Switched capacitor filter

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Switched capacitor filter

  1. 1. Switched Capacitor Filter (SC filters) By student : EE562 Minh Anh Nguyen
  2. 2. Outline • Introduction • Basic building blocks (OTAS, Capacitor,Switches and non-overlapping). • Basic operation and analysis (resistor equivalence of switched capacitor filters and integrators). • Definition of switched-capacitor filters. • Basics circuit for Switched-capacitor filters • Disadvantage & advantage of switched-capacitor filters. • Compared switched-capacitor filter circuit with other circuit • Summary and reference of switched-capacitor filters
  3. 3. Introduction • There are three main types filters, in integrated analog filters. 1. Switched capacitor filter (SC filter) -resistor replaced by switch capacitor -sample time but analog values 2. R-C filter -“Standard” active filter RC and Opamp with feed back -Resistor often implemented with MOS, so called MOSFET C filter 3.gm-C filter -resistors replaced by trans-conductor used open loop -two latter types are continuous time filter
  4. 4. Historical background • Due to the difficulty in making fully integrated resistors the active RC filters were not able to fabrication in monolithic form on one silicon chip. • Switched capacitor filters characterized in the z domain were developed late 70s and earlier 80s. • The origin of SC principle was first report by Maxwell around 1873. • The first book fully dedicated to switches capacitor was published in 1948 by P.E.Allen and E.SanchezSinenico “Switches capacitor circuit” Van Nostrand Reinhold,NY,1984.
  5. 5. Basic building blocks • The ideal operational amplifier is a voltagecontrolled voltage source with: • Infinite gain and input impedance • Zero output impedance. • Vo=A(Vi)
  6. 6. Basic Building Block of OTAS • Often realized as single-stage load compensated OTAs since the load is purely capacitive. • Low dc gain affect the accuracy of the transfer function • The unity-gain frequency should be at least five time higher than the clock frequency. • Dc offset can result in high output dc offset depending on the topology chosen. The techniques exist that can significantly reduce this offset and at the same time reduce 1/f noise. • Not so low output impedance • Still used as voltage amplifier
  7. 7. Opamps Vs. OTA
  8. 8. Building block of capacitors • Double poly capacitors • A highly linear capacitance is usually constructed between two poly-silicon layers • Substantial parasitic with large bottom plate capacitor (20% of C1) • Metal-metal capacitors are used but have even large parasitic capacitances
  9. 9. Building block of switches • MOSFET switches are good switches • Should have as high off resistance Roff as possible. At T=300K, MOS switches have Roff on the order of giga ohms. The finite value is caused by finite leakage currents that is typically dominated by reverse biased diodes. • Should have as low on resistance Ron as possible. Ron can be made arbitrarily small by increasing the width of the transistors. But parasitic capacitance and leakage current increase with increasing width. • MOS switches does not introduce any offset • BJT switches does introduce offset
  10. 10. MOS Switches • Nonlinear capacitance on each side of the switch. • Charge injection effects • Capacitive coupling from the logic signal to each side of the switch.
  11. 11. Charge injection • An additional charge, coming from the MOS channel when the switch is turn off, stored on the CL • Charge store in the channel when switch is on. • Direct coupling capacitance Cgd. (Mainly to overlap capacitance Cgdov). • When phase1 switches charge injection into Vi and Vo
  12. 12. Charge injection • Input node vi is typical low impedance node • When phase1 switched high(off-on) charge injected into Vi and Vo node collected by input impedance (in this phase the output require follow the input voltage Vi) • When phase1 switched low(on-off) charge injected into Vi
  13. 13. Charge injection (Const.) • For nMOS charge during the on state • Charge stored in the channel Qch CoxWL VDD ( Vt h Vi) charge due to overlap capacitance Vi Qgsov Cgsov ( VDD Vi) Qgdov Cgdov ( VDD Vo Vi) • Charge during the off state; Qch 0 charge due to overlap capacitance Vi Qgsov Cgsov ( VDD Vi) Qgdov Cgdov ( VDD Vi) Vo
  14. 14. Non-overlapping clocks • To guarantee that charge is not lost in SC circuits, non overlapping clocks are used. • Both clocks are never on at the same time. • Integer values occur at end of phase 1 • End of phase2 is ½ off integer value
  15. 15. Resistor equivalence to a switched capacitor • The capacitor is the “switched capacitor” • Non-overlapping clocks phase1 and phase2 controlled M1and M2, respectively. • Vi is the sample at falling edge of phase1 • And sample frequency is f
  16. 16. Resistor equivalence to a switched capacitor (Const.) • The charge transferred from V1 to V2 is Q C( V1 • The average current flow from V1 to V2 is Ieq • With the current flow through the switch capacitor resistor proportional to the voltage across it, the equivalent “switch capacitor resistance is V2) Q T
  17. 17. Resistor equivalence to a switched capacitor (Const.) T Ieq 1 f Q T Q C V1 C V2 T T Q C ( V1 T T Q V2) C V1 C V2 1 T f Q C ( V1 V2) f T Ieq Req Req C ( V1 V V2) f where V V1 V2 Ieq V1 C ( V1 V2 V2) f Req 1 C f
