 db Low-pass High-pass Band-pass Band-reject -3dB Series -85 frequenzy

8 Switch Capacitor Filter Final +2Vcc V2 2Vdc V3 2Vdc Q V = V4 V = V5 V2 = 5 V2 = 5 TD = TR = TD = 5n TR = TF = PW = 5n PER = u TF = PW = 5n PER = u Q -2Vcc Q J C3 J2 J3 J4 p C4 Vac Vdc V Q J5 Q C5 p J6-2Vcc p TL V- 3 + UA V+ 4 +2Vcc OUT Q Q J2 C 62.5p J22 TL UB C n -2Vcc V- V+ 4 +2Vcc OUT 7 Q J23 C2 62.5p J24 TL UC C2 n -2Vcc V- V+ 4 +2Vcc OUT 8 Q Q J7 J8 C9 p J9 J2 C 9p J J2 C7 p J5 J6 C8 p Q -2Vcc J7 C6 p J8 TL V- 2 + UD V+ 4 +2Vcc OUT 4

9 Switch Capacitor Filter output 5 db State Variable Filter with Switch Capacitor Final High-pass Band-pass Low-pass Band-reject -3 db -5-4 db frequency E8

10 Advantage & Disadvantage of Switch Capacitor Filter Advantage Take up very little space of the chip area The accuracy of the capacitor it typically within,% Is very reliable for temperature changes Low supply use Disadvantage Because the f S has to be 5 to 2 times bigger then the f C the SC filter technology can t be used at very high frequencies Has to uses two clock frequencies

11 Questions.