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EE 4900: Fundamentals of Sensor Design
Lecture 13
Sensors and Digital Signal Processing (DSP)
Part 1: ADCs and DACs
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Digital Signal Processing (DSP): Basics
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Q: Why use DSP over Analog Signal Processing?
A:
1) Cost effective for complex signal processing techniques
2) Some techniques are impossible/difficult without DSP
3) Compact, more reliable and less sensitive to
environmental effects
Q: What is needed to be DSP designer/engineer?
A:
1)Knowledge of signal processing techniques
2)Knowledge of DSP microprocessors, C/Matlab
programming, A/D&D/A converters, filters, common
algorithms, sensors
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Digital Signal Processing (DSP): Basics
3
DSP Chain
Ideal
ADC
Discrete-Time
System
Ideal
DAC
Clock (T)
Clock (T)
Analog
Input
Sensor
Digital Input
from ADC
Signal
Conditioning
ADC
DSP
Clock (T)
Digital Output
to DAC
DAC
Analog
Output
Analog
Filter
Clock (T)
Ref: Applied Digital Signal Processing: Theory and Practice, by D. Manolakis and V. K. Ingle
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Digital Signal Processing (DSP): Basics
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DSP in a generic Sensor ASIC
Ref: Applied Digital Signal Processing: Theory and Practice, by D. Manolakis and V. K. Ingle
EE 4900 Fundamentals of Sensor Design
Suketu Naik
DSP: Basic Modules
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Analog to Digital Converter (A/D)
EE 4900 Fundamentals of Sensor Design
Suketu Naik
DSP: Basic Modules
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Analog to Digital Converter (A2D, A/D, ADC)
 Ranges from discrete circuits, monolithic ICs, modules, boxes
 ADC converts analog data such as voltage into digital data compatible
with DSP devices
 Characteristics: accuracy, linearity, resolution, conversion speed,
stability, price
 Speed and accuracy are inversely related
Example: Image Processing
Ref: Applied Digital Signal Processing: Theory and Practice, by D. Manolakis and V. K. Ingle
EE 4900 Fundamentals of Sensor Design
Suketu Naik
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A/D and D/A Basics
ADC Symbol
A/D: convert continuous-time signal into discrete-domain signal
D/A: convert discrete-time signal into continuous-time signal
EE 4900 Fundamentals of Sensor Design
Suketu Naik
DSP: Basic Modules
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Analog to Digital Converter (A2D, A/D, ADC)
Quantization Errors
EE 4900 Fundamentals of Sensor Design
Suketu Naik
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A/D Basics
Sample and Hold Amplifier (SHA)
1) The sample-and-hold amplifier captures an analog signal and holds it during
analog-digital conversion): SHA plays a major role in determining the Spurious
Free Dynamic Range (SFDR), Signal to Noise Ratio (SNR)
2) Input amplifier provides current gain to charge the cap, voltage on the cap
follows the input signal, the switch is opened, and the capacitor retains the voltage
present before it was disconnected from the input buffer, the output buffer offers
a high impedance to the hold capacitor to keep the held voltage from discharging
prematurely
Ref: http://www.analog.com/media/en/training-seminars/tutorials/MT-090.pdf
EE 4900 Fundamentals of Sensor Design
Suketu Naik
A/D Basics: SNR and SFDR
Signal to Noise Ratio (SNR)
Signal to Noise Ratio (SNR) is the
ratio of the value of the signal to
the rms value noise floor
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Spurious Free Dynamic Range (SFDR)
Spurious free dynamic range
(SFDR) is the ratio of the rms
value of the signal to the rms value
of the worst spurious signal
(regardless of where it falls in the
frequency spectrum)
EE 4900 Fundamentals of Sensor Design
Suketu Naik
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A/D Basics
Few of the ADC Parameters
1) Resolution: 8-bit, 12-bit, 16-bit, etc
2) Digital output code
