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Built-In Self-Test and Calibration of Mixed-signal Devices
Wei JiangPh.D. Dissertation Proposal
June 11, 2009
Advisor: Vishwani D. Agrawal
Committee Members: Fa F. DaiVictor P. NelsonAdit D. Singh
Outline
• Overview• Background• Built-in Test and Calibration Approach• Current Progress• Future Work• Conclusion
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Overview
• Issues– Parameter deviation– Process variation
• Problem– Design variation-tolerant process-independent
technique for mixed-signal devices
• Approach– Test and characterization of mixed-signal
devices– Output calibration
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Mixed-signal Device
• Analog and digital circuitry• Digital controllable• Typical devices
– Converters, digital-to-analog/analog-to-digital
– Amplifier
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Testing of Mixed-signal Devices• Defects and faults
– Catastrophic faults (hard faults)– Parametric faults (soft faults)
• Test approaches– Functional test (specification oriented)– Structural test (defect oriented)
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Challenges
• Analog circuitry– No convincing fault models– Difficult to identify faults– Device parameters more susceptible to
process variation than digital circuitry– Fault-free behavior based on a known range
of acceptable values for component parameters
• Large statistical process variation effects in deep sub-micron MOSFET devices
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Process Variation
• Parameter variation in nanoscale process• Yield, reliability and cost• Feature size scaling down and performance
improvement• Effects on digital and analog circuitry
– Analog circuitry more affected by process variation– Parameter deviation severed in nanoscale process– System performance degraded when parameter
deviation exceeds beyond tolerant limits
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Outline
• Overview• Background• Built-in Test and Calibration Approach• Current Progress• Future Work• Conclusion
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Typical Mixed-Signal Architecture
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Mixed-Signal System Test Architecture
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* F. F. Dai and C. E. Stroud, “Analog and Mixed-Signal Test Architectures,” Chapter 15, p. 722 in System-on-Chip Test Architectures: Nanometer Design for Testability, Morgan Kaufmann, 2008.
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Mixed-Signal System Test Architecture• Digital system
– Digital I/O– Digital signal processor (DSP)– TPG and ORA and test control unit– Digital loopback
• Mixed-signal system– DAC and ADC– Analog loopback
• Analog system– Analog circuitry– Analog signal I/O– Analog I/O loopback
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Test Criteria
• Digital circuitry test– Defect-oriented test– Defects cat be detected by wrong
output response for specific test pattern
• Analog circuitry test– Specific-oriented test– Parameter deviations vs. the acceptable
tolerant limit
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Typical Mixed-Signal Devices
• DAC – digital-to-analog converter– Digital inputs; analog outputs
• ADC – analog-to-digital converter– Analog inputs; digital inputs
• Digital Controlled Amplifier– Analog inputs/outputs with digital controlling
inputs– Analog transfer function controlled by digital
device, e.g. microcontroller– Gain/distortion/nonlinearity respond to
digital controlling signalWei Jiang 13
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Existing Testing Approach
• Oscillation BIST• LFSR-based TPG• FFT-based BIST
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Linearity Problem
• LSB – least significant bit– The minimum measurement for of
analog value– Represented by 1 digital bit
• Non-linearity Error– DNL – differential non-linearity
– INL – integral non-linearity
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11
LSBDNL kk
k
kLSB
DNLINL kk
kk
0
0
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Analog input
Dig
ital c
ode
outp
ut
Ideal
Actual (νK)
Ideal
Actual (νK)
Ana
log
outp
ut
Digital code input
Non-linearity error of ADC Non-linearity error of DAC
k
kν
ν
Non-linearity Error of ADC/DAC
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Non-linearity error
Non-linearity
error
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Other Characteristics
• Frequency response– Bandwidth
• Noise– SNR – signal-to-noise ratio– SINAD – signal-to-noise and distortion
ratio
• Offset, gain, harmonic distortion• Intermodulation distortion
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Outline
• Overview• Background• Built-in Test and Calibration
Approach• Current Progress• Future Work• Conclusion
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Typical Mixed-Signal System with DAC/ADC
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Proposed Test and Calibration Architecture
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Components Description• Digital circuitry (including DSP) as BIST control
unit– Test pattern generation (TPG) and output response
analysis (ORA) • Measuring ADC
– First-order 1-bit Sigma-Delta modulator– Digital low-pass filter– Measuring outputs of DAC-under-test
• Dither DAC– Low resolution DAC– Generating correcting signal for calibration– Calibrated DAC for test of ADC-under-test
• ADC Polynomial Fix– Digital process to revise ADC output codes
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Testing Procedure• Self-test of testing and calibrating components
– Self-test of BIST control unit (including DSP, TPG/ORA)
– Self-test of measuring ADC– Test of dithering DAC by measuring ADC
• Test of On-chip DAC– Ramp test of on-chip DAC– Characterizing on-chip DAC by DSP– Calibration of on-chip DAC by dithering DAC
• Test of on-chip ADC– Ramp test of on-chip ADC– Characterizing and fixing on-chip ADC outputs by
DSP
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Faulty Mixed-Signal Circuitry
