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Lecture 010 – Introduction (3/24/10)
Page 010-1
LECTURE 010 - INTRODUCTION TO CMOS ANALOG CIRCUIT
DESIGN
LECTURE ORGANIZATION
Outline
• Introduction
• What is Analog Design?
• Skillset for Analog IC Circuit Design
• Trends in Analog IC Design
• Notation, Terminology and Symbols
• Summary
CMOS Analog Circuit Design, 2nd Edition Reference
Pages 1-16
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
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INTRODUCTION
Course Objective
This course teaches analog integrated circuit design using CMOS technology.
VDD
VPB1
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SPECIFICATIONS
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CMOS Analog Circuit Design
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Course Prerequisites
• Basic understanding of electronics
- Active and passive components
- Large and small signal models
- Frequency response
• Circuit analysis techniques
- Mesh and loop equations
- Superposition, Thevenin and Norton’s equivalent circuits
• Integrated circuit technology
- Basics process steps
- PN junctions
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
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Course Organization – Based on 2nd Ed. of CMOS Analog Circuit Design
Chapter 9
Switched Capacitor Circuits
Chapter 10
D/A and A/D
Converters
Systems
Chapter 6
Simple CMOS &
BiCMOS OTA's
Chapter 7
High Performance
OTA's
Chapter 8
CMOS/BiCMOS
Comparators
Complex
Simple
Chapter 4
CMOS
Subcircuits
Chapter 5
CMOS
Amplifiers
Chapter
Chapter10
2
CMOS/BiCMOS
D/A and A/D
Technology
Converters
Chapter
Chapter11
3
CMOS/BiCMOS
Analog
Modeling
Systems
Circuits
Devices
Introduction
CMOS Analog Circuit Design
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© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
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References
1.) P.E. Allen and D.R. Holberg, CMOS Analog Circuit Design – 2nd Ed., Oxford
University Press, 2002.
2.) P.R. Gray, P.J. Hurst, S.H. Lewis and R.G. Meyer, Analysis and Design of Analog
Integrated Circuits – 4th Ed., John Wiley and Sons, Inc., 2001.
3.) B. Razavi, Design of Analog CMOS Integrated Circuits, McGraw-Hill, Inc., 2001.
4.) R.J. Baker, H.W. Li and D.E. Boyce, CMOS Circuit Design, Layout, and
Simulation, IEEE Press, 1998.
5.) D. Johns and K. Martin, Analog Integrated Circuit Design, John Wiley and Sons,
Inc., 1997.
6.) K.R. Laker and W.M.C. Sansen, Design of Analog Integrated Circuits and Systems,
McGraw-Hill, Inc., 1994.
7.) R.L. Geiger, P.E. Allen and N.R. Strader, VLSI Techniques for Analog and Digital
Circuits, McGraw-Hill, Inc., 1990.
8.) A. Hastings, The Art of Analog Layout – 2nd Ed., Prentice-Hall, Inc., 2005.
9.) J. Williams, Ed., Analog Circuit Design - Art, Science, and Personalities,
Butterworth-Heinemann, 1991.
10.) R.A. Pease, Troubleshooting Analog Circuits, Butterworth-Heinemann, 1991.
CMOS Analog Circuit Design
Lecture 010 – Introduction (3/24/10)
© P.E. Allen - 2010
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Course Philosophy
This course emphasizes understanding of analog integrated circuit design.
Although simulators are very powerful, the designer must understand the circuit before
using the computer to simulate a circuit.
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-7
WHAT IS ANALOG DESIGN?
Analysis versus synthesis (design)
System 1
System 2
System
ANALYSIS
Properties
System 3
DESIGN
Properties
System 4
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• Analysis: Given a system, find its properties. The solution is unique.
• Design: Given a set of properties, find a system possessing them. The solution is rarely
unique.
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
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The Analog IC Design Process
Conception of the idea
Electrical
Design
Definition of the design
Comparison
with design
specifications
Implementation
Comparison
with design
specifications
Simulation
Physical Definition
Physical
Design
Physical Verification
Parasitic Extraction
Fabrication
Fabrication
Testing and
Product
Development
Testing and Verification
Product
Fig. 1.1-2
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-9
What is Electrical Design?
