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Analog Circuit Design on Digital CMOSWhy it is difficult, and which ideas help. Presented by HP. Schmid.
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 2
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.
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 3
Tutorial Philosophy
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 4
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
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 5
Philosophy II: Be a child
– open for everything
– playful
– does not dowhat she should do
– a child has got time!
– Advice for scientists byDouglas Adams:
See first, think later, then test. But always see first, or you will only see what you expect to see!
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 6
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! (André Gide)
– Will the fool not fall down?
Not if the fool also is a child.
The most exciting phrase in science, the one that heralds new discoveries, is not Eureka! (I found it!), but
That's funny ...
(Isaac Asimov)
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 7
Tutorial Contents
Image from http://www.beatenbergbilder.ch/
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 8
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
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 9
Multi-metal cross section
Example: 6 Metal layers. Lateral dimensions are smaller than vertical dimensions!
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 10
Transconductance in Strong and Weak Inversion
Strong Inversion
Weak Inversion
Moderate Inversion: Superposition
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 11
Maximum gain of single stage is reached in weak inversion
For a given supply current: gain is proportional to supply voltage!
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 12
Weak inversion = matching problems?
For a 0.25u process:
Voltage offset Current offsetfor identical supply current for identical gate-source voltage
Therefore: Differential pairs in weak inversionTherefore: Current mirrors in strong inversion
from [Kinget07]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 13
Summary
– Thinner gates (and higher gate tunnelling currents!)
– more gate (overlap, ...) capacitance per area
– No buried channels anymore pMOS is not better anymore in terms of flicker noise!
– Less supply voltage less signal
– Less gain
– same white noise at same supply current; less flicker noise
– Sub-threshold leakage
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 14
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 Deliberation-Without-Attention Effect," Science, vol. 311, pp. 1005–1007, 2006.
[Simons99] Daniel Simons et. al., "Gorillas in our midst: sustained inattentionalblindness for dynamic events," Perception, vol. 28, pp. 1059–1074, 1999.
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 15
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 16
Signal Integrity
– Ground and Power Routing
– Star Connections
– Tapered Stars
– Signal Grounds and Refs
– Improving PSR (theory)
– Finger capacitors andMIM-capacitors
– Demodulation by nonlinearity
– Decoupling
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 17
Why correct ground and power routing are important
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 18
On PCB: Power plane? No!
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 19
On PCB: Split ground plane? Dangerous!
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 20
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!
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 21
The problem of the star connection on chip
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 22
Calculation example: hearing aid system
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 23
16μΩ is not a lot!
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 24
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.
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 25
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
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 26
How to improve PSRR and CMRR in a system?
CMRR and PSRR are connected!Proof: Gauge transformation
from [Säckinger91]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 27
Solution: Additional input from quiet ground
Now we have one more degree of freedom
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 28
Example: additional signal path
from [Loikkanen06]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 29
Example: additional signal path
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 30
Recommendations for chip routing
Use "tapered" star connections
For every differential signal node, make sure that the signal isreferred to a clean signal.
Problem:
the references can change within a single circuit
Input reference
Output reference
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 31
Multi-metal Finger-Cap MIM-Cap combination
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 32
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?
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 33
Normal Operation with HF-Signal on Pad(weak inversion)
Gives DC Offset! Inputs must be protected against this ...
Demodulation by a nonlinearity I: DC offset
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 34
Normal Operation with amplitude-modulatedHF-Signal on Pad (weak inversion)
Demodulates the signal and gives more DC offset!
Demodulation by a nonlinearity II: receiver
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 35
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
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 36
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
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 37
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 38
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
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 39
Weak Inversion = Sub-threshold Operation
from [Tsividis99]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 40
Zero-Vgs folded-cascode opamp in 0.18μm technology
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 41
Zero-Vgs folded-cascode opamp in 0.18μm technology
VT=230 mV (!), L=min, ID=5uA
VGS
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 42
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
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 43
Normal current mirror
Output resistance
Increase this with feedback!
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 44
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.
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 45
Low-voltage cascode current mirror
Same feedback loop!
Careful design needed such thatM3 and M1 are always saturated
Bias voltage necessary
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 46
Self-biased low-voltage cascode current mirror
Still same feedback loop!
But: for the same current, Vgs3 < Vgs1!
– M1, M2 in strong inversionM3, M4 in weak inversion(makes Aloop small and M3,M4 huge)
– M1, M2 normal-Vt transistorsM3, M4 low-Vt transistors(requires low-Vt transistors, whichmost submicron processes have)
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 47
Different view: build super transistors
Then: build good super transistors!
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 48
The regulated cascode
Increasing the loop gain ...
... gives much higher output resistance
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 49
The "original" by Säckinger
simplest loop amplifier, but needs a lot of supply voltage
from [Säckinger90]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 50
Matryoshka-style regulated-cascode amplifier
from [Treichler06]
one OTA Sliceseveral OTA Slices
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 51
Matryoshka slice layout!
