ADC

Report
Analog-to-Digital Converter (ADC)
Introduction to Mechatronics
Fall 2012
Craig Woodin
Ali AlSaibie
Ehsan Maleki
Background Information
What is ADC?
Conversion Process
Accuracy
Examples of ADC applications
Presenter: Craig Woodin
Signal Types
Analog Signals
 Any continuous signal that a
time varying variable of the
signal is a representation of
some other time varying
quantity
 Measures one quantity in
terms of some other quantity
 Examples
• Speedometer needle as
function of speed
• Radio volume as function of
knob movement
t
Signal Types
Digital Signals
 Consist of only two states
 Binary States
 On and off
 Computers can only
perform processing on
digitized signals
1
0
Analog-Digital Converter (ADC)
An electronic integrated circuit which converts
a signal from analog (continuous) to digital
(discrete) form
Provides a link between the analog world of
transducers and the digital world of signal
processing and data handling
Analog-Digital Converter (ADC)
An electronic integrated circuit which converts
a signal from analog (continuous) to digital
(discrete) form
Provides a link between the analog world of
transducers and the digital world of signal
processing and data handling
Analog-Digital Converter (ADC)
An electronic integrated circuit which converts
a signal from analog (continuous) to digital
(discrete) form
Provides a link between the analog world of
transducers and the digital world of signal
processing and data handling
ADC Conversion Process
Two main steps of process
1. Sampling and Holding
2. Quantization and Encoding
Analog-to-Digital Converter
Quantizing
and
Encoding
Sampling and
Hold
t
Input: Analog Signal
t
ADC Process
Sampling & Hold
 Measuring analog signals
at uniform time intervals
 Ideally twice as fast as
what we are sampling
Continuous Signal
 Digital system works with
discrete states
 Taking samples from each
location
 Reflects sampled and
hold signal
 Digital approximation
t
ADC Process
Sampling & Hold
 Measuring analog signals
at uniform time intervals
 Ideally twice as fast as
what we are sampling
 Digital system works with
discrete states
 Taking samples from each
location
 Reflects sampled and
hold signal
 Digital approximation
t
ADC Process
Sampling & Hold
 Measuring analog signals
at uniform time intervals
 Ideally twice as fast as
what we are sampling
 Digital system works with
discrete states
 Taking a sample from each
location
 Reflects sampled and
hold signal
 Digital approximation
t
ADC Process
Sampling & Hold
 Measuring analog signals
at uniform time intervals
 Ideally twice as fast as
what we are sampling
 Digital system works with
discrete states
 Taking samples from each
location
 Reflects sampled and
hold signal
 Digital approximation
t
ADC Process
Quantizing
 Separating the input signal
into a discrete states with K
increments
 K=2N
 N is the number of bits of the
ADC
 Analog quantization size
 Q=(Vmax-Vmin)/2N
 Q is the Resolution
Encoding
 Assigning a unique
digital code to each
state for input into the
microprocessor
ADC Process
Quantization & Coding
 Use original analog
signal
ADC Process
Quantization & Coding
 Use original analog
signal
 Apply 2 bit coding
11
10
01
00
K=22
00
01
10
11
ADC Process
Quantization & Coding
 Use original analog
signal
 Apply 2 bit coding
11
10
01
00
K=22
00
01
10
11
ADC Process
Quantization & Coding
 Use original analog
signal
 Apply 3 bit coding
K=23
000
001
010
011
100
101
110
111
ADC Process
Quantization & Coding
 Use original analog
signal
 Apply 3 bit coding
 Better representation of
input information with
additional bits
 MCS12 has max of 10
bits
K=23
000
001
010
011
100
101
110
111
K=16
0000
.
.
.
1111
K=…
ADC Process-Accuracy
The accuracy of an ADC can be improved by increasing:
Sampling Rate, Ts
 Based on number of steps
required in the conversion
process
 Increases the maximum
frequency that can be
measured
t
Resolution, Q
 Improves accuracy in
measuring amplitude of
analog signal
 Limited by the signal-tonoise ratio (~6dB)
t
ADC Process-Accuracy
The accuracy of an ADC can be improved by increasing:
Sampling Rate, Ts
 Based on number of steps
required in the conversion
process
 Increases the maximum
frequency that can be
measured
t
Resolution (bit depth), Q
 Improves accuracy in
measuring amplitude of
analog signal
t
ADC-Error Possibilities
 Aliasing (sampling)
 Occurs when the input signal is changing much faster
than the sample rate
 Should follow the Nyquist Rule when sampling
• Answers question of what sample rate is required
• Use a sampling frequency at least twice as high as the
maximum frequency in the signal to avoid aliasing
• fsample>2*fsignal
 Quantization Error (resolution)
 Optimize resolution
 Dependent on ADC converter of microcontoller
ADC Applications
ADC are used virtually everywhere where an
analog signal has to be processed, stored, or
transported in digital form