Electrical design is the process of going from the specifications to a circuit solution. The
inputs and outputs of electrical design are:
;;
L
W
W/L ratios
Circuit or
systems
specifications
Analog
Integrated
Circuit Design
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VDD
M6
M4
M1
Cc
vout
CL
M2
+
VBias
-
M7
M5
VSS
Topology
DC Currents
Fig. 1.1-3
The electrical design requires active and passive device electrical models for
- Creating the design
- Verifying the design
- Determining the robustness of the design
CMOS Analog Circuit Design
© P.E. Allen - 2010
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Steps in Electrical Design
1.) Selection of a solution
- Examine previous designs
- Select a solution that is simple
2.) Investigate the solution
- Analyze the performance (without a computer)
- Determine the strengths and weaknesses of the solution
3.) Modification of the solution
- Use the key principles, concepts and techniques to implement
- Evaluate the modifications through analysis (still no computers)
4.) Verification of the solution
- Use a simulator with precise models and verify the
solution
- Large disagreements with the hand analysis and
computer verification should be carefully examined.
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Lecture 010 – Introduction (3/24/10)
M11
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M2
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NB1
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+
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−
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CMOS Analog Circuit Design
© P.E. Allen - 2010
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What is Physical Design?
Physical design is the process of representing the electrical design in a layout consisting
of many distinct geometrical rectangles at various levels. The layout is then used to
create the actual, three-dimensional integrated circuit through a process called
fabrication.
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FABRICATION
LAYOUT
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CMOS Analog Circuit Design
Lecture 010 – Introduction (3/24/10)
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What is the Layout Process?
1.) The inputs are the W/L values and the schematic (generally from schematic entry
used for simulation).
2.) A CAD tool is used to enter the various geometries. The designer must enter the
location, shape, and level of the particular geometry.
3.) During the layout, the designer must obey a set of rules called design rules. These
rules are for the purpose of ensuring the robustness and reliability of the technology.
4.) Once the layout is complete, then a process called layout versus schematic (LVS) is
applied to determine if the physical layout represents the electrical schematic.
5.) The next step is now that the physical dimensions of the design are known, the
parasitics can be extracted. These parasitics primarily include:
a.) Capacitance from a conductor to ground
b.) Capacitance between conductors
c.) Bulk resistance
6.) The extracted parasitics are entered into the simulated database and the design is resimulated to insure that the parasitics will not cause the design to fail.
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-13
Packaging†
Packaging of the integrated circuit is an important part of the physical design process.
The function of packaging is:
1.) Protect the integrated circuit
2.) Power the integrated circuit
3.) Cool the integrated circuit
4.) Provide the electrical and mechanical connection between the integrated circuit and
the outside world.
Packaging steps:
Dicing
the wafer
Attachment
of the chip to
a lead frame
Connecting
the chip to
a lead frame
Encapsulating the
chip and lead
frame in a package
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Other considerations of packaging:
• Speed
• Parasitics (capacitive and inductive)
†
Rao Tummala, “Fundamentals of Microsystems Packaging,” McGraw-Hill, NY, 2001.
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-14
What is Test Design?
Test design is the process of coordinating, planning and implementing the
measurement of the analog integrated circuit performance.
Objective: To compare the experimental performance with the specifications and/or
simulation results.
Types of tests:
• Functional – verification of the nominal specifications
• Parametric – verification of the characteristics to within a specified tolerance
• Static – verification of the static (AC and DC) characteristics of a circuit or system
• Dynamic – verification of the dynamic (transient) characteristics of a circuit or system
Additional Considerations:
Should the testing be done at the wafer level or package level?
How do you remove the influence (de-embed) of the measurement system from the
measurement?
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-15
ANALOG INTEGRATED CIRCUIT DESIGN SKILLSET
Characteristics of Analog Integrated Circuit Design
• Done at the circuits level
• Complexity is high
• Continues to provide challenges as technology evolves
• Demands a strong understanding of the principles, concepts and techniques
• Good designers generally have a good physics background
• Must be able to make appropriate simplifications and assumptions
• Requires a good grasp of both modeling and technology
• Have a wide range of skills - breadth (analog only is rare)
• Be able to learn from failure
• Be able to use simulation correctly
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-16
Understanding Technology
Understanding technology helps the analog IC designer to know the limits of the
technology and the influence of the technology on the design.