-e.g. the largest output value that the 3-bit
A/D converter can produce is 7/8th of the full-scale
reference voltage, VREF
Ideal Transfer Function for
3-bit ADC
3) Code width
-This is range of analog input voltages for which the
code is produced: measured in weight of 1 LSb
-e.g. 1 LSb = VREF/2N, where N is the number of bits
For example, if VREF=4.096 V and N=12-bits
1 LSb will have a weight of 4.096 V/212 = 1 mV
4) Accuracy
-how close the actual digital output code represents useful information
-function of its internal circuitry and noise from external sources
Ref: http://www.analog.com/library/analogDialogue/archives/39-06/Chapter%202%20Sampled%20Data%20Systems%20F.pdf
http://ww1.microchip.com/downloads/en/appnotes/00693a.pdf
EE 4900 Fundamentals of Sensor Design
Suketu Naik
A/D Basics
Example: 4-bit ADC Output Table
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Ref: http://www.analog.com/library/analogDialogue/archives/39-06/Chapter%202%20Sampled%20Data%20Systems%20F.pdf
http://ww1.microchip.com/downloads/en/appnotes/00693a.pdf
EE 4900 Fundamentals of Sensor Design
Suketu Naik
A/D Basics
ADC selection: Bits vs Bandwidth
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Ref: http://www.ti.com/europe/downloads/Choose%20the%20right%20data%20converter%20for%20your%20application.pdf
EE 4900 Fundamentals of Sensor Design
Suketu Naik
DSP: Basic Modules
Types of A/D Converters
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1) Successive Approximation Register (SAR)
2) ΣΔ (Sigma-Delta)
3) Flash
4) Pipelined
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Successive Approximation Register (SAR) ADC
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SARs determine the digital word by
– Sampling the input signal
– Using an iterative process
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Successive Approximation Register (SAR) ADC
Application Areas (other than General Purpose)
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1) Multiple Channel Data Acquisition
2) Pen Digitizers
3) CMOS Image Sensors
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Successive Approximation Register (SAR) ADC
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 Successive-approximation register (SAR)
performs bit-weighing conversion
 Comparator compares the input
voltage to the output of an N-bit
DAC
 Using the DAC output as a reference
this process approaches the final result as a
sum of N weighting steps
 SHA keeps the signal constant during the
conversion cycle
 First, the comparator determines whether the SHA
output is greater or less than the DAC output, and MSB (1 or 0) is stored in SAR
 DAC is then set either to 1/4 scale or 3/4 scale by the control logic (depending on
the value of the MSB) and the comparator makes the decision for the second bit of
the conversion: the result (1 or 0) is stored in the register, and the process
continues...
 At the end of the conversion process, a logic signal (EOC, DRDY, BUSY, etc.) is
asserted
 As each bit is determined, it is latched into the SAR as part of the ADC's output
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Successive Approximation Register (SAR) ADC
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SAR Algorithm using binary scale
and binary weights
Ref: http://www.analog.com/library/analogdialogue/archives/39-06/architecture.html
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Successive Approximation Register (SAR) ADC
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Timing Diagram
SAR Conversion Process
EE 4900 Fundamentals of Sensor Design
Suketu Naik
DSP: Basic Modules
Types of A/D Converters
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1) Successive Approximation Register (SAR)
2) ΣΔ (Sigma-Delta)
3) Flash
4) Pipelined
EE 4900 Fundamentals of Sensor Design
Suketu Naik
ΣΔ (Sigma-Delta) ADC
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Delta-Sigma converters determine the digital word by
– Oversampling
– Applying Digital Filtering