• Good circuitry– All parameters and characteristics are within
pre-defined specified range
• Fault-tolerant factor– Post-fabrication and software-controllable– Fault-tolerant factor varies for different
application– Trade-off between fault-tolerance of
parameter deviation and calibration resolution
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Determine Faulty DAC/ADC
• Coefficients representing offset, gain and harmonic distortion exceeding specific limit
• Maximum INL error exceeding calibration range (depending on fault-tolerant factor)– ±4LSB for fault-tolerant factor 3
• INL errors of all calibrated outputs must be within ±0.5LSB
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Device Test and Calibration
• During BIST– Test DAC/ADC with ramp signals– Measure response of each test code– Obtain INL error for each code– Characterize device by INL error
• After BIST– Determine faulty devices by deviation of
parameters– Generate correcting signal/data (identical
to INL error) for each code– Calibrate DAC/ADC output using correcting
signal/data by removing INL errorWei Jiang 25
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The ONLY Problem
• Storing all INL errors for every input code of DAC/ADC is impossible– Requiring huge amount of memory– Needing lots of access time to retrieve
specific data from memory– Prohibiting cost
• Solution– Polynomial fitting– Storing several coefficients instead of all
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Test Pattern
• Test pattern– Ramp code– Least value to most value– Testing time for each pattern depends
on the converting speed of measuring ADC
• Single-tone and multi-tone test patterns can also be used
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Test of Digital Circuitry
• Conventional digital BIST technology• LFSR-based random test; Scan-based
deterministic test• Digital loopback conducted• Fault-free digital circuitry then used
for mixed-signal test• May be hardware- or software-based
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Measuring ADC
• First-order 1-bit sigma-delta ADC• Perform self-test before any other
mixed-signal test• Make sure each components of
sigma-delta ADC working• Quantization noise• Bit-stream output pattern
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Sigma-Delta Modulator
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Sigma-Delta Modulator (cont.)• Advantage
– Oversampling and noise-shaping– High resolution and linear results– Resolution depends on OSR (oversampling
ratio)– Simple structure and low cost
• Disadvantage– Very slow converting speed– Bit-stream output pattern issue for low-order
modulation– Requiring high-speed clock
• Higher order and/or multi-bit modulation
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Selection of Sigma-Delta Modulator
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First-order
Second-order
Third-order
17-bit ENOB104.1LSB
Oversampling ratio (OSR)General Oral Examination
Digital Filter
• Sigma-delta ADC consists of sigma-delta modulator and digital filter
• Low-pass filter (LPF)• Integrator• Comb filter
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Dithering DAC
• Low-cost low-resolution DAC• Better linearity output with DEM
technique• Must be tested by measuring ADC
before test of on-chip mixed-signal devices
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Resolution of Dithering DAC
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3
α=1
17bits
Resolution of dithering-DAC (bits)
Overs
am
plin
g r
ati
o (
OS
R)
2
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Polynomial Fitting Algorithm• Introduced by Sunter et al.
in ITC’97 and A. Roy et al. in ITC’02
• Summary:– Divide DAC transfer
function into four sections
– Combine function outputs of each section (S0, S1, S2, S3)
– Calculate four coefficients (b0, b1, b2, b3) by easily-generated equations
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33
2210 xbxbxbby
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Third-order Polynomial
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343
232
3121
200
01233
01232
01231
01230
33
2210
3
128
163
443
41
33
Bn
b
Bn
b
BBn
b
BBn
b
SSSSB
SSSSB
SSSSB
SSSSB
xbxbxbby
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Adaptive Polynomial Fitting
• Dynamically choose polynomial degree• Low-order polynomial
– Simple to design and implement– Less area and performance overhead– Large fitting error
• High-order polynomial– Better fitting results– More coefficients to store– Much more complicated polynomial
evaluation circuitry design and heavy area and performance overhead
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Test and Calibration of On-Chip DAC
BIST CONTROL
DACunder-test
1st-order 1-bit ΣΔ Modulator
LPFDigital Filter
Ramp code generator
Characteristics analysis
Pass/fail indicator
Offset,Gain,2nd and 3rd harmonic distortion
Coefficientsfor polynomial evaluation
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ΣΔ ADC
Polynomial evaluation
Dithering DAC
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Test and Calibration of On-Chip ADC
ADCunder-test
DACunder-test
Polynomial Fix
Polynomial evaluation
Dithering DAC
coefficients
BIST CONTROL
Pass/fail indicator
Offset,Gain,2nd and 3rd harmonic distortion
Coefficientsfor polynomial fit
Ramp code generator
Characteristics analysis
14
14
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General Mixed-Signal Test
• Variation-tolerant design• Digital controlled BIST• Digitalized TPG/ORA• Self-testable measuring components• Characterization of device-under-test by
DSP• Faulty circuitry determined by
characterized parameters• Coefficients of output fix/correction
signals calculated by DSPWei Jiang 41
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Outline
• Overview• Background• Built-in Test and Calibration Approach• Current Progress• Future Work• Conclusion
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Publications• W. Jiang and V. D. Agrawal, “Built-In Test and
Calibration of DAC/ADC Using A Low-Resolution Dithering DAC,” NATW’08, pp. 61-68.
