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FLASH ADCS
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FLASH ADC ARCHITECTURE
• Reference ladder consists of 2N
matched resistors
• Input is compared to 2N-1 reference
voltages
• Massive parallelism
• Very fast ADC architecture
• Latency = 1 Ts = 1/fs
• Throughput = fs
• Complexity = 2N
… …
En
co
de
r
VFS Vi
fs
Strobe
Dout
2N-1
comparators
…
Do 0
Vi
Δ
2Δ
5Δ
6Δ
7Δ
VFS
……
0
1
5
6
7
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THERMOMETER CODE
… …
VFS Vi
fs
Strobe
2N-1
comparators
…
1
1
1
0
Thermometer code
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THERMOMETER CODE
1
1
0
1
b2 b1 b0
001
010
110
111
000
ROM encoder
…
… ……
… …
VFS Vi
fs
Strobe
2N-1
comparators
…
1
1
1
0
Thermometer code
1-of-n code
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FLASH ADC CHALLENGES: EXAMPLE
• VDD = 1.8 V
• 10-bit
• VFS = 1 V
• DNL < 0.5 LSB
• 0.5 mV = 3-5 σ
→ 1023 comparators
→ 1 LSB = 1 mV
→ Vos < 0.5 LSB
→ σ = 0.1-0.2 mV
• 2N-1 very large comparators
• Large area, large power consumption
• Very sensitive design
• Limited to resolutions of 4-8 bits
1V
1mV
σ
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FLASH ADC CHALLENGES
4 6 8 10
2
8
32
128
N [bits]
Vo
s, m
ax [m
V]
VFS = 1V
VFS = 2V
0.5
• DNL < 0.5 LSB
• Large Full-scale voltage (VFS) relaxes offset tolerance
• Small VFS benefits conversion speed
(settling, linearity of building blocks)
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ADC INPUT CAPACITANCE
2
2 2Vthth g
Aσ V C 10 fF /
WLμm
• N = 6 bits
• VFS = 1 V
• σ = LSB/4
• AVT0 = 10 mV·μm
→ 63 comparators
→ 1 LSB = 16 mV
→ σ = 4 mV
→ L = 0.24 μm,
W = 26 μm
N (bits) # of comp. Cin (pF)
6 63 3.9
8 255 250
10 1023 ??!
• Small Vos leads to large device sizes, hence large area and power
• Large comparator leads to large input capacitance, difficult to drive and difficult to achieve sufficient tracking bandwidth
M. Pelgrom, A. Duinmaijer, and A. Welbers, “Matching properties of MOS transistors,” IEEE Journal of Solid-State Circuits, vol. 24, no. 5, pp. 1433-1439, 1989.
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FULLY-DIFFERENTIAL ARCHITECTURE
• VFS is effectively doubled
• 3-dB gain in SNR
• Better CMRR
• Noise immunity
• Input feedthrough is cancelled
• Cin nonlinearity partially removed
…… … …
En
co
de
r
…
VR+
VR-
Vi+
Vi-
PA Latch
Dout… … …
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FLASH ADC DESIGN CONSIDERATIONS
• Use a dedicated S/H (or T/H) for better dynamic performance
– Can be avoided when using the A/D inside a ΔΣ loop
• Large input range for the quantizer has several benefits
– Increased step-size (VLSB) relaxes offset requirements on the comparators
– Reduced matching requirements result in small input cap to the S/H, easier to drive
– Reduced input cap results in smaller clock routing parasitics – power savings in clock drivers
• Comparator Design
– See comparator design notes/slides
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FLASH ADC: REFERENCE LADDER
• Differential reference ladder
• Decaps on the reference taps
– large RC time-constant will not allow reference restoration after kickback noise
– Small R will lead to power dissipation
– Optimize RC time-constant value
• Subtract references from the input in a differential manner
– Several topologies are possible
• Several architectures for the digital backend
– May need to pipeline digital logic at high sampling rates >500 MS/s
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REFERENCE GENERATOR (FOR VREFP AND VREFM)
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FLASH ADC: REFERENCE SUBTRACTION
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REFERENCE SUBTRACTION: SCHEME I
• Employ reference ladder for subtraction
• Choose current (I) such that differential voltage drop across R = 1 VLSB
• Ladder is part of the signal path
– Comparator input cap load the resistor taps
– Excess delay
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REFERENCE SUBTRACTION: SCHEME II
• Source followers to buffer vin
– reduced swing, varies with PVT
• Ladder is part of the signal path
