INF4420 Switched-Capacitor Circuits Jørgen Andreas Michaelsen Spring 2014
Outline • Introduction (why and how) • Integrators and filters • Gain circuits • Noise and charge injection Spring 2014 Switched-Capacitor Circuits 2
Introduction Discrete time analog signal processing Why? Spring 2014 Switched-Capacitor Circuits 3
Introduction The arrangement of switches and the capacitor approximates a resistor. Spring 2014 Switched-Capacitor Circuits 4
Introduction Spring 2014 Switched-Capacitor Circuits 5
Introduction RC accuracy (matching). Large time constants implies large passive components. With SC the time constant is set by capacitor ratio and clock frequency (both precisely controlled). Spring 2014 Switched-Capacitor Circuits 6
Introduction Spring 2014 Switched-Capacitor Circuits 7
Building blocks Spring 2014 Switched-Capacitor Circuits 8
Integrators Spring 2014 Switched-Capacitor Circuits 9
Discrete integrators Analyze each clock phase separately Spring 2014 Switched-Capacitor Circuits 10
Discrete integrators Spring 2014 Switched-Capacitor Circuits 11
Discrete integrators Discrete time time constant Spring 2014 Switched-Capacitor Circuits 12
Discrete integrators The discrete time equivalent time constant is defined by the capacitor ratio and clock frequency. • Allows precise time constant definition. • Allows large time constants without excessively large passive components. Spring 2014 Switched-Capacitor Circuits 13
Integrator parasitic capacitance Poorly controlled and non-linear Spring 2014 Switched-Capacitor Circuits 14
Parasitic insensitive integrators Critical for performance Turn off first (bottom plate sampling) Again, we analyze the charge transfer from one clock phase to the next to find the transfer function. Spring 2014 Switched-Capacitor Circuits 15
Parasitic insensitive integrators Spring 2014 Switched-Capacitor Circuits 16
Parasitic insensitive integrators The parasitic capacitors still affect settling, but not the signal charge transfer. Spring 2014 Switched-Capacitor Circuits 17
Delay free integrator Same circuit as before, but modified clocking of the switches. Spring 2014 Switched-Capacitor Circuits 18
Signal flow graph analysis • Now we have the fundamental building blocks (discrete time integrators), to realize filters. • We need a more convenient tool to analyze large systems. • Signal flow graph (SFG) analysis allows us to graphically analyze SC systems. Spring 2014 Switched-Capacitor Circuits 19
Signal flow graph analysis Spring 2014 Switched-Capacitor Circuits 20
SC filters A simple design strategy: • Start with a continuous time prototype • Replace resistors with SC resistor equivalents The resulting circuit is similar for input frequencies much lower than the sampling frequency • Use SFG to determine the z -domain transfer function Accurate description of the transfer function Spring 2014 Switched-Capacitor Circuits 21
First order filters Filter design example. Start with the continuous time circuit. In this case: Spring 2014 Switched-Capacitor Circuits 22
First order filters Replace the resistors with SC elements Spring 2014 Switched-Capacitor Circuits 23
First order filters Spring 2014 Switched-Capacitor Circuits 24
Switch sharing Some switches are redundant, we use this to simplify the circuit: Spring 2014 Switched-Capacitor Circuits 25
Biquad filters Spring 2014 Switched-Capacitor Circuits 26
Low-Q biquad Spring 2014 Switched-Capacitor Circuits 27
Low-Q biquad Spring 2014 Switched-Capacitor Circuits 28
Low-Q biquad Spring 2014 Switched-Capacitor Circuits 29
High-Q biquad Spring 2014 Switched-Capacitor Circuits 30
High-Q biquad Spring 2014 Switched-Capacitor Circuits 31
Gain • Resettable gain circuit • Samples offset voltage during reset (reduces flicker noise) Spring 2014 Switched-Capacitor Circuits 32
Gain Amplifier slew-rate requirement is high. Spring 2014 Switched-Capacitor Circuits 33
Capacitive-reset gain Include a capacitor to hold the output during the reset phase. Avoid excessive slewing. Configurable positive or negative gain. Spring 2014 Switched-Capacitor Circuits 34
Noise Spring 2014 Switched-Capacitor Circuits 35
Noise Spring 2014 Switched-Capacitor Circuits 36
Correlated double sampling (CDS) Spring 2014 Switched-Capacitor Circuits 37
Fully differential circuits Real circuits are almost always fully differential. Coupled noise, power supply noise, substrate noise will mostly affect the common mode, while our signal is in the differential mode. Also, cancels even order harmonics. Spring 2014 Switched-Capacitor Circuits 38
Charge injection Spring 2014 Switched-Capacitor Circuits 39
Bootstrapped switch Spring 2014 Switched-Capacitor Circuits 40
SC amplifier design Spring 2014 Switched-Capacitor Circuits 41
Further reading Sansen, Analog Design Essentials, Springer, 2006, Ch. 17 Spring 2014 Switched-Capacitor Circuits 42
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