EECT 7327 Fall 2014 Successive Approximation SA ADC Successive Approximation ADC 2 Data Converters Successive Approximation ADC Professor Y Chiu EECT 7327 Fall 2014 Binary search algorithm N ID: 675038
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Slide1
– 1 –
Data Converters Successive Approximation ADC Professor Y. ChiuEECT 7327 Fall 2014
Successive Approximation
(SA) ADCSlide2
Successive Approximation ADC
– 2 –Data Converters Successive Approximation ADC Professor Y. Chiu
EECT 7327
Fall 2014
Binary search algorithm → N*
T
clk to complete N bitsConversion speed is limited by comparator, DAC, and digital logic (successive approximation register or SAR)Slide3
Binary Search Algorithm–
3 –Data Converters Successive Approximation ADC Professor Y. Chiu
EECT 7327
Fall 2014
DAC output gradually approaches the input voltage
Comparator differential input gradually approaches zeroSlide4
Charge Redistribution SA ADC
– 4 –Data Converters Successive Approximation ADC Professor Y. Chiu
EECT 7327
Fall 2014
4-bit binary-weighted capacitor array DAC
Capacitor array samples input when
Φ1 is asserted (bottom-plate)Slide5
Charge Redistribution (MSB)
– 5 –Data Converters Successive Approximation ADC Professor Y. Chiu
EECT 7327
Fall 2014Slide6
Comparison (MSB)–
6 –Data Converters Successive Approximation ADC Professor Y. Chiu
EECT 7327
Fall 2014
If V
X
< 0, then Vi > VR/2, and MSB = 1, C4
remains connected to V
R
If V
X
> 0, then V
i
< V
R
/2, and MSB = 0, C
4
is switched to groundSlide7
Charge Redistribution (MSB-1)
– 7 –Data Converters Successive Approximation ADC Professor Y. Chiu
EECT 7327
Fall 2014Slide8
Comparison (MSB-1)–
8 –Data Converters Successive Approximation ADC Professor Y. Chiu
EECT 7327
Fall 2014
If V
X
< 0, then Vi > 3VR/4, and MSB-1 = 1, C3
remains connected to V
R
If V
X
> 0, then V
i
< 3V
R
/4, and MSB-1 = 0, C
3
is switched to groundSlide9
Charge Redistribution (Other Bits)
– 9 –Data Converters Successive Approximation ADC Professor Y. Chiu
EECT 7327
Fall 2014
Test completes when all four bits are determined w/ four charge redistributions and comparisonsSlide10
After Four Clock Cycles…
– 10 –Data Converters Successive Approximation ADC Professor Y. Chiu
EECT 7327
Fall 2014
Usually, half T
clk
is allocated for charge redistribution and half for comparison + digital logicVX always converges to 0 (V
os
if comparator has nonzero offset)Slide11
Summing-Node Parasitics
– 11 –
Data Converters Successive Approximation ADC Professor Y. Chiu
EECT 7327
Fall 2014
If
Vos = 0, CP has no effect eventually; otherwise, C
P
attenuates V
X
Auto-zeroing can be applied to the comparator to reduce offsetSlide12
Summary of SA ADC–
12 –Data Converters Successive Approximation ADC Professor Y. Chiu
EECT 7327
Fall 2014
Power efficiency – only comparator consumes DC power
DAC nonlinearity limits the INL and DNL of the SA ADC
N-bit precision requires N-bit matching from the cap arrayCalibration can be performed to remove mismatch errors (Lee, JSSC’84)
Comparator offset
V
os
introduces an input-referred offset
~ (1+
C
P
/
Σ
C
j
)*
V
os
C
P
in general has little effect on the conversion (V
X
→
0 at the end of the search); however, V
X
is always attenuated due to charge sharing of CPBinary search is sensitive to intermediate errors made during search – if an intermediate decision is wrong, the digitization process cannot recoverDAC must settle into ±
½ LSB bound within the time allowedComparator offset must be constant (no hysteresis or time-dependent offset)Non-binary search algorithm can be used (
Kuttner
, ISSCC’02
)Slide13
References
– 13 –
Data Converters Successive Approximation ADC Professor Y. Chiu
EECT 7327
Fall 2014
J. L. McCreary and P. R. Gray, JSSC, pp. 371-379, issue 6, 1975.
R. E. Suarez, P. R. Gray, and D. A. Hodges, JSSC, pp. 379-385, issue 6, 1975.H.-S. Lee, D. A. Hodges, and P. R. Gray, JSSC, pp. 813-819, issue 6, 1984.M. de Wit, K.-S. Tan, and R. K. Hester, JSSC, pp. 455-461, issue 4, 1993.C. M. Hammerschmied and H. Qiuting, JSSC, pp. 1148-1157, issue 8, 1998.
S.
Mortezapour
and E. K. F. Lee, JSSC, pp. 642-646, issue 4, 2000.
G.
Promitzer
, JSSC, pp. 1138-1143, issue 7, 2001.
F.
Kuttner
, ISSCC 2002, pp. 176-177.
S. M. Chen and R. W.
Brodersen
, JSSC, pp. 2350-2359, issue 2, 2006.
N.
Verma
and A.
Chandrakasan
, ISSCC 2006, pp. 222-223.
G. Van der
Plas
et al., ISSCC 2008, pp. 242-243.
M. van
Elzakker
et al., ISSCC 2008, pp. 244-245.
W.
Liu, P. Huang, and Y. Chiu , JSSC, pp
. 2661-2672,
issue 11,
2011.