– Comparator input cap load the resistor taps
– Excess delay
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REFERENCE SUBTRACTION: SCHEME III• Differential difference amplifier for subtracting the
reference
– followed by a zero crossing detector
• Relaxes the impedance requirements on the ladder
• Mismatch in differential pairs and tail current sources results in comparator offset
– current trimming (see the reference below)
• Finite BW of the amplifier causes excess delay
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REFERENCE SUBTRACTION: SCHEME IV
• Switched-capacitor reference subtraction
– Pay attention to charge injection
• ADC can handle large input swing
• Slow when auto-zeroing preamp is used
– Large settling time constant
– Reference subtraction in background
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EXAMPLE: FULLY-DIFFERENTIAL COMPARATOR
• Double-balanced, fully-differential preamp
• Switches (M7, M8) added to stop input propagation during regeneration
• Active pull-up PMOS added to the latch
M1 M2
Vi+
M5 M6
M9
Φ
Vo+
Vo-
Fully-diff. PA Latch
M3 M4
Vi-
VR+
VR-
M7 M8Φ
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FLASH ADC: ERRORS
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FLASH ADC ERRORS
… …
En
co
de
r
VFS Vi
Dout
2N-1
PA + Latch
…
Do 0
Vi
Δ
2Δ
5Δ
6Δ
7Δ
VFS
……
0
1
5
6
7
…
fs
Strobe• SHA-less
• Signal and clock propagation delay
• 2N-1 PAs + latches must be matched
• Synchronized clock strobe signal is critical
Going parallel is fast, but also gives rise to inherent problems…
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PREAMP INPUT COMMON MODE
Vi
…
VR1
VRj
PA 1
S1
PA j
Sj
PA j+1
… …
……
Input CM difference creates systematic mismatch (offset, gain, Cin, tracking BW, and CMRR) among preamps
j
R
1
R VV
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SAMPLING APERTURE ERROR
• Preamp delay and VTHN of sampling switch (M9) are both signal-dependent → signal-dependent sampling point (aperture error)
• A major challenge of distributing clock signals across 2N-1 comparators in flash ADC with minimum clock skew (routing, VTHN mismatch of M9, etc.)
M1 M2
Vin
M3 M4
Cgs1 Cgs2
CS
VR
RS
M5 M6
M8M7
M9
Φ
Vo+
Vo-
Cgd1 Cgd2Φ Mode
“high” Track
“low” Regenerate
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NONLINEAR INPUT CAPACITANCE
Vin Cin(Vout)
RS
Vout
t
Vin,
Vout
Signal-dependent input bandwidth (1/RSCin) introduces distortion
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INPUT SIGNAL FEEDTHROUGH
Feedthrough of Vin to the reference ladder through the serial connection of Cgs1
and Cgs2 disturbs the reference voltages
……
Vi
M1 M2
Vin
M3 M4
Cgs1 Cgs2
VRj
RS
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COMPARATOR METASTABILITY
• Cascade preamp stages (typical flash comparator has 2-3 PA stages)
• Use pipelined multi-stage latches; PA can be pipelined too
• Avoid branching off comparator logic outputs
LSB1
ΔBER
LmV2V1io /CgtexpAA0VtV
Vi
DoΔ
j
Vos
j+1 Assuming that the input is uniformly distributed over VFS, then
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COMPARATOR METASTABILITYVi
1
x
0
0
0
1 1 1
011
100
Logic levels can be misinterpreted by digital gates (branching off, diff. outputs)
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BUBBLES (SPARKLES) IN THERMOMETER CODE
Vi
0
0
1…
…
1
1
0
1 1 0
011
100
0010
Static/dynamic comparator errors cause bubbles in thermometer code
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BUBBLES (SPARKLES)
j+1
……
Vi
j
0
1
0
1
VRj
VRj+1
t
Vj
Vj+1
VRj
VRj+1
Δt
1 LSB
Comparator offset Timing error
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BUBBLE-TOLERANT BOUNDARY DETECTORVi
1
0
1
0
1
1
……
1
0
1
1
Ref: J. G. Peterson, “A monolithic video A/D converter,” IEEE Journal of Solid-State Circuits, vol. 14, pp. 932-937, issue 6, 1979.
• 3-input NAND
• Detect “011” instead of “01” only
• “Single” bubble correction
• Biased correction
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BUILT-IN BIAS
0
0
0
0
1
0
1
1
1
1
0
0
0
0
0
1
1
0
1
1
0
0
1
0
0
1
1
1
1
1
0
0
0
1
1
0
0
1
1
1
A CB D
1
2
3
Case“011”
Det.
