ADC STUDENT
LECTURE
Andrew Brown
Jonathan Warner
Laura Strickland
Table of Contents
• Signals
• Applications of ADC’s
• Types of ADC’s
• Successive Approximation Example
• The ADC on the MC9S12C32
Introduction
• Analog to digital converters convert analog, or “real world”
signals to a series of 1’s and 0’s, able to be stored or
transmitted through computers or digital systems.
Introduction cont.
• Reasons why this would be needed:
• Digital storage of a non-digital signal
• (ex: recording light intensity of a lightning strike using sensors, mapping
a flight path of an aircraft onto a computer for analysis)
• Transmitting data over a digital system
• (ex: sending your voice through a telephone system, Skype chatting,
etc…)
Analog Signals
• Analog signals are the smooth, “real”, signals of the world.
• These signals can contain any and all values needed to
represent the data in question.
Digital Signals
• Digital signals, however, contain series of discrete values,
with interpolation occurring between data points to
recreate the signal.
• Digital signals are meant to be used in digital systems,
and therefore are composed simply of 0’s and 1’s.
Benefits of Digital Signals over Analog
• Can be stored in digital system.
• Can be compressed.
• Can filter out frequencies you don’t want, analog noise is
removed.
How does it work?
• ADC’s work in two steps:
• Sampling
• Quantization
Sampling
• Let’s look again at our last graph:
• Our discrete values on the y axis are taken at spaced-out
time steps on the x axis.
• These are the “sampling points”.
Sampling cont.
• Larger number of
sampling points during
the same amount of
time = smoother
looking graph.
• “Sampling rate” is this
frequency at which
sampling will occur.
• Nyquist Theorem:
Sampling rate should
be 2*highest frequency
you want to capture.
Sampling Question
• If you use a sampling rate of 50,000 Hz for 2 seconds,
how many data points are you capturing?
• What is the distance between each point on the resulting
graph of Voltage vs. time?
Quantization
• “Sampling for y axis”
• Assigning a binary
code value to discrete
measurements, stored
on a fixed-length
variable.
Quantization Noise
• Since values are rounded to the nearest possible digital
value, a certain level of “quantization noise” will occur.
• Example: In an 8-bit resolution system, a value of 236.4
will be stored as the digital value 236.
• Signal to noise ratio measures the noise level by the
equation:
• SNR = 6.02*n + 1.761 dB, for n-bit resolution
Disadvantages of Digital
Signals
• Not a perfect representation
Example: Phone
systems use a sampling
rate of 8kHz, so all
frequencies above 4kHz
are canceled. As a
result, playing music
through a phone sounds
muffled and low quality.
of the analog signal
• Low memory systems give
you bad quality output, as
resolution or sampling may
be low
Aliasing
• Aliasing occurs when a signals frequency is above the
Nyquist Frequency.
• The data points captured suggest a lower frequency
signal than the one that actually exists.
ADC APPLICATIONS
Sound recording
• ADC’s are used to convert sound waves into digital
signals through the use of computer microphones or
sensors.
• This allows digital storage and transmission of music,
voice, and other sound data.
• Ex: Telephones convert your voice using 8kHz sampling.
Sensors and Data Acquisition
• Digital sensors output an analog voltage when reading
data.
• Examples:
• light sensors
• pressure sensors
• accelerometers
• Computers store this data by converting the signal to
digital values, used later by computers.
Digital Cameras
• Photo-sensors on
cameras convert
photon impacts into
voltage outputs.
• These are then
converted to digital
values and stored on
your camera’s memory
card to be recreated
later on a computer.
Circuit representation of ADC
• The general representation of an ADC is shown below.
• But what is inside the ADC block? How is the data
recorded and stored?
