Analog Circuit Design on Digital CMOS
Why it is difficult, and which ideas help. Presented by HP. Schmid.
Background on Hanspeter Schmid
– Dissertation on video-frequency integrated filters (ETH Zürich)
– Analog IC Designer at Bernafon / William Demant Holding:
– Analog electronics: LNAs, amplifiers, regulators, filters, standard
cells, circuits for wireless communication system.
– System design, analog signal processing and signal integrity.
– Communication facilitator between Danish and Swiss Teams.
– IME: research projects (sensor systems, sigma-delta, etc.),
consulting, teaching.
– ETH Zürich: teaching analog (integrated) signal processing
– IEEE CAS:
– Chair Analog Signal Processing Tech. Comm.
– Associate Editor of TCAS-I
– Hobbies: going for walks,
playing trombone, reading.
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Tutorial Philosophy
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Philosophy I: Be a fool!
– multiparameter optimization
–
–
–
–
–
–
–
–
–
noise
distortion
power consumption
signal delay
chip area
offset
yield
mask costs
…
– conscious vs. subconscious
– conscious mind: 4…5 criteria
– subconscious: 100? 200?
– what it means to be a fool
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Philosophy II: Be a child
– open for everything
– playful
– does not do
what she should do
– a child has got time!
– Advice for scientists by
Douglas Adams:
See first, think later, then
test. But always see first, or
you will only see what you
expect to see!
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Philosophy III: Be a climber
– works hard to achieve a goal
– is well trained
– normally gets to the intended
goal
– Is the intention good?
The direct path leads only to
the
goal!
Gide) new
The most exciting phrase in science,
the
one (André
that heralds
discoveries, is not Eureka! (I found it!), but
– Will
That's funny
... the fool not fall down?
(Isaac Asimov)
Not if the fool also is a child.
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Tutorial Contents
Image from http://www.beatenbergbilder.ch/
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Introduction: What is new?
– More metal layers
– Small lateral distances
– Thinner gates
– more C
– less Vdd
– less gain
– more weak inversion
Image from http://www.ndl.org.tw/cht/ndlcomm/P10_2/7.pdf
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Multi-metal cross section
Example: 6 Metal layers.
Lateral dimensions are smaller than vertical dimensions!
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Transconductance in Strong and Weak Inversion
Strong Inversion
Weak Inversion
Moderate Inversion: Superposition
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Maximum gain of single stage is reached in weak inversion
For a given supply current: gain is proportional to supply voltage!
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Weak inversion = matching problems?
For a 0.25u process:
Voltage offset
for identical supply current
Current offset
for identical gate-source voltage
Therefore: Differential pairs in weak inversion
Therefore: Current mirrors in strong inversion
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
from [Kinget07]
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Summary
– Thinner gates (and higher gate tunnelling currents!)
– more gate (overlap, ...) capacitance per area
– No buried channels anymore
Æ pMOS is not better anymore in terms of flicker noise!
– Less supply voltage Æ less signal
– Less gain
– same white noise at same supply current; less flicker noise
– Sub-threshold leakage
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Literature: What is new?
[Annema99] Anne-Johan Annema, "Analog Circuit Performance and Process Scaling",
IEEE Trans. Circuits and Systems—II, vol. 46, no. 6,
pp. 711–725, June 1999.
[Huang98] Qiuting Huang et. al., "The Impact of Scaling Down to Deep Submicron on
CMOS RF Circuits," IEEE J. Solid-State Circuits, vol. 33, no. 7, pp. 1023–1036, July
1998
[Kinget07] Peter Kinget, "Device Mismatch: An Analog Design Perspective", ISCAS, New
Orleans, pp. 1245–1248, May 2007.
[Tsividis02] Yannis Tsividis, Mixed Analog-Digital VLSI Devices and Technology, World
Scientific Publishers, 2002.
[Tsividis99] Yannis Tsividis, Operation and Modelling of the MOS Transistor, ed. 2,
McGraw-Hill 1999.
[Dijksterhuis06] Ap Dijksterhuis et. al., "On Making the Right Choice: The DeliberationWithout-Attention Effect," Science, vol. 311, pp. 1005–1007, 2006.
[Simons99] Daniel Simons et. al., "Gorillas in our midst: sustained inattentional
blindness for dynamic events," Perception, vol. 28, pp. 1059–1074, 1999.