Device Parasitics:
Drain
Collector
RD
CGD
Gate
RG
RC
Cμ
CBD
RB Bulk
CGB
Base
CJS
RB
RSub
CGS
Connection Parasitics:
+5V
vin
M2
vout
Cπ
CBS
Substrate
RS
RE
Source
Emitter
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vout
vin
+5V
M1
M1
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CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-17
Implications of Smaller Technology on IC Design
The good:
• Smaller geometries
• Smaller parasitics
• Higher transconductance
• Higher bandwidths
The bad:
• Reduced voltages
• Smaller channel resistances (lower gain)
• More nonlinearity
• Deviation from square-law behavior
The challenging:
• Increased substrate noise in mixed signal applications
• Threshold voltages are not scaling with power supply
• Reduced dynamic range
• Poor matching at minimum channel length
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-18
Understanding Modeling
Modeling:
Modeling is the process by which the electrical properties of an electronic circuit or
system are represented by means of mathematical equations, circuit representations,
graphs or tables.
Models permit the predicting or verification of the performance of an electronic
circuit or system.
Electronic
Circuits
and
Systems
Equations,
Circuit
representations,
graphs, tables
Prediction or
verification of
circuit or system
performance
Electronic Modeling Process
030130-02
Examples:
Ohm’s law, the large signal model of a MOSFET, the I-V curves of a diode, etc.
Goal:
Models that are simple and allow the designer to understand the circuit performance.
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-19
Key Principles, Concepts and Techniques of Analog IC Design
• Principles mean fundamental laws that
Techniques
are precise and never change.
"Tricks"
(Webster – A comprehensive and
fundamental law, doctrine, or
assumption. The laws or facts of nature
underlying the working of an artificial
Concepts Analog
device.)
Information
IC Design
that enhances
Process
• Concepts will include relationships,
design
“soft-laws” (ones that are generally
true), analytical tools, things worth
remembering.
(Webster – An abstract idea generalized
Principles (laws)
from particular instances.)
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used in design
• Techniques will include the assumptions,
“tricks”, tools, methods that one uses to simplify and understand.
(Webster – The manner in which technical details are treated, a method of
accomplishing a desired aim or goal.)
CMOS Analog Circuit Design
Analog
Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-20
Complexity in Analog Design
Analog design is normally done in a non-hierarchical manner and makes little use of
repeated blocks. As a consequence, analog design can become quite complex and
challenging.
How do you handle the complexity?
Systems
Systems Level (ADC)
1.) Use as much hierarchy as possible.
2.) Use appropriate organization
Circuits Level (op amps)
techniques.
3.) Document the design in an efficient
manner.
Circuits
Block Level (amplifier)
4.) Make use of assumptions and
simplifications.
Sub-block Level (current sink)
5.) Use simulators appropriately.
Components
Components (transistor)
031030-03
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-21
Assumptions
Assumptions:
An assumption is taking something to be true without formal proof. Assumptions in
analog circuit design are used for simplifying the analysis or design. The goal of an
assumption is to separate the essential information from the nonessential information
of a problem.
The elements of an assumption are:
1.) Formulating the assumption to simplify the problem without eliminating the
essential information.
2.) Application of the assumption to get a solution or result.
3.) Verification that the assumption was in fact appropriate.
Examples:
Neglecting a large resistance in parallel with a small resistance
Miller effect to find a dominant pole
Finding the roots of a second-order polynomial assuming the roots are real and
separated
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-22
WHERE IS ANALOG IC DESIGN TODAY?
Analog IC Design has Reached Maturity
There are established fields of application:
• Digital-analog and analog-digital conversion
• Disk drive controllers
• Modems - filters
• Bandgap reference
• Analog phase lock loops
• DC-DC conversion
• Buffers
• Codecs
• Etc.
Existing philosophy regarding analog circuits:
“If it can be done economically by digital, don’t use analog.”
Consequently:
Analog finds applications where speed, area, or power have advantages over a digital
approach.
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-23
Analog IC Design Challenges
Technology:
• Digital circuits have scaled well with technology
• Analog does not benefit as much from smaller features
- Speed increases
- Gain decreases
- Matching decreases
- Nonlinearity increases
- New issues appear such as gate current leakage
Analog Circuit Challenges:
• Trade offs are necessary between linearity, speed, precision and power
• As analog is combined with more digital, substrate interference will become worse
CMOS Analog Circuit Design
Lecture 010 – Introduction (3/24/10)
© P.E. Allen - 2010
Page 010-24
Digitally Assisted Analog Circuits
Use digital circuits which work better at
scaled technologies to improve analog
circuits that do not necessarily improve
with technology scaling.
