Department of Electrical and Computer Engineering
Delta-Sigma Analog-to-Digital
Converters
Feedback DAC Design
Vishal Saxena, Boise State University
(vishalsaxena@boisestate.edu)
Aug 5, 2012
© Vishal Saxena
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Delta-Sigma ADC
Array of unit-weighted current steering DACs to pull and
push current from the opamp current summing node
Use high crossover pre-drivers to reduce DAC glitching
noise
Aug 5, 2012
© Vishal Saxena
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Feedback DAC Architecture
Array of unit-weighted current steering DACs to pull and
push current from the opamp current summing node
Use high crossover pre-drivers to reduce DAC glitching
noise
Aug 5, 2012
© Vishal Saxena
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Feedback DAC Nonlinearity
In a multibit DAC, element mismatch leads to nonlinearity
v is related to the input (u) by inverse non-linearity of the DAC
A single-bit DAC is always linear
Aug 5, 2012
© Vishal Saxena
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Feedback DAC Nonlinearity
Leads to distortion
intermodulation of quantization noise into the signal
band
Aug 5, 2012
© Vishal Saxena
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Dynamic Element Matching (DEM)
Randomize the DAC elements
Distortion components
converted to noise
Increased noise floor
Aug 5, 2012
© Vishal Saxena
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Data Weighted Averaging (DWA)
Cycle through all the current elements
15
as fast as possible
Accumulate the input code and move the10
pointer
5
First-order mismatch noise shaping
0
0
Thermometer-Coding
10
20
First-Order Shaping
15
10
5
0
0
Aug 5, 2012
© Vishal Saxena
10
20
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Data Weighted Averaging (DWA)
N=2B
fs
u(t)
vc(t)
v
L(s)
N
Thermometer
to Binary
B
B
Accumulator
DAC
Barrel
Shifter
N
Barrel shifter delay in the signal path increases loop delay
Not viable at higher sampling rates (>400 MHz)
Aug 5, 2012
© Vishal Saxena
-8-
DAC Calibration
Digital calibration – estimate element error and subtract from the
output
• Increased Decimation filter complexity
Analog calibration – calibrate the elements with respect to a master
• Need to calibrate at every cycle
Aug 5, 2012
© Vishal Saxena
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References
1. R. Schreier, Matlab Delta-Sigma Toolbox, 2009 [Online].
2. S. Pavan, N. Krishnapura, EE658: Data Conversion Circuits Course at
IIT Madras [Online].
Aug 5, 2012
© Vishal Saxena
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