EE 4900 Fundamentals of Sensor Design
Suketu Naik
ΣΔ (Sigma-Delta)ADC
Application Areas
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1) Process Control Systems
-Water supply & Sewage
-Oil & Gas
-Power
-Heating and Cooling
-Commuter Trains
2) Precision temperature measurements
3) Weighing scales
EE 4900 Fundamentals of Sensor Design
Suketu Naik
ΣΔ (Sigma-Delta) or Oversampling ADC
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Functional Block Diagram
 Σ-Δ is
modulator followed
by a digital decimation filter
 ΣΔ modulator: Difference
Amplifier, an integrator and a
comparator with a feedback loop
that contains a 1-bit DAC
 The internal DAC is simply
a switch that connects the
comparator input to a positive or
negative reference voltage VREF
 The Σ-Δ ADC also includes a
clock unit that provides proper timing for the modulator (oversampling at Kfs)
and digital filter (fs)
 Digital-and-decimation filter decimates the oversampled bit stream and reduces
the data rate
 The digital filter averages the 1-bit data stream, improves the ADC resolution,
and removes quantization noise
Ref: Demystifying Delta-Sigma ADCs, https://www.maximintegrated.com/en/app-notes/index.mvp/id/1870
EE 4900 Fundamentals of Sensor Design
Suketu Naik
ΣΔ (Sigma-Delta) or Oversampling ADC
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Oversampling, Noise Shaping, Digital Filtering, and Decimation
Ref: Choose the right A/D converter for your application, Texas Instruments
http://www.ti.com/europe/downloads/Choose%20the%20right%20data%20converter%20for%20your%20application.pdf
EE 4900 Fundamentals of Sensor Design
Suketu Naik
ΣΔ (Sigma-Delta) or Oversampling ADC
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Oversampling and Noise Shaping
More '1's created
during the
positive cycle
More '0's created
during the
negative cycle
Ref: Choose the right A/D converter for your application, Texas Instruments
http://www.ti.com/europe/downloads/Choose%20the%20right%20data%20converter%20for%20your%20application.pdf
EE 4900 Fundamentals of Sensor Design
Suketu Naik
ΣΔ (Sigma-Delta) or Oversampling ADC
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Digital Filtering
High Frequency Noise Reduction
Sinc 3 Filter
Ref: Choose the right A/D converter for your application, Texas Instruments
http://www.ti.com/europe/downloads/Choose%20the%20right%20data%20converter%20for%20your%20application.pdf
EE 4900 Fundamentals of Sensor Design
Suketu Naik
ΣΔ (Sigma-Delta) or Oversampling ADC
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Decimation
Ref: Choose the right A/D converter for your application, Texas Instruments
http://www.ti.com/europe/downloads/Choose%20the%20right%20data%20converter%20for%20your%20application.pdf
EE 4900 Fundamentals of Sensor Design
Suketu Naik
DSP: Basic Modules
Types of A/D Converters
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1) Successive Approximation Register (SAR)
2) ΣΔ (Sigma-Delta)
3) Flash
4) Pipelined
EE 4900 Fundamentals of Sensor Design
Suketu Naik
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Flash (Parallel) ADC
Application Areas (requiring very large bandwidths)
1) Data acquisition systems
- typically contain signal conditioning circuitry, analog-todigital converter (ADC), computer bus, DACs), digital I/O
lines, counter/timers
2) Satellite communication
3) Radar processing
4) Sampling oscilloscopes
EE 4900 Fundamentals of Sensor Design
Suketu Naik
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Flash (Parallel) ADC
Flash ADC Architecture
 Fastest way to convert an analog signal
to a digital signal
 Flash ADCs are made by cascading
high-speed, low gain (trade-off between
bandwidth and gain) comparators
 N-bit Flash ADC has 2N-1 comparators
connected in parallel, with reference voltages
set by a resistor network and spaced
VFS/2N (1-LSB) apart
 A change of input voltage usually causes
a change of state in more than one comparator
output
 Output changes are combined in a
decoder-logic unit that produces a
parallel N-bit output from the converter.
