Mixed-Signal-Electronics - Technische Universität · PDF fileChapter 3 . Stephan Henzler...

Stephan Henzler Mixed-Signal-Electronics 2011/12

Mixed-Signal-Electronics

PD Dr.-Ing. Stephan Henzler

1


Switched Capacitor Circuits

2

Chapter 3


Resistor Realizations in MOS Technologies: Poly-Resistor

3

Sheet resistance:

200-400Ω□

(metal <1 Ω□)

Highly variable

resistance

Good linearity

Isolated from

substrate


Resistor Realizations in MOS Technologies: Diffusion Resistor

Sheet resistance: 50-150Ω□

Highly variable resistance (up to +/- 20%)

Parasitic diode

High parasitic capacitances

4


Resistor Realizations in MOS Technologies

5

Sheet resistance:

Poly: 200-400Ω□

Diffusion: 50 -150Ω□

Metal: <1Ω□

Drawbacks:

area intensive expensive

high variability (20-30%)

W

LR

Wd

LR []


Effect of Resistor between two Nodes

6

continuous time point-of-view discrete time point-of-view


Switched Capacitor Resistor Emulation 1

7


Switched Capacitor Resistor Emulation 2

8


Resistor Equivalents

9


Non-Overlapping Multiphase Clocking

both clocks active at the same time

would cause short circuit currents

10


Parameter Variations in SC Circuits 1

What is the variation of a RC time constant?

i.e. the relative error of the time constant is equal to the sum of

the relative error of the resistance and the capacitance

11


Parameter Variations in SC Circuits 2

What happens for the switched-capacitor equivalent?

i.e. the relative error of the time constant is equal to the relative

error of a capacitance ratio

very small if matching is considered in layout

12


Implementation of Integrated Capacitors

13


Alternative Capacitor Implementation

Junction capacitance

– Parasitic diode leakage

– Voltage and temperature dependent

MOS capacitance

– MOS transistor in accumulation or inversion

– Alternatively: gate over diffusion

– Strongly non-linear

– Very high capacitance/area

– God for decoupling

Vertical parallel plate capacitor

– Matrix of wires exploiting vertical and lateral capacitance

– Ref. next slide

14


Vertical Parallel Plate Capacitor

Kim, VLSI Symposium 2003

15


Limitations of SC-Equivalence

Continuous Time RC-Low-Pass

16



17

S1 and S2 are

closed alternately



Switched Capacitor Low-Pass

z-Transformation:

18



Switched Capacitor Equivalence

19


Switched Capacitor Integrators

20


Switched Capacitor Integrators

21


Analysis of Switched-Capacitor Circuits

– SC-Cookbook –

Mark polarity of all capacitors (arbitrarily)

Do not change this polarity during the analysis

Draw equivalent circuit for each clock phase

Charge balance analysis for full cycle

– assume steady state conditions for each time step

– compute ∆Q that occurs during phase transitions

z-transformation

Calculate transfer function

Evaluate transfer function on unit circle of z-plane

22


Spectral Analysis

z-Transformation

Approximation on unit circle

23


Spectral Analysis (cont.)

24


Analysis of Switched-Capacitor Circuits

– SC-Cookbook –

Mark polarity of all capacitors (arbitrarily)

Do not change this polarity during the analysis

Draw equivalent circuit for each clock phase

Charge balance analysis for full cycle

– assume steady state conditions for each time step

– compute ∆Q that occurs during phase transitions

z-transformation

Calculate transfer function

Evaluate transfer function on unit circle of z-plane

25


Impact of Parasitic Capacitances

Cp1||C1 direct impact on transfer behavior

Cp2 shorted no impact

Cp3 shorted between GND and VGND

Cp4 at opamp output negligible impact

26

1

1

2

11

1

z

z

C

CCzH

p


Alternative Integrator Architectures

Sign of transfer function can be changed by switching the

control signal of the switches.

Less sensitive to parasitic capacitances

27


Switched Capacitor Amplifier

28


Switched Capacitor Amplifier: Impact of Offset

29


SC Amplifier: Output Signal

30

opamp offset


Switched Capacitor Amplifier

(without output reset)

31

Mixed-Signal-Electronics - Technische Universität · PDF fileChapter 3 . Stephan Henzler...

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