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Signal Integrity
– Ground and Power Routing
– Star Connections
– Tapered Stars
– Signal Grounds and Refs
– Improving PSR (theory)
– Finger capacitors and
MIM-capacitors
– Demodulation by nonlinearity
– Decoupling
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Why correct ground and power routing are important
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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On PCB: Power plane? No!
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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On PCB: Split ground plane? Dangerous!
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Recommendations for PCB routing
[National05] recommend
– Use a single, unified ground plane
– use separate power planes for analog and digital
– let trace routing control ground currents.
Low-power low-noise circuits:
require controlled power/gnd routing!
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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The problem of the star connection on chip
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Calculation example: hearing aid system
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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16μΩ is not a lot!
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Solution: Tapered star
This means: we have full control of where the noise currents flow.
But: more chip area or more supply / ground wire resistance!
Paradox: most sensitive nodes are farthest away from pad.
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Local decoupling is sometimes needed
The question is: where shall the decoupling capacitor go?
Answer: to the reference of the signal!
But this may not be so easy.
Many "PSR problems" are really coupling problems or problems
with dirty references
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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How to improve PSRR and CMRR in a system?
CMRR and PSRR are connected!
Proof: Gauge transformation
from [Säckinger91]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Solution: Additional input from quiet ground
Now we have one more degree
of freedom
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Example: additional signal path
from [Loikkanen06]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Example: additional signal path
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Recommendations for chip routing
Use "tapered" star connections
For every differential signal node, make sure that the signal is
referred to a clean signal.
Input reference
Problem:
the references can change
within a single circuit
Output reference
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Multi-metal Finger-Cap MIM-Cap combination
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Comparison for a six-metal 0.18um CMOS process
MIM capacitor (Metal 5 and Metal 6):
1.0 fF/μm2
Finger structure (Metal 1 … Metal 4):
1.3 fF/um2
MIM capacitor on top of Finger structure (all Metal):
2.3 fF/um2
MOSFET gate capacitance (non-linear):
10.0 fF/um2
Can we use a MOSFET gate capacitor for decoupling?
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Demodulation by a nonlinearity I: DC offset
Normal Operation with HF-Signal on Pad
(weak inversion)
Gives DC Offset! Inputs must be protected against this ...
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Demodulation by a nonlinearity II: receiver
Normal Operation with amplitude-modulated
HF-Signal on Pad (weak inversion)
Demodulates the signal and gives more DC offset!
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Realistic? Yes!
In all digitally driven class-D (PWM) amplifiers, the signal is
amplitude-modulated on the system clock frequency.
The square of this signal appears in the supply current.
If this strays back into a high-gain audio system:
huge distortion or even instability!
Solution:
decouple all inputs
... to the respective reference of the signal
... as close to the pad as possible
... with as big a capacitor as possible
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Literature: Signal Integrity
[Loikkanen06] Mikko Loikkanen et. al., "PSRR Improvement
Technique for Amplifiers with Miller Capacitor," ISCAS 2006,
Kos, Greece, pp. 1394–1397.
[National05] National Semiconductor Analog University, Meeting
Signal-Path Design Challenges, High-performance seminar
series 2005, part no. 570012-001. (Can be ordered from
National for free.)
[Säckinger91] Eduard Säckinger et. al., "A General Relationship
Between Amplifier Parameters, And Its Application to PSRR
Improvement," IEEE Trans. Circuits and Systems—I, vol. 38,
no. 10, pp. 1173–1181, Oct 1991
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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An amp within an amp
– Weak inversion
– Zero-Vgs amplifiers
– Super-Transistors
– Cascode current mirrors
– Self-biased cascodes
– Regulated cascodes
– Matryoshka amplifiers
– Regulated cascode OTAs
– Nested Miller amplifiers
Image from http://www.souvenironline24.de/shop.aspx
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Weak Inversion = Sub-threshold Operation
from [Tsividis99]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Zero-Vgs folded-cascode opamp in 0.18μm technology
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Zero-Vgs folded-cascode opamp in 0.18μm technology
VGS
VT=230 mV (!), L=min, ID=5uA
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Maximum gain of single stage is reached in weak inversion
For a given supply current: gain is proportional to supply voltage!
Less gain on (deep)
submicron
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Normal current mirror
Output resistance
Increase this with feedback!
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Cascode current mirror
Feedback loop:
For a constant signal current, the transistor M4 tries to keep the
drain voltage of M2 constant. The loop gain around M4 is
and the output resistance:
Problem: high voltage drop.
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Low-voltage cascode current mirror
Same feedback loop!