  18. 18. Resistor equivalence example What is the equivalent resistance of 10nF capac itance sample at a clock f requency of 100kHz. Req 1 Cf 1 Req 1 10 Req 1 9 3 100 10 4 10 • This equivalence is very large • Requires only 2 transistors, a clock and relatively small capacitance • In a CMOS process, large resistor would normally require a huge amount of silicon area
  19. 19. What is Switched capacitor filter? • The switched capacitor filter is technique based on the realization that a capacitor switched between two circuit nodes at a sufficiently high rate is equivalent to a resistor connecting these two nodes. • Used a miller integrator circuit, replaces the input resistor by a ground capacitor together with two MOS transistors acting as switches. • The switches are driven by a non-overlapping two phase clock. • SC filters operate on the principle of transferring analog signal samples ( represented as charges on capacitors) from one storage element to another
  20. 20. Switched capacitor filter • Let built an SC filter • We’ll start with a simple miller integrate circuit • Replaced the physical resistor by an equivalent SC resistor.
  21. 21. SC filter Wave form • The typical Waveforms
  22. 22. Transfer function • The basic idea to calculated the transfer function H( s ) Zf Zi
  23. 23. RC active filters • Calculated the transferred function for RC active filters ( V2 Vi) R C2 dVo Vi dt R C2 Vo Vi (s ) 1 R C2 d dt ( V2 Vo ) 0
  24. 24. SC filters (non-inverting) • During phase1(S1 on,S2 off) • C1 charge up to the current of vi • During phase2(S1 off, S2 on) Discharge into C2 or A charge packet C1Vi is remove from C2
  25. 25. SC filters • Calculated the transferred function for SC filter ( V2 0 Vi) C2 Req dVo dt Vi dt d Req C2 1 Req Vo Vi C1 f (s ) 1 1 C1 f Vo Vi (s ) C2 s C1 f C2 s ( V2 Vo )
  26. 26. SC filters (inverting) • Phase1:S1 on, S2 off Vi is store in C1, S1 is driven by Vi, S2 is maintained at 0, by the virtual ground. • Phase2: S1 off, S2 on Vi is disconnected, C1 is complete discharge for the next cycle.
  27. 27. SC filters (inverting) • Calculated the transfer function ( V2 0 Vi) C2 Req dVo dt Vi dt d Req C2 1 Req Vo Vi C1 f (s ) 1 1 C1 f Vo Vi (s ) C2 s C1 f C2 s ( V2 Vo )
  28. 28. Gm-C filter • An ideal transconductor is described by the following expression io Gm Vi The ouput voltage of the integrator is Vo Vo Io sC1 Gm Vi sC1 Vo Gm Vi sC1 H( s ) Gm C1
  29. 29. First order low pass filter • Calculated the transfer function Zf H( s ) Zi 1 R2 H( s ) H( s ) K w H( s ) S C2 R1 K w s w R2 R1 1 R2 C2 1 R1 C2( s R2 C2 1)
  30. 30. First order high pas filters • Calculated the transfer function Zf H( s ) Zi R2 H( s ) 1 R1 H( s ) K w H( s ) K sC1 s s w R2 R1 1 R1 C1 R2 s R1( s R1 C1 1)
  31. 31. Comparison • This is the table compare the transfer function for some of the filter
  32. 32. SC filter Noise • The resistance of switch M1 produce a noise voltage on C with variance kT/C 2 2 Q C • The corresponding noise charge is 2 Q 2 V KT C • This charge is sample when M1 is open • The resistance of switch M2 contribute to an uncorrelated noise charge C at the end of phase 2 • The mean square of charge transfer from v1 to v2 each sample period is • 2 Q 2KT C
  33. 33. SC filters noise (const.) • The mean square noise current M1 and M2 KT/C 2 2 2 noise is I Q f 2 I 2 KT C f 2 • The noise spectrum are single sided by convention, the distributed between 0 and f/2.The spectra density noise is 2 I 2 KT C f 2 f f 4 KT C f f 2 C f Req f 2 2 1 1 C 2 I Req I f 4K T Req • The noise from an SC resistor is equal to the noise of physical resistor
  34. 34. SC resistor noise spectrum
  35. 35. Advantage • • • • • Reduction of power consumption for filters IC High integration density Area(switches + capacitor) << area resistor Switch capacitor integrator R is replaced by C and 2 switched (MOS transistor)
  36. 36. Disadvantage • Sample data effect (noise) • Need clock circuit and anti-aliasing filters • Not suited for high frequency
  37. 37. Why Switched-capacitors(SC) circuits? • Resistors occupy inordently large amount of area in integrated circuits • AC resistors can be simulated by periodically switching a capacitor between slow varying voltages • Area(switches + capacitor)<< Area resistor
  38. 38. Application of SC filter • Over sampled A/D and D/A converter • Analog front-end (CDs) • Stand alone filter (eg. National Semiconductor LMF100) • Replaced by ADC and DSP in many cases
  39. 39. Summary • A miller integrator • Replaces the input resistor R by a ground capacitor C together with two MOS transistors acting as switches. • The switches are driven by a non-overlapping two phase clock • Pole and zero frequencies proportional to sample frequency and capacitor ratios • Bandwidth required less than the continuous time filter • “analog” sample data filters
  40. 40. Reference • http://www.ics.ee.nctu.edu.tw/~jtwu/publications/pdf/96isc -lvsc.pdf • Microelectronic circuit by Sedra/Smith • Switched Capicitor Filters: Theory, Analysis and Design by Anandmohan, Concorde University Ramachandran, Concorde University and Swamy Thank You

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