TYPES OF
ANALOG TO DIGITAL
CONVERTERS
Jonathan Warner
Overview
1. Parallel Design (Flash)
2. Successive Approximation
3. Dual-Slope
4. Sigma-Delta
Parallel Design (Flash ADC)
• Vref set to Vmax
• Resistors used to divide reference
voltage into intervals
• Comparators used to compare Vin to
the reference voltages
• Encoder uses logic gates to convert
control logic to binary digital output
2^n-1 comparators
Parallel Design (Flash ADC)
Advantages
• Fastest ADC
(gigahertz)
• Simple Design
• Can achieve
non-linear output
•
•
•
•
Disadvantages
2^n-1 comparers
Low resolution
Large Die size
Prone to glitches
(out of sequence
output)
Successive Approximation
DAC-Based Design
• Starts by setting MSB D(n-1)
to 1
• Uses DAC and op amp to
determine if bit should
remain 1 or be set to zero
(greater or less than
Vres * 2^(n-1))
• Next, bit D(n-2) set to 1 and
comparison is repeated
Output Buffer allows the circuit
to read the digital data while the
ADC is working on the next
sample
Successive Approximation
DAC-Based Design
Advantages
Disadvantages
• Speed, worst case
• Resolution tradeoff
n clock cycles
with speed
• Conversion time
independent of
amplitude of Vin
• Capable of outputting
the binary number in
serial (one bit at a time)
format.
Dual-Slope
Integrator-Based Design
• Switch connects Vin with
•
•
•
•
integrator
Switch held for fixed
number of clock cycles
Analog switched at set time
to –Vref
T2 clock cycles
proportional to Vin
Vin = Vref x T2/T1
Dual-Slope
Integrator-Based Design
Dual-Slope
Integrator-Based Design
Advantages
• Insensitive to
components value
errors
• Can achieve high
resolution (but at the
cost of speed)
• Useful for highly
accurate
measurements
Disadvantages
• Speed, 2^n-1 clock
cycles
• Limited applications
Sigma-Delta
• Analog signal set to
integrator
• Resulting “sawtooth”
waveform compared with
zero volts
• Output either high or low
• Output converted to positive or
negative Vres and fed back to be
added to next sample’s Vin
• Resulting stream of 0’s and 1’s
represents the analog signal
average voltage
Clock rate used is very high,
results in “oversampling” of data
Sigma-Delta
Advantages
• High Resolution
• No need for precision
components
Disadvantages
• Speed,
Oversampling
• Only applicable for
low bandwidth
ADC Comparison
Type
Speed
(relative)
Cost
(relative)
Resolution
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
Successive Approximation
Example
Given:
8 bit ADC
Vin = 0.2 V
Vref = 2 V
Find:
n bit digital
output
2n = 28 = 256
Vres = Vref / 256
Vres = 0.0078125 V (Resolution)
Bit
Voltage
7
1
6
0.5
5
0.25
4
0.125
3
0.0625
2
0.03125
1
0.015625
0
0.0078125
Successive Approximation
Example (cont.)
1
0
0
0
0
0
0
0
0.4 < 1
0
1
0
0
0
0
0
0
0.4 < 0.5
0
0
1
0
0
0
0
0
0.4 > 0.25
0
0
1
1
0
0
0
0
0.4 > 0.375
0
0
1
1
1
0
0
0
0.4 <0.4375
0
0
1
1
0
1
0
0
0.4 <0.4063
0
0
1
1
0
0
1
0
0.4 > 0.39
0
0
1
1
0
0
1
1
0.4 > 0.398
0
0
1
1
0
0
1
1
Digital
Output
The ATD10B8C on the MC9S12C32
Input Pins
ATD10B8C
MC9S12C32 Block Diagram
The Basics of the ATD10B8C
• Resolution: 8- or 10-bit (manually chosen)
• 8-channel multiplexed inputs
• Successive Approximation architecture
• Can perform single or continuous sampling
• Can sample single or multiple channels
• Conversion time: 7 µs (in 10-bit mode)
• Optional external trigger
ATD10B8C Block Diagram
Pin
Purposes
AN7/
ETRIG/
PAD7
Analog input channel
7/
External trigger for
ADC/
General purpose
digital I/O
AN6/PAD6 –
AN0/PAD0
Analog input/
General purpose
digital I/O
VRH, VRL
High, low reference
voltages
VDDA, VSSA