 Upto 1 GHz sampling rate, low resolution,
large die size, excessive input capacitance and power consumption
 Very precise matching required, prone to sporadic and erratic outputs
Ref: Understading Flash ADCs, https://www.maximintegrated.com/en/app-notes/index.mvp/id/810
EE 4900 Fundamentals of Sensor Design
Suketu Naik
DSP: Basic Modules
Types of A/D Converters
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1) Successive Approximation Register (SAR)
2) ΣΔ (Sigma-Delta)
3) Flash
4) Pipelined
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Pipeline ADC
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Pipeline converters determine digital word by
– Undersampling
– Multiple stages / Larger Cycle-latency
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Pipelined (Subranging Quantizers) ADC
Application Areas
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1) Communication systems:
-total harmonic distortion (THD),
-spurious-free dynamic range (SFDR)
2) CMOS Image Sensor/CCD-based imaging systems
- noise, bandwidth, and fast transient response
3) Data-acquisition systems
-time and frequency-domain characteristics are both
important
-low spurs and high input bandwidth
EE 4900 Fundamentals of Sensor Design
Suketu Naik
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Pipelined (Subranging Quantizers) ADC
 Provides an optimum balance of size, speed, resolution, power
dissipations
 Consists of numerous consecutive stages, each containing a
track/hold (T/H), a low-resolution ADC and DAC, and a summing
circuit that includes an interstage amplifier to provide gain
Ref: Understading Pipelined ADCs, https://www.maximintegrated.com/en/app-notes/index.mvp/id/1023
EE 4900 Fundamentals of Sensor Design
Suketu Naik
References
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Recommended ADCs
 Choose the right A/D converter for your application
http://www.ti.com/europe/downloads/Choose%20the%20right%20data%20converter%20f
or%20your%20application.pdf
 Texas Instruments, Analog Devices, Maxim Integrated
 Low power acquisition (SAR)
 ADS7042 (12-bit, 1 msps)
 MAX1116 (8-bit, 100 ksps)
 Pressure and Temperature Sensing (delta sigma)
 MAX11270 (24-bit, 2 ksps)
 ADS1146 (16-bit, 64 ksps)
 AD7797 (24-bit, 123 sps)
EE 4900 Fundamentals of Sensor Design
Suketu Naik
DSP: Basic Modules
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Analog to Digital Converter (A2D, A/D, ADC) Alternatives
1) Pulse Width Modulator (PWM)
2) Voltage-to-Frequency (V/F) Converter
3) Direct Digitization
4) Resistance to Frequency Converter
EE 4900 Fundamentals of Sensor Design
Suketu Naik
DSP: Basic Modules
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Digital to Analog Converter (D/A)
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Which Type of DAC to choose?
EE 4900 Fundamentals of Sensor Design
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Suketu Naik
Digital to Analog Converter (DAC)
Types of D/A Converters
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1) R-2R Ladder
2) ΣΔ (Sigma-Delta)
3) Multiplying
4) Current Steering
EE 4900 Fundamentals of Sensor Design
Suketu Naik
R-2R DAC
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Application Areas
1) Automatic Test Equipment
2) Precision Instrumentation
3) Industrial Control Systems
EE 4900 Fundamentals of Sensor Design
Suketu Naik
R-2R Ladder DAC
DAC=Resistors and Summing Amp
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Problem: Matching
For large number of bits, summing resistors need to be very precise
Solution: R-2R Ladder
EE 4900 Fundamentals of Sensor Design
Suketu Naik
R-2R Ladder DAC
DAC=Resistors and Summing Amp
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D's = digital data
The digital inputs can be control signals
for switches that connect the resistors to
Vref (high or logical 1) or ground (low or
logical 0)
EE 4900 Fundamentals of Sensor Design
Suketu Naik
R-2R Ladder DAC
R-2R DAC is the classical and most common type of DAC
Voltage Mode
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Current Mode
Voltage Mode
The arms of the ladder are switched between VREF and ground, and the output is
taken from the end of the ladder
Current Mode
The end of the ladder, with its code-independent impedance, is used as the VREF
terminal and the ends of the arms are switched between ground
The output line is held at ground and the network is followed by a current-tovoltage (I/V) converter
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Digital to Analog Converter (DAC)
Types of D/A Converters
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1) R-2R Ladder
2) ΣΔ (Sigma-Delta)
3) Multiplying
4) Current Steering
EE 4900 Fundamentals of Sensor Design
Suketu Naik
ΣΔ (Sigma-Delta) DAC
Application Areas
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1) Audio (USB DAC)
2) Motion Control Systems
-More Resolution
-Less Accuracy
3) Sonar Electronics
EE 4900 Fundamentals of Sensor Design
Suketu Naik
ΣΔ (Sigma-Delta) DAC
Reverse of ΣΔ ADC
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Oversampling of the digital input, digital filtering, demodulation,
analog filtering
EE 4900 Fundamentals of Sensor Design
Suketu Naik
References
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Recommended DACs
 How to select a precision DAC?
http://www.ti.com/general/docs/video/watch.tsp?entryid=0_l2147gog
 Texas Instruments (TI), Analog Devices (AD), Maxim Integrated
(MI)
 Delta-Sigma DACs
 DAC1220 (TI, 20-bit, 2.5 mW, low power)
 AD1833A (AD, 24-bit, 192 kHz, audio)
 MAX5134 (MI, 16-bit, highly-linear, process control)
EE 4900 Fundamentals of Sensor Design
Suketu Naik