Microphones
Strain Gages
Thermocouple
Digital Multimeters
Types of ADC
Successive Approximation A/D Converter
Flash A/D Converter
Dual Slope A/D Converter
Delta-Sigma A/D Converter
Presenter: Ali AlSaibie
Successive Approximation ADC
 Elements
•
•
•
•
•
•
•
DAC = Digital to Analog Converter
EOC = End of Conversion
SAR = Successive Approximation Register
S/H = Sample and Hold Circuit
Vin = Input Voltage
Comparator
Vref = Reference Voltage
Successive Approximation ADC
 Algorithm
•
•
•
•
•
•
•
•
Uses an n-bit DAC and original analog results
Performs a binary comparison of VDAC and Vin
MSB is initialized at 1 for DAC
If Vin < VDAC (VREF / 2^n=1) then MSB is reset to 0
If Vin > VDAC (VREF / 2^n) Successive Bits set to 1 otherwise 0
Algorithm is repeated up to LSB
At end DAC in = ADC out
N-bit conversion requires N comparison cycles
Successive Approximation ADC Example DAC bit/voltage
 5-bit ADC, Vin=0.6V, Vref=1V
 Cycle 1 => MSB=1
SAR = 1 0 0 0 0
VDAC = Vref/2^1 = .5
Bit
4
3
Voltage
.5 .25
2
1
0
.125
.0625
.03125
Vin > VDAC SAR unchanged = 1 0 0 0 0
 Cycle 2
SAR = 1 1 0 0 0
VDAC = .5 +.25 = .75
Vin < VDAC SAR bit3 reset to 0 = 1 0 0 0 0
 Cycle 3
SAR = 1 0 1 0 0
VDAC = .5 + .125 = .625
Vin < VDAC SAR bit2 reset to 0 = 1 0 0 0 0
 Cycle 4
SAR = 1 0 0 1 0
VDAC = .5+.0625=.5625
Vin > VDAC SAR unchanged = 1 0 0 1 0
 Cycle 5
SAR = 1 0 0 1 1
VDAC = .5+.0625+.03125= .59375
Vin > VDAC SAR unchanged = 1 0 0 1 1
Flash ADC
 Also known as parallel ADC
 Elements
• Encoder – Converts output
of comparators to binary
• Comparators
Flash ADC
 Algorithm
 Vin value lies between two comparators
 Resolution ∆ =

2
;
 N= Encoder Output bits
 Comparators => 2N-1
 Example: Vref 8V, Encoder 3-bit
• Resolution ∆ =
8
23
= 1.0V
• Comparators 23-1=7
 1 additional encoder bit -> 2 x # Comparators
Flash ADC Example
Vin = 5.5V, Vref= 8V
0
Vin lies in between Vcomp5 & Vcomp6
Vcomp5 = Vref*5/8 = 5V
Vcomp6 = Vref*6/8 = 6V
0
1
1
Comparator 1 - 5 => output 1
Comparator 6 - 7 => output 0
1
1
Encoder Octal Input = sum(0011111) = 5
Encoder Binary Output = 1 0 1
5.5V
1
Dual Slope A/D Converter
Also known as an Integrating ADC
+
_
Start
Clock
Control
Logic
Counter
Stop
Dual-Slope ADC – How It Works
 An unknown input voltage is applied to the input of the integrator and allowed to
ramp for a fixed time period (tu)
 Then, a known reference voltage of opposite polarity is applied to the integrator
and is allowed to ramp until the integrator output returns to zero (td)
 The input voltage is computed as a function of the reference voltage, the constant
run-up time period, and the measured run-down time period
 The run-down time measurement is usually made in units of the converter's clock,
so longer integration times allow for higher resolutions
 The speed of the converter can be improved by sacrificing resolution
Vin  Vref
td
tu
Delta-Sigma A/D Converter
Analog
Input
Delta-Sigma
Modulator
Low-Pass
Filter
Digital
Output
Delta-Sigma ADC – How It Works
 Input over sampled, goes to integrator
 Integration compared with ground
 Iteration drives integration of error to zero
 Output is a stream of serial bits
Comparison of ADC’s
Type
Speed
(relative)
Cost
(relative)
Resolution
(bits)
Dual Slope
Slow
Med
12-16
Flash
Very Fast
High
4-12
Successive
Approx
Medium –
Fast
Low
8-16
Sigma – Delta
Slow
Low
12-24
ADC Subsystem of MC9S12C32
Input Pins
ADC Built-into
MC9S12C32
Presenter: Ehsan Maleki
ADC - Schematic Diagram
ATD
Port AD
ATD 10B8C - Block Diagram
High/Low
Ref Voltage
Power
Supplies
Analog Input
General Purpose I/O
External Trigger
Analog Input
General Purpose I/O
ATD 10B8C – Key Features
Resolution: 8/10 bits
Conversion time: 7 μsec (10 bit)
8-channel multiplexed inputs
Successive Approximation ADC
External trigger control
Conversion Modes:
 Single or continuous conversion
 Single channel or multiple channels
Operating Modes
Modes:
 Stop Mode: All clocks halt; conversion aborts; minimum
recovery delay (~ 20μs)
 Wait Mode: Reduced MCU power; can resume
 Freeze Mode: Breakpoint for debugging an application
Registers
MC9S12C Family Reference Manual: Ch. 8
REGISTERS






