EE247 Lecture 8 Continuous time filter design considerations Active bandpass filter design continued Gm C bandpass filter using simple diff pair continued Various Gm C filter implementations Performance comparison of various continuous time filter topologies Switched capacitor filters Emulating a resistor by using a switched capacitor Tradeoffs in choosing sampling rate Effect of sample and hold Switched capacitor network electronic noise EECS 247 Lecture 8 Filters Continuous Time Switched Capacitor 2009 H K Page 1 Summary Lecture 7 Automatic on chip filter tuning continued from previous lecture Continuous tuning continued DC tuning of resistive timing element Periodic digitally assisted filter tuning Systems where filter is followed by ADC DSP existing hardware can be used to periodically update filter freq response Continuous time filter design considerations Monolithic highpass filters Active bandpass filter design Lowpass to bandpass transformation Example 6th order bandpass filter Gm C bandpass filter using simple diff pair EECS 247 Lecture 8 Filters Continuous Time Switched Capacitor 2009 H K Page 2 Linearity of the Source Coupled Pair CMOS Gm Cell 3a3 2 25a5 4 v v 4a1 i 8a1 i Su bstitu ting for a1 a3 IM3 2 4 v i v i IM3 3 25 32 VGS Vth 1024 VGS Vth v i max 4 VG S Vth 2 IM 3 3 I M 3 1 VG S Vth 1V V inrms 23 0m V Note that max signal handling capability function of gate overdrive voltage EECS 247 Lecture 8 Filters Continuous Time Switched Capacitor 2009 H K Page 3 Dynamic Range for Source Coupled Pair Based Filter IM 3 1 VGS Vth 1V Vinrms 230mV Minimum detectable signal determined by total noise voltage It can be shown for the 6th order Butterworth bandpass filter fundamental noise contribution is given by vo2 3Q k T Assumin g Cint g Q 10 Cint g 5 pF rms 160 V vnoise rms since vm a x 230mV 3 Dynamic Range 20log 230x10 6 63dB 160x10 EECS 247 Lecture 8 Filters Continuous Time Switched Capacitor 2009 H K Page 4 Simplest Form of CMOS Gm Cell Pros Capable of very high frequency performance highest Simple design Cons Tuning affects max signal handling capability can overcome Limited linearity possible to improve Tuning affects power dissipation Ref H Khorramabadi and P R Gray High Frequency CMOS continuous time filters IEEE Journal of Solid State Circuits Vol SC 19 No 6 pp 939 948 Dec 1984 EECS 247 Lecture 8 Filters Continuous Time Switched Capacitor 2009 H K Page 5 Gm Cell Source Coupled Pair with Degeneration Id Cox W 2 2 V Vth Vds Vds gs g ds geff 2 L I d W V V Cox gs th L Vds Vds small 1 1 M3 g ds 2 M 1 2 gm M 1 2 M3 g ds for g m M3 geff g ds M3 operating in triode mode source degeneration determines overall gm Provides tuning through varing Vc DC voltage source EECS 247 Lecture 8 Filters Continuous Time Switched Capacitor 2009 H K Page 6 Gm Cell Source Coupled Pair with Degeneration Pros Moderate linearity Continuous tuning provided by varying Vc Tuning does not affect power dissipation Cons Extra poles associated with the source of M1 2 3 Low frequency applications only Ref Y Tsividis Z Czarnul and S C Fang MOS transconductors and integrators with high linearity Electronics Letters vol 22 pp 245 246 Feb 27 1986 EECS 247 Lecture 8 2009 H K Page 7 Filters Continuous Time Switched Capacitor BiCMOS Gm Cell Example MOSFET operating in triode mode M1 Id Cox W 2 V 2 M1 gm L Is 2 gs Vth Vds Vds Iout I d W Cox Vds Vgs L Note that if Vds is kept constant gm stays constant Linearity performance keep gm constant as Vin varies function of how constant VdsM1 can be held Need to minimize gain node X V M 1 B1 Ax x g m gm Vin Since for a given current gm of BJT is larger compared to MOS preferable to use BJT Extra pole at node X could limit max freq EECS 247 Lecture 8 Vb B1 X Vcm Vin M1 Varying Vb changes VdsM1 Changes gmM1 adjustable overall stage gm Filters Continuous Time Switched Capacitor 2009 H K Page 8 Alternative Fully CMOS Gm Cell Example BJT replaced by a MOS transistor with boosted gm Lower frequency of operation compared to the BiCMOS version due to more parasitic capacitance at nodes A B EECS 247 Lecture 8 A Filters Continuous Time Switched Capacitor B 2009 H K Page 9 BiCMOS Gm C Integrator Differential needs common mode feedback ckt Freq tuned by varying Vb Cintg 2 Design tradeoffs Extra poles at the input device drain junctions Input devices have to be small to minimize parasitic poles Results in high input referred offset voltage could drive ckt into non linear region Small devices high 1 f noise EECS 247 Lecture 8 Filters Continuous Time Switched Capacitor Vout Cintg 2 2009 H K Page 10 7th Order Elliptic Gm C LPF For CDMA RX Baseband Application Vin A B A B A B A B A B A C B A B Vout Gm Cell in previous page used to build a 7th order elliptic filter for CDMA baseband applications 650kHz corner frequency In band dynamic range of 50dB achieved EECS 247 Lecture 8 2009 H K Page 11 Filters Continuous Time Switched Capacitor Comparison of 7th Order Gm C versus Opamp RC LPF Vin Gm C Filter A B A B B A A A A C B B A B B Vout Opamp RC Filter Vin Gm C filter requires 4 times less intg cap area compared to Opamp RC For low noise applications where filter area is dominated by Cs could make a significant difference in the total area Opamp RC linearity superior compared to Gm C Power dissipation tends to be lower for Gm C since OTA load is C and thus no need for buffering Vo EECS 247 Lecture 8 Filters Continuous Time Switched Capacitor 2009 H K Page 12 BiCMOS Gm OTA C Integrator Used to build filter for disk drive applications Since high frequency of operation timeconstant sensitivity to parasitic caps significant Opamp used M2 M3 added provides phase lead to compensate for phase lag due to amp extra poles Ref C Laber and P Gray A 20MHz 6th Order BiCMOS Parasitic Insensitive Continuous time Filter Second Order Equalizer Optimized for Disk Drive Read Channels IEEE Journal of Solid State Circuits Vol 28 pp 462 470 April 1993 EECS 247 Lecture 8 Filters Continuous Time Switched Capacitor 2009 H K Page 13 6th Order BiCMOS Continuous time Filter Second Order Equalizer for Disk Drive Read Channels Gm C opamp of the previous page used to build a 6th order filter for Disk Drive Filter consists of cascade of 3 biquads with max Q of 2 each Tuning DC tuning of gm cells Lect 7 page 3 trimming of Cs Performance in the order of 40dB SNDR achieved for up to 20MHz corner frequency Ref C Laber and P Gray A 20MHz 6th Order BiCMOS Parasitic Insensitive Continuous time Filter Second Order …
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