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Continuous Time Domain Filters
A few very simple active filters. YLD 10/2/99 ESINSA
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Continuous Time Domain LPF
2nd order Sallen and Key low-pass filter R1 R3 C4 C2 K Generic Architecture Check Stability! YLD 10/2/99 ESINSA
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Continuous Time Domain LPF
R C 2nd order Sallen and Key low-pass filter Practical realization YLD 10/2/99 ESINSA
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Continuous Time Domain LPF
2nd order Multiple FeedBack low-pass filter R1 R3 C2 C5 R4 Generic Architecture Stable! YLD 10/2/99 ESINSA
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Continuous Time Domain LPF
2nd order Multiple FeedBack low-pass filter R C Practical realization YLD 10/2/99 ESINSA
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Continuous Time Domain LPF
159.2 Hz R*C = 1.0e-3 -50 -40 -30 -20 -10 10 1 100 1000 10000 Sallen and Key MFB 2nd order 1st order YLD 10/2/99 ESINSA
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Continuous Time Domain HPF
LPF HPF C R C R 2nd order Sallen and Key filter YLD 10/2/99 ESINSA
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Continuous Time Domain HPF
LPF HPF R C C R 2nd order Multiple FeedBack filter YLD 10/2/99 ESINSA
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Differential LP Filters
Single-ended Differential R C Inappropriate 2nd order Sallen and Key low-pass filter YLD 10/2/99 ESINSA
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Differential LP Filters
Single-ended Differential R C R C/2 C 2nd order Multiple FeedBack low-pass filter YLD 10/2/99 ESINSA
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Continuous Domain Filters
It is important to evaluate the sensivity to electrical parameters, (absolute values, matchings) especially if high Q is expected. Parameters mismatches introduce transfer function errors. Opamp gain and slew-rate have the most dangerous effects. Slew-rate, being a non linear effect, makes the analysis more difficult. Extend the analysis outside the bandwidth of the filter! YLD 10/2/99 ESINSA
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Continuous Domain Filters
DO NOT UNDERESTIMATE THE NOISE ANALYSIS. YLD 10/2/99 ESINSA
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Continuous Domain Filters
Input Noise Noise! Filtered Noise YLD 10/2/99 ESINSA
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Continuous Domain Filters
Input Noise Filtered Noise Noise! freq dB Output Noise! Input Noise YLD 10/2/99 ESINSA
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Continuous Domain Filters
Kickback Noise Still a Risk ! YLD 10/2/99 ESINSA
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Interfacing SWC modules
YLD 10/2/99 ESINSA
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Interfacing SWC modules
At the input of a SWC module, the signal bandwidth must be limited below the Nyquist frequency of the module. AntiAliasing filter At the output of a SWC module, the signal must be smoothed to reconstruct a continuous-time domain signal. Smoothing filter YLD 10/2/99 ESINSA
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AA Filter Signal Noise Brick wall AA filter Simpler AA filter freq Nyquist Sampling It is difficult to build linear and accurate brick wall filter Practical antialiasing filter has a more limited frequency range. YLD 10/2/99 ESINSA
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AA Filter This is the first challenge to solve.
A quasi brick wall filter could use the bandwidth more efficiently. It has a very complex structure with many poles and zeroes. It uses a lot of active devices. It will not be very flat and will not generally respect the phase of the signal. Distortion, noise, power dissipation, size, … are also prohibitive. Spread of the transfer function is a killer. YLD 10/2/99 ESINSA
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AA Filter Oversampling is often a good solution. Antialiasing filters are then simpler and safer! But this is not using the bandwidth very efficiently. AA freq Nyquist Sampling Signal spread YLD 10/2/99 ESINSA
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AA Filter Oversampling? SWC modules must run faster!
SWC modules are not necessarily able to be run at much higher frequencies. At least they consume much more power. YLD 10/2/99 ESINSA
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AA Filter A solution: An antialiasing filter could be a cascade of 2 LP filters. First filter is a loose continuous time domain LPF. Second filter is a high order, oversampled, precise, SWC LPF. Simple LPF filter Continuous Time Domain High order LPF SWC filter SWC Module Antialiasing filter @ FS * OSR @ FS YLD 10/2/99 ESINSA
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AA Filter We have now to build a loose low order LPF in the
continuous time domain. Specifications: As steep as possible Flat response transfer at low frequencies Linear phase at low frequencies Very precise gain Very low distortion High TSNR Low power, small size, ... YLD 10/2/99 ESINSA
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AA Filter The Sallen and Key and the Multiple FeedBack LP filters
make good building blocks for antialiasing filters. They are not very steep (2nd order) but uses only one Opamp. Higher order versions exist. They could be advantageously cascaded with other filters. They request a fair OSR. YLD 10/2/99 ESINSA
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AA Filter Expected qualities of an antialiasing filter:
it has good rejection of out of band frequencies it respects signal integrity spread of passive devices does not lower the yield appropriate settling time low power low cost YLD 10/2/99 ESINSA
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AA Filter Very often, the antialiasing filter is expected also to:
have a high input impedance build an isolation of input from IC feedback noise accurately sample the input signal makes a programmable gain or attenuation makes the single-ended to differential conversion etc… (It makes the life of the analog designer more interesting) YLD 10/2/99 ESINSA
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Smoothing Filter Nyquist Sampling Signal freq Brick wall Smoothing filter Simpler Smoothing filter It is difficult to build linear and accurate brick wall filter Practical smoothing filter has a more limited frequency range. YLD 10/2/99 ESINSA
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Smoothing Filter Again, oversampling is often a good solution. Smoothing filters are then simpler and safer! But this is not using the bandwidth very efficiently. Nyquist Sampling Signal freq Smoothing filter YLD 10/2/99 ESINSA
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Smoothing Filter A solution: A smoothing filter could be a cascade of 2 LP filters. First filter is a high order, oversampled, precise, SWC LPF. Second filter is a loose continuous time domain LPF. Smoothing filter SWC Module High order LPF SWC filter Simple LPF filter Continuous Time Domain @ FS @ FS * OSR YLD 10/2/99 ESINSA
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Smoothing Filter The Sallen and Key and the Multiple FeedBack LP filters make again good building blocks for smoothing filters. They are not very steep (2nd order) but uses only one opamp. Higher order versions exist. They could be advantageously cascaded with other filters. They request a fair OSR. YLD 10/2/99 ESINSA
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Smoothing Filter Expected qualities of an smoothing filter:
it has good rejection of out of band frequencies it respects signal integrity spread of passive devices does not lower the yield low power low cost YLD 10/2/99 ESINSA
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Smoothing Filter Smoothing filters are difficult: the continuous LP filter is not sampling the end of the phase but filter the complete waveform. This waveform is not necessarily perfect! YLD 10/2/99 ESINSA
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Smoothing Filter Very often, the smoothing filter is expected also to:
have a (very) low output impedance makes a programmable gain or attenuation makes the differential to single-ended conversion etc… (It makes again the life of the analog designer more interesting) YLD 10/2/99 ESINSA
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Smoothing Filter Be very careful when an output amplifier is driving an external load. As this impedance is fairly unknown, it is dangerous to use an Opamp essential in the construction of the transfer function of a filter as an output driver. YLD 10/2/99 ESINSA
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Smoothing Filter External Load Interaction between Opamp and Load C
Modified feedback Modified filter R C External Load R R C Interaction between Opamp and Load YLD 10/2/99 ESINSA
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