“001”
Det.
A
B Fail
C Fail
D Fail Fail
Inspecting more neighboring comparator outputs improves performance
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MAJORITY VOTING LOGIC
Ref: C. W. Mangelsdorf, “A 400-MHz input flash converter with error correction,” IEEE Journal of Solid-State Circuits, vol. 25, pp. 184-191, issue 1, 1990.
0
0
0
0
0
1
1
1
1
1
0
0
0
0
0
1
1
1
1
1
0
0
0
0
0
1
1
1
1
1
0
0
0
1
1
0
0
1
1
1
Case“011”
Det.
Majority
voting
A
B Fail
C
D Fail Fail
0
0
0
0
1
0
1
1
1
1
0
0
0
0
0
1
1
0
1
1
0
0
1
0
0
1
1
1
1
1
0
0
0
1
1
0
0
1
1
1
A CB D
1
2
3
1j1j1jjj1j
*
j CCCCCCC
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GRAY ENCODING
43
622
75311
TG
TTG
TTTTG
0
0
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
0
0
0
1
0
1
1
0
0
1
1
0
0
1
1
1
1
1
1
1
0
0
0
0
0
1
1
1
0
0
0
0
1
1
1
1
0
0
0
1
1
1
1
1
0
0
0
0
0
0
1
1
0
0
1
1
1
1
0
0
0
1
0
1
0
1
0
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
Thermometer Gray Binary
G3 G2 G1 B3 B2 B1T1 T2 T3 T4 T5 T6 T7
• One comparator output is ONLY used once → No branching!
• Gray encoding fails benignly in the presence of bubbles
• Codes are also robust over metastability errors
Only one transitionb/t adjacent codes
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GRAY ENCODING
Conversion of Gray code to binary code is quite time-consuming → “quasi” Gray code
43
622
75311
TG
TTG
TTTTG
0
0
1
1
0
1
1
1
1
1
0
0
1
1
1
0
1
0
0
0
0
1
0
0
13
15
12
Thermometer Gray Decimal
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Ref: Y. Akazawa, et al., “A 400MSPS 8b flash AD conversion LSI,” in IEEE International Solid-State Circuits Conference, Dig. Tech. Papers, 1987, pp. 98-99.
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FLASH ADC: BINARY DECODERS
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GRAY ENCODED ROM DECODER
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WALLACE TREE ADDER (1S ADDER)
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FOLDED WALLACE TREE DECODER
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MUX-BASED DECODER
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COMPARISON OF DECODER SCHEMES
• Mostly custom design at higher speeds
• Pipeline the digital backend at high-sampling rates to meet clock timing
• Tpd<TCK-(tpcq+tsetup)
• Tcd>thold-tccq
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FLASH ADC: OTHER TECHNIQUES
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OFFSET AVERAGING (1)
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OFFSET AVERAGING (2)
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FLASH ADC WITH AVERAGING
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OFFSET CALIBRATION
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COMPARATOR WITH OFFSET CORRECTING DAC
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HIGH-PERFORMANCE FLASH WITH CALIBRATION (1)
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COMPARATOR REDUNDANCY (1)
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COMPARATOR REDUNDANCY (2)
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STOCHASTIC FLASH ADC
• Fully synthesized ADC using ‘digital’ comparator cells (large offsets)
• Use more than 1 comparator for a reference threshold
– Use ‘detection theory’ to make accurate decisions around a threshold, by using more than one observation
• Low speed designs (<20 MS/s)
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FLASH ADC: CASE STUDY
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CASE STUDY: DISTORTION COMPENSATING FLASH ADC
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T/H DISTORTION
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PRE-DISTORTED REFERENCES
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ADC ARCHITECTURE
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T/H CIRCUIT
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COMPARATOR
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NON-LINEAR REFERENCE GENERATOR
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ADC TESTING
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TEST RESULTS
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REFERENCES
1. Rudy van de Plassche, “CMOS Integrated Analog-to-Digital and Digital-to-Analog Converters,” 2nd Ed., Springer,
2005.
2. N. Krishnapura, Analog IC Design Lectures, IIT Madras, 2008.
3. Y. Chiu, Data Converters Lecture Slides, UT Dallas 2012.
4. B. Boser, Analog-Digital Interface Circuits Lecture Slides, UC Berkeley 2011.