Careful design needed such that
M3 and M1 are always saturated
Bias voltage necessary
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Self-biased low-voltage cascode current mirror
Still same feedback loop!
But: for the same current, Vgs3 < Vgs1!
– M1, M2 in strong inversion
M3, M4 in weak inversion
(makes Aloop small and M3,M4 huge)
– M1, M2 normal-Vt transistors
M3, M4 low-Vt transistors
(requires low-Vt transistors, which
most submicron processes have)
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Different view: build super transistors
Then: build good super transistors!
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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The regulated cascode
Increasing the loop gain ...
... gives much higher output resistance
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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The "original" by Säckinger
simplest loop amplifier, but needs a lot of supply voltage
from [Säckinger90]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Matryoshka-style regulated-cascode amplifier
several OTA
Slices
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
one OTA Slice
from [Treichler06]
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Matryoshka slice layout!
One OTA Slice
Full OTA
[Treichler06]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Matryoshka Miller OpAmp: Two stages
from [Huijsing01]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Matryoshka Miller OpAmp: Three stages
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Matryoshka Miller OpAmp: Four stages
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Conclusion
On modern digital technologies, we lose
– supply voltage
– gain
If we need gain:
– we need to combine more gain stages
– and, if possible, use weak inversion
Intuitive way to think about it:
An Amp within an Amp within an Amp
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Literature: New uses of old parts
[Burger96] Thomas Burger and Qiuting Huang, "A 100dB 480MHz OTA in
0.7um CMOS for sampled-data applications," Proc. CICC,
pp. 101–104, 1996.
[Huijsing01] Johan H. Huijsing, Operational Amplifiers—Theory and Design,
Kluwer Academic Publishers, 2001.
[Säckinger90] Eduard Säckinger et. al, "A High-Swing, High-Impedance MOS
Cascode Circuit," IEEE J. Solid-State Circuits, vol. 25, no. 1,
pp. 289–298, Feb 1990.
[Treichler06] Jürg Treichler et. al., "A 10-bit ENOB 50-MS/s Pipeline ADC in
130-nm CMOS at 1.2 V Supply," Proc. ESSCIRC, Montreux, Switzerland,
pp. 552–555, 2006.
[Tsividis99] Yannis Tsividis, Operation and Modelling of the MOS Transistor,
ed. 2, McGraw-Hill 1999.
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Switched capacitors
– Speed limit of SC filters
– SC noise filtering
– Switches and T-gates
– Voltage doublers
– for clock signals
– for OTA tails
– for control voltages
– Flicker Noise
– Autozero, CDS and Chopping
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Simple SC resistor
Pole frequency of SC resistor
loaded with capacitance:
from [Gregorian86]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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SC becomes much faster on modern processes
from [Johns97]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Huge SC resistor for noise filtering
"Bucket Chain" technique
Possible: 1s time constant!
Requires RC filters for antialiasing
e.g., 80fF, 160kHz, 13 elements Æ 1 GΩ
Beware of offset!!!
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Bad Layout: asymmetries of clock lines!
This can give huge offset.
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Good Layout: as symmetrical as possible
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Types of switches
from [Johns97]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Voltage-level limitation
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Benefitting from Narrow-Channel Effects
from [Tsividis96]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Reduce switch threshold voltage by slicing
VT=610mV
0.18u Process
Normal-VT Transistors
VT=540mV
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Clock voltage doubler
"Doubling" pMOS gate voltages below VSS is also possible!
from [Basu99]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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What is flicker noise?
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Fllicker noise comes from a process with memory!
from [Keshner82]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Why is it called "flicker" noise?
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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On flicker noise:
the Yahoo Aaaaaargh!
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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On flicker noise: the Yahoo Aaaaaargh!
from [Schmid07]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Nature of Memory in MOSFETs
Mainly interface traps at the channel-to-oxide and gate-to-oxide
interfaces:
– Spectrum caused by a single trap with time constant τ:
– Distribution of the time constants:
– Flicker noise slope is a physical property.
– Flicker noise magnitude is related to the absolute number of
interface traps.
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Model of scaling-invariant memory
from [Schmid08]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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RMS behaviour of flicker noise
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Reducing flicker noise by deleting memory I
from [Klumperink00]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Sampling noise
from [Schmid08]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Reducing offset and flicker noise by auto-zeroing
Autozero
1: Vos
2: Vin+Vos
2−1: Vin
Correlated
Double
Sampling
1: −Vin+Vos
2: Vin+Vos
2−1: 2Vin
from [Enz96]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Reducing offset and flicker noise by auto-zeroing
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Reducing offset and flicker noise by chopping
from [Enz96]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Simulated chopped noise
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Chopper circuit
from [Schmid08]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Matryoshka Chopper
from [Schmid08]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Multipath Chopper
Chopped
High
Gain
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Reducing offset and 1/f noise by correlated double sampling
– Auto-zeroing: sample offset in one phase; sample signal in
other phase while compensating offset.