• W. Jiang and V. D. Agrawal, “Built-in Self-Calibration of On-Chip DAC and ADC,” ITC’08, paper 32.2.
• W. Jiang and V. D. Agrawal, “Built-in Adaptive Test and Calibration of DAC,” NATW’09, pp. 3-8.
• W. Jiang and V. D. Agrawal, “Designing Variation-Tolerance in Mixed-Signal Components of a System-on-Chip,” ISCAS’09, pp. 126-129.
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Progress
• Presenting a novel approach to test and calibration DAC/ADC
• Presenting a method to dynamically determine the order of curve fitting polynomial for INL errors
• Proved by Matlab simulation theoretically
• Applicable for digitally controllable mixed-signal devices
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Simulation of DAC Test
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• 14-bit DAC• 16K ramp
codes• INL error up
to ±1.5LSB
Indices of 14-bit DAC-under-test
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Simulation (Cont.)
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• Fitting results by different order polynomial
Indices of 14-bit DAC-under-test
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Best-matching Polynomial
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Fitting Algorithm
• Third-order polynomial fitting algorithm
• Adaptive polynomial fitting algorithm• Determination of best matching
polynomial degree
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Measuring ADC / Dithering DAC• Measuring ADC
– First-order 1-bit Sigma-Delta ADC– Higher-order multi-bit Sigma-Delta ADC– Non-Sigma-Delta ADC– Digital low-pass filter
• Dithering DAC– Binary weighted DAC
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Current Tasks
• Modeling and hardware verification of proposed testing approach
• Programming of third-order polynomial fitting algorithm
• Implementation and optimization of digital polynomial evaluation circuit
• Design and verification of the whole test and calibration system
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Outline
• Overview• Background• Built-in Test and Calibration Approach• Current Progress• Future Work• Conclusion
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Future Tasks and Schedule
• Testability of measuring ADC (~2 months)• Best matching polynomial (~2 months)• At-speed characterization (~1 month)• Dynamic Element Matching (~1 month)• Other testing techniques (~2 months)• Dissertation and defending (~2 months)
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Testability of Measuring ADC
• Measuring ADC must be self-testable• Testability of Sigma-Delta modulator• Problem
– Using digital circuitry and DSP to test analog circuitry
– Measuring ADC must tell faulty or healthy by itself
– Test of measuring ADC must be done before test of all other mixed-signal components
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Best Matching Polynomial
• Degree determination of best matching polynomial
• Problem– Find cut-off degree of INL errors– Higher-order polynomial brings heavy
hardware overhead– Lower-order polynomial gives more
fitting error
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At-Speed Characterization
• An approach to utilize idle time of DSP to re-calibrate mixed-signal devices
• Performance of analog components varies by environment, i.e. temperature, usage time
• Characterization during boot time may be inaccurate and need continuous revision during its lifetime
• Dynamic re-characterization of mixed-signal devices may reflect real condition of the devices and generate better results
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Dynamic Element Matching (DEM)• Reduce non-linearity error of DAC/ADC• Fault-tolerant for analog elements• Approaches
– Dynamically change/rotate matching elements to generate desired outputs for specific inputs
– Reduce non-linearity of mismatching elements
• Disadvantage– Output pattern issue for low-order
matching algorithm– Requiring high-speed clock
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Other Things…
• Frequency response test• Noise test and removal• Delay test• Single-tone and multi-tone test
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Outline
• Overview• Background• Built-in Test and Calibration Approach• Current Progress• Future Work• Conclusion
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Conclusion• A post-fabrication built-in test and calibration
approach for mixed-signal devices is proposed• This approach relies on digital circuitry and DSP
for TPG/ORA and BIST control• Digital circuitry is testable by conventional
digital testing approaches and therefore guarantee the testability of analog circuitry
• The approach has been applied to test of DAC/ADC
• The same idea can be widely used for other digital-controlled mixed-signal devices
• Calibration on mixed-signal devices will significantly reduce defects, improve die yield and lower manufacturing cost
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Q&ATHANKS
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