[Treichler06]
One OTA Slice
Full OTA
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 52
Matryoshka Miller OpAmp: Two stages
from [Huijsing01]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 53
Matryoshka Miller OpAmp: Three stages
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 54
Matryoshka Miller OpAmp: Four stages
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 55
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
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 56
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.
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 57
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 58
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
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 59
Simple SC resistor
from [Gregorian86]
Pole frequency of SC resistor loaded with capacitance:
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 60
SC becomes much faster on modern processes
from [Johns97]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 61
Huge SC resistor for noise filtering
"Bucket Chain" technique Requires RC filters for antialiasing
Possible: 1s time constant! e.g., 80fF, 160kHz, 13 elements 1 GΩ
Beware of offset!!!
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 62
Bad Layout: asymmetries of clock lines!
This can give huge offset.
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 63
Good Layout: as symmetrical as possible
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 64
Types of switches
from [Johns97]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 65
Voltage-level limitation
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 66
Benefitting from Narrow-Channel Effects
from [Tsividis96]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 67
Reduce switch threshold voltage by slicing
VT=610mV
VT=540mV
0.18u Process
Normal-VT Transistors
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 68
Clock voltage doubler
"Doubling" pMOS gate voltages below VSS is also possible!
from [Basu99]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 69
What is flicker noise?
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 70
Fllicker noise comes from a process with memory!
from [Keshner82]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 71
Why is it called "flicker" noise?
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 72
On flicker noise:the Yahoo Aaaaaargh!
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 73
On flicker noise: the Yahoo Aaaaaargh!
from [Schmid07]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 74
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.
Nature of Memory in MOSFETs
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 75
Model of scaling-invariant memory
from [Schmid08]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 76
RMS behaviour of flicker noise
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 77
Reducing flicker noise by deleting memory I
from [Klumperink00]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 78
Sampling noise
from [Schmid08]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 79
Reducing offset and flicker noise by auto-zeroing
from [Enz96]
Autozero
1: Vos
2: Vin+Vos
2−1: Vin
CorrelatedDoubleSampling
1: −Vin+Vos
2: Vin+Vos
2−1: 2Vin
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 80
Reducing offset and flicker noise by auto-zeroing
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 81
Reducing offset and flicker noise by chopping
from [Enz96]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 82
Simulated chopped noise
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 83
from [Schmid08]
Chopper circuit
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 84
from [Schmid08]
Matryoshka Chopper
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 85
Multipath Chopper
ChoppedHigh Gain
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 86
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!
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 87
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 Quarter2007.
[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.
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 88
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 89
Feedback or no feedback
– The benefit of feedback
– Current mode andvoltage mode
– Example: Open-Loop Sigma-Delta A/D converter
– Case study with CSEM Zürich:Low-feedback approach applied to buffer design
Image from [Black34]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 90
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
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 91
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]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 92
High-Impedance node in AD844 current-feedback amplifier
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 93
Simple example: voltage-controlled current source
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 94
AD844: the first stage is a Current Conveyor (CCII)
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 95
Current Amplifier without high-impedance node
from [Schmid00]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 96
Real difference
from [Schmid03]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 97
Very simple, very fast voltage integrator
from [Nauta92]
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 98
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
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 99
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 100
Case Study: Low-feedback approach applied to buffer designHanspeter Schmid, IME/FHNWSimon Neukom and Yue-Li Schrag, CSEM Zürich
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 101
Standard SC amplifier
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 102
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.
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 103
Operation principle: (with matched resistors)
Stage 1: single-ended voltage to differential currentStage 2: current to voltage
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 104
Offset compensation with current-output Track&Hold
Signal is processed in "Hold" mode
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 105
Offset compensation with current-output Track&Hold
Offset is compensated in "Track" mode individually for each output path
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 106
The remaining offset comes only from the T&H OTA!
All other offsets, including random offsets in the gnd references, are cancelled.
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 107
Input transconductor
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 108
Output transresistance amplifier
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 109
Track&Hold amplifier
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 110
Static offset: value settled at the end of calibration cycle
Dynamic offset: mean value of full-scale settled values
Durch Bild oder Grafik ersetzen
(Grösse und Position beibehalten)
Static Offset
Dynamic Offset
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 111
Static and dynamic offsetcorrelate very well
digital correction possible!
Offsets of two channels donot correlate well
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 112
Supply current for full-scale steps
The current peaks are much smaller than for SC amplifiers
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 113
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 CircuitsWednesday May 27, 2009 from 15:30 - 17:00 in Room 101B.
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 114
What causesnon-idealities?
odd-order distortion
offsetNOISE
even-order distortion
gain error
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 115
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 116
Design time!
– two weeks including allsimulations and layout
– has been used onthree chips
– first time right;meets specs
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 117
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.
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 118
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 119
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 120
ISCAS 2009© Hanspeter Schmid, Institute of Microelectronics, FHNW, Windisch, Switzerland 121
Thank you for coming!
Hanspeter SchmidInstitute of MicroelectronicsSteinackerstrasse 15210 WindischSwitzerland
Tel +41 56 462 46 25Fax +41 56 462 46 15
Lab: http://www.fhnw.ch/technik/ime/Publications: http://www.schmid-werren.ch/hanspeter/