Auto-zeroing works in sampled time.
– Chopping: modulate input signal to a higher frequency;
modulate signal back after amplifier, and therefore modulate
offset and 1/f noise to higher frequencies.
Chopping works in continuous time!
– Correlated double sampling combines both: first sample signal,
then sample inverse, then subtract.
Correlated double sampling works in sampled time.
CDS can be used most effectively in capacitive sensor systems
where the sensor can be controlled to give normal or inverse
output signals! Then sensor offset and 1/f noise is reduced too.
– In auto-zero and CDS, the transistor bias history must be
the same for both samples!
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Literature: Switched capacitors
[Basu99] S. Basu and G. Temes, "Simplified Clock Voltage Doubler," Electronics Letters, vol. 35, no. 22, pp. 1901–1902, Oct 1999.
[Duisters98] Tonny A. F. Duisters and Eise Carel Dijkmans, "A −90-dB THD rail-to-rail input opamp using a new local charge pump in
CMOS," IEEE J. Solid-State Circuits, vol. 33, no. 7, pp. 947–955, Jul. 1998.
[Enz96] Christian Enz and Gabor Temes, "Circuit Techniques for Reducing the Effects of Op-Amp Imperfections: Autozeroing, Correlated
Double Sampling, and Chopper Stabilization," Proc. IEEE, vol. 84, no. 11, pp. 1584–1614, Nov 1996.
[Gregorian86] Roubik Gregorian and Gabor Temes, Analog MOS Integrated Circuits for Signal Processing, John Wiley & Sons 1986.
[Johns97] David Johns and Ken Martin, Analog Integrated Circuit Design, John Wiley & Sons 1997.
[Keshner82] Marvin Keshner, "1/f Noise," Proc. IEEE, vol. 70, no. 3, pp. 212–218, March 1982.
[Klumperink00] Eric Klumperink et. al., "Reducing MOSFET 1/f Noise and Power Consumption by Switched Biasing," IEEE J. Solid-State
Circuits, vol. 35, no. 7, pp. 994–1001, Jul. 2000.
[Schmid02] Hanspeter Schmid, "An 8.25-MHz 7th-Order Bessel Filter Built with Single-Amplifier Biquadratic MOSFET C Filters", Analog
Integrated Circuits and Signal Processing, NORCHIP special issue, vol. 30, no. 1, pp. 69–81, January 2002.
[Schmid07] Hanspeter Schmid , "Aaargh! I Just Loooove Flicker Noise," IEEE Circuits and Systems Magazine, pp. 32–35, First Quarter
2007.
[Schmid08] Hanspeter Schmid, "Offset, flicker noise, and ways to deal with them": Chapter in Circuits at the Nanoscale, CRC Press,
2008, edited by Krzysztof Iniewski.
[Wel07] Arnoud P. van der Wel et. al., "Low-Frequency Noise Phenomena in Switched MOSFETs," IEEE J. Solid-State Circuits, vol. 42,
no. 3, pp. 540–550, March 2007.
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Feedback or no feedback
– The benefit of feedback
– Current mode and
voltage mode
– Example: Open-Loop SigmaDelta A/D converter
– Case study with CSEM Zürich:
Low-feedback approach
applied to buffer design
Image from [Black34]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Feedback (in Black's words)
Advantages:
constancy of amplification
freedom from nonlinearity
reduced delay and delay distortion, reduced noise disturbance from
the power supply circuits
Disadvantages:
[difficult] because of the [] special control required of phase shifts
Unless these relations are maintained, singing will occur
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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No free lunch!
The famous no-free-lunch theorem states that even if we say, e.g.,
"A system with feedback gives us low distortion for free", it is not
really for free, we just cannot possibly optimize power by trading in
distortion or other parameters.