6 Control Registers (first 2 are reserved!)
2 Status Registers
2 Test Registers
1 Digital Input Enable Register
1 Digital Port Data Register
8 Result Registers
Control Register (2)
This register controls power down, interrupt, and external
trigger.
Writes to this register will abort current conversion sequence
but will not start a new sequence.
ATD
Power
External Trigger
(Tab. 8-2)
Interrupt
Enable
Control Register (3)
This register controls the conversion sequence length, FIFO for
results registers and behavior in Freeze Mode.
Writes to this register will abort current conversion sequence
but will not start a new sequence.
Conversion
Sequence length
(Tab. 8-4)
Background Debug
Freeze Enable
(Tab. 8-5)
Control Register (4)
This register selects the conversion clock frequency, the length
of the second phase of the sample time and the resolution of
the A/D conversion (i.e.: 8-bits or 10-bits).
Writes to this register will abort current conversion sequence
but will not start a new sequence.
Resolution
(0=10 bit)
Clock Prescaler
(Default=5)
(Tab. 8-8)
Control Register (5)
This register selects the type of conversion sequence and the
analog input channels sampled.
Writes to this register will abort current conversion sequence
and start a new conversion sequence.
Single (0) / Continuous (1) Analog Input Channel Select
(Tab. 8-12)
Conversion Mode
Result Register
Data Justification
RRD Unsigned (0)
/ Signed (1)
(Tab. 8-10/11)
Single (0) / Multi (1)
Channel Mode
Status Register (0)
This read-only register contains the sequence complete flag,
overrun flags for external trigger and FIFO mode, and the
conversion counter.
Sequence
Complete Flag
Conversion
Counter
Status Register (1)
This read-only register contains the Conversion Complete Flags.
Test Registers
Reserved
This register contains the SC bit used to enable special channel conversions.
Port Data Register
The data port associated with the ATD is general purpose I/O.
Digital Input Enable Register
This bit controls the digital input buffer from the analog input
pin to PTADx data register.
Results Registers – Left Justified
Results Registers – Right Justified
Setting Up & Starting the ADC
 Step 1: Power up ATD and define settings in ATDCTL2
 ADPU = 1 (power up the ATD)
 ASCIE = 1 (enables interrupt, if needed)
 Step 2: Wait for ATD recovery time (~ 20μs)
 Step 3: Set up # of conversions in ATDCTL3
 Step 4: Configure resolution, sampling time, and ATD
clock speed in ATDCTL4
 Step 5: Configure starting channel, single/multiple
channel, single or continuous sequence, and result
data format in ATDCTL5
QUESTIONS?
Appendix
Table 8-2
BACK
Tables 8-4 & 8-5
BACK
Table 8-8
Table 8-10
Table 8-11
Table 8-12
References
 http://en.wikipedia.org/wiki/Analog-to-digital_converter
 http://www.grin.com/object/external_document.259394/fb1fe2e3b955672eca34
58c9116d595b_LARGE.png
 http://en.wikipedia.org/wiki/Successive_approximation_ADC
 http://www.maximintegrated.com/app-notes/index.mvp/id/810
 http://en.wikipedia.org/wiki/Delta-sigma_modulation
 http://www.beis.de/Elektronik/DeltaSigma/DeltaSigma.html
 http://www.allaboutcircuits.com/vol_4/chpt_13/9.html
 http://en.wikipedia.org/wiki/Integrating_ADC
 MC9S12C Family Reference Manual

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