Principles and Techniques:
• Open-loop vs. closed loop
- Open loop is less accurate but smaller Faster, less power
- Closed-loop is more accurate but larger Slower, more power
• Averaging
- Increase of accuracy Smaller devices, more speed
• Calibration
- Accuracy increases Increased resolution with same area
• Dynamic Element Matching
- Enhancement of component precision
• Doubly correlated sampling
- Reduction of dc influences (noise, offset) Smaller devices, more speed
• Etc.
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-25
What is the Future of Analog IC Design?
• More creative circuit solutions are required to achieve the desired performance.
• Analog circuits will continue to be a part of large VLSI digital systems
• Interference and noise will become even more serious as the chip complexity increases
• Packaging will be an important issue and offers some interesting solutions
• Analog circuits will always be at the cutting edge of performance
• Analog designer must also be both a circuit and systems designer and must know:
Technology and modeling
Analog circuit design
VLSI digital design
System application concepts
• There will be no significantly new and different technologies - innovation will combine
new applications with existing or improved technologies
• Semicustom methodology will eventually evolve with CAD tools that will allow:
- Design capture and reuse
- Quick extraction of model parameters from new technology
- Test design
- Automated design and layout of simple analog circuits
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-26
NOTATION, TERMINOLOGY AND SYMBOLOGY
Definition of Symbols for Various Signals
Drain Current
Signal Definition
Quantity
Subscript Example
Total instantaneous value of the signal Lowercase Uppercase
qA
DC value of the signal
Uppercase Uppercase
QA
AC value of the signal
Lowercase Lowercase
qa
Complex variable, phasor, or rms value Uppercase Lowercase
Qa
of the signal
Example:
Idm
id
ID
iD
t
Fig. 1.4-1
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-27
MOS Transistor Symbols
D
S
Enhancement
NMOS with
VBS = 0V.
G
Enhancement
PMOS with
VBS = 0V.
G
S
D
D
S
Enhancement
B NMOS with
VBS 0V.
G
Enhancement
B PMOS with
VBS 0V.
G
S
D
D
S
Simple
NMOS
symbol
G
Simple
PMOS
symbol
G
S
D
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-28
Other Schematic Symbols
+
V
-
V
-
Differential amplifier,
op amp, or comparator
+
AvV1
V1
+
-
I1
+
+
+
V
-
-
Independent
voltage sources
+
+
V2
V1
-
-
Voltage-controlled,
voltage source
Independent
current source
I2
GmV1
I1
Voltage-controlled,
current source
I2
+
RmI1 +-
V2
AiI1
Current-controlled,
voltage source
CMOS Analog Circuit Design
Current-controlled,
current source
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-29
Three-Terminal Notation
QABC
A = Terminal with the larger magnitude of potential
B = Terminal with the smaller magnitude of potential
C = Condition of the remaining terminal with respect to terminal B
C = 0 There is an infinite resistance between terminal B and the 3rd terminal
C = S There is a zero resistance between terminal B and the 3rd terminal
C = R There is a finite resistance between terminal B and the 3rd terminal
C = X There is a voltage source in series with a resistor between terminal B
and the 3rd terminal in such a manner as to reverse bias a PN
junction.
Examples
IDSS
S
VGS
S
D
-
CDGS
+
G
(a.)
G
D
+
D
IDS
S
(b.)
BVDGO
G
-
(c.)
(a.) Capacitance from drain to gate with the source shorted to the gate.
(b.) Drain-source current when gate is shorted to source (depletion device)
(c.) Breakdown voltage from drain to gate with the source is open- circuited to the gate.
CMOS Analog Circuit Design
© P.E. Allen - 2010
Lecture 010 – Introduction (3/24/10)
Page 010-30
SUMMARY
• Successful analog IC design proceeds with understanding the circuit before simulation.
• Analog IC design consists of three major steps:
1.) Electrical design Topology, W/L values, and dc currents
2.) Physical design (Layout)
3.) Test design (Testing)
• Analog designers must be flexible and have a skill set that allows one to simplify and
understand a complex problem
• Analog IC design has reached maturity and is here to stay.
• The appropriate philosophy is “If it can be done economically by digital, don’t use
analog”.
• As a result of the above, analog finds applications where speed, area, or power result in
advantages over a digital approach.
• Deep-submicron technologies will offer exciting challenges to the creativity of the
analog designer.
CMOS Analog Circuit Design
© P.E. Allen - 2010