A more scientific version of the no-free-lunch theorem states:
A general-purpose optimization strategy is impossible, and
the only way one strategy can outperform another is if it is
specialized to the structure of the specific problem under
consideration.
from [Ho01]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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High-Impedance node in AD844 current-feedback amplifier
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Simple example: voltage-controlled current source
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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AD844: the first stage is a Current Conveyor (CCII)
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Current Amplifier without high-impedance node
from [Schmid00]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Real difference
from [Schmid03]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Very simple, very fast voltage integrator
from [Nauta92]
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Impedance mismatch
– to decouple
– feedback couples again
– no FB
–
–
–
–
–
decoupled
optimization is much faster
optimization space becomes tidier
the child finds out more in a shorter time
the fool won't fall
– Example
– aggressive design time
– first time right
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Case Study: Low-feedback approach applied to buffer design
Hanspeter Schmid, IME/FHNW
Simon Neukom and Yue-Li Schrag, CSEM Zürich
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Standard SC amplifier
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Why an open-loop solution?
We needed
– Voltage level shift from arbitrary low voltage to 1.6V
– Less supply current variation (lowered by 20dB)
– 12-bit precise settling at 4 MHz sample rate, 12-bit precise offset
Our open-loop continuous-time solution gave
– less offset (3σ=3.3mV compared to SC amp's 3σ=11.4mV)
– less power (14mW compared to SC amp's 63.5mW)
Disadvantages are:
– more harmonic distortion
– more noise
but since this is an output driver after high-gain pre-amplifier chain,
both disadvantages do not matter in our application.
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Operation principle: (with matched resistors)
Stage 1: single-ended voltage to differential current
Stage 2: current to voltage
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Offset compensation with current-output Track&Hold
Signal is processed in "Hold" mode
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Offset compensation with current-output Track&Hold
Offset is compensated in "Track" mode individually for each output path
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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The remaining offset comes only from the T&H OTA!
All other offsets, including random offsets in the gnd references, are cancelled.
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Input transconductor
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Output transresistance amplifier
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Track&Hold amplifier
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Static offset: value settled at the end of calibration cycle
Dynamic offset: mean value of full-scale settled values
Static Offset
Durch Bild oder Grafik
ersetzen
(Grösse und Position beibehalten)
Dynamic Offset
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Static and dynamic offset
correlate very well
Offsets of two channels do
not correlate well
digital correction possible!
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Supply current for full-scale steps
The current peaks are much smaller than for SC amplifiers
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Monte-Carlo simulation of third-order (left)
and second-order (right) harmonic distortion (full scale, full speed)
Efficient Simulation of Harmonic Distortion in Discrete-Time Circuits
Wednesday May 27, 2009 from 15:30 - 17:00 in Room 101B.
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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What causes
non-idealities?
odd-order distortion
even-order distortion
gain error
NOISE
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
offset
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© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Design time!
– two weeks including all
simulations and layout
– has been used on
three chips
– first time right;
meets specs
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Literature: Feedback or no feedback
[Black34] Harold S. Black, "Stabilized Feed-Back Amplifiers," Electrical Engineering, vol. 53, no. 1,
pp. 114–120, Jan 1934. Reprinted in Proc. IEEE, vol. 87, no. 2, pp. 379–385, Feb 1999.
[Ho01] Y-C. Ho, D. Pepyne, "Simple Explanation of the No Free Lunch Theorem of Optimization",
Proc. 40th IEEE Conf. on Decision and Control, Orlando, pp. 4409–4414, Dec. 2001.
[Mahattanakul98] Jirayuth Mahattanakul, "Current-Mode Versus Voltage-Mode Gm-C Biquad Filters:
What the Theory Says," IEEE Trans. CAS–I, vol. 45, no. 2, pp. 173–186, Feb 1998.
[Nauta92] Bram Nauta, "A CMOS Transconductance-C Filter Technique for Very High Frequencies,"
IEEE J. Solid-State Circ., vol. 27, no. 2., pp. 142–153, Feb 1992.
[Schmid00] Hanspeter Schmid, "Approximating the Universal Active Element." IEEE Trans. CAS–I,
vol. 47, no. 11, pp. 1160–1169, Nov 2000.
[Schmid03] Hanspeter Schmid, "Why 'Current Mode' Does Not Guarantee Good Performance,"
Analog Integrated Circuits and Signal Processing, vol. 35, no. 1, pp. 79–90, April 2003.
© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland
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Thank you for coming!
Hanspeter Schmid
Institute of Microelectronics
Steinackerstrasse 1
5210 Windisch
Switzerland
Tel +41 56 462 46 25
Fax +41 56 462 46 15
hanspeter.schmid@fhnw.ch
Lab: http://www.fhnw.ch/technik/ime/
Publications: http://www.schmid-werren.ch/hanspeter/
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