Supply power for analog
circuitry
Control Register 2
Pin
Description
7
0 – Power down ATD; has recovery time period
1 – Normal ATD functionality
6
0 – Normal clearing (read CCF before reading result register)
1 – Fast Flag Clearing (auto-clear CCF after result register is
accessed)
5
0 – Continue running in Wait Mode
1 – Halt conversion and power ATD down while in Wait Mode
4
0 – External Trigger Edge
1 – Trigger Level
3
0 – Low/falling trigger polarity
1 – High/rising trigger polarity
2
0 – Disable external trigger mode
1 – Enable external trigger mode
1
0 – ATD Sequence Complete Interrupt Request disabled
1 – ATD Sequence Complete Interrupt Request enabled
0
0 – No ATD interrupt occurred
1 – ATD sequence complete interrupt pending
Control Register 3
Pin
Description
6-3
Controls the number of conversions per sequence
2
0 – ATD Conversion calculation goes to
corresponding result register
1 – Current ATD conversion put in consecutive result
registers; wraps around sequentially at end
1-0
Determines how ATD responds to a breakpoint (see
Table 8.5)
Control Register 4
Pin
Description
7
0 – 10-bit resolution
1 – 8-bit resolution
6-5
Selects the length of the second phase of the
sample time in units of ATD conversion clock
cycles. (See Table 8-7)
4-0
ATD Clock Prescaler (PRS) (5 bits long). ATD
conversion clock frequency is calculated by:
ATDclock =
BusClock
* 0.5
PRS +1
Control Register 5
Pin
Description
7
0 – Data in the result registers is left-justified
1 – Data in the result registers is right-justified
6
0 – Result register data is unsigned
1 – Result register data is signed
5
Continuous Conversion Sequence Mode
0 – Single conversion sequence
1 – Continuous conversion sequences (scan mode)
4
Multi-Channel Sample Mode
0 – Sample only one channel
1 – Sample across multiple channels
2-1
Selects the analog input channel(s) whose signals are
sampled and converted to digital codes (See Table 8-12)
Single Channel (MULT = 0)
Single Conversion (SCAN = 0)
ATDDR7
ATDDR6
7
6
5
4
3
2
1
0
ATDDR5
ATDDR4
ATDDR3
Port AD
Result
Register
Interface
ATD Converter
ATDDR2
ATDDR1
ATDDR0
Single Channel (MULT = 0)
Continuous Conversion (SCAN = 1)
ATDDR7
ATDDR6
7
6
5
4
3
2
1
0
ATDDR5
ATDDR4
ATDDR3
Port AD
Result
Register
Interface
ATD Converter
ATDDR2
ATDDR1
ATDDR0
Multiple Channel (MULT = 1)
Single Conversion (SCAN = 0)
ATDDR7
ATDDR6
7
6
5
4
3
2
1
0
ATDDR5
ATDDR4
ATDDR3
Port AD
Result
Register
Interface
ATD Converter
ATDDR2
ATDDR1
ATDDR0
Single Channel (MULT = 1)
Continuous Conversion (SCAN = 1)
ATDDR7
ATDDR6
7
6
5
4
3
2
1
0
ATDDR5
ATDDR4
ATDDR3
Port AD
Result
Register
Interface
ATD Converter
ATDDR2
ATDDR1
ATDDR0
Status Register 0
Pin
Description
7
0 – Conversion sequence not completed
1 – Conversion sequence completed (set to 1 after each
sequence complete when SCAN mode is on)
5
0 – No external trigger overrun error has occurred
1 – External trigger overrun error has occurred
4
0 – No overrun in results
1 – A overrun in results
3-0
Conversion Counter (read-only; points to result register that
will receive the result of the current conversion)
Status Register 1
Pin
Description
7-0
Conversion complete flag (one bit is set at the end
of every conversion in a conversion sequence,
going from CCF0 in order to CCF7)
0 – Conversion # x is not completed
1 – Conversion # x is completed and results are
available
Left-Justified Result Register
• Right-Justified Result Register is similar.
• Each register has a high and a low byte.
• 8 Result Registers total ($0090 - $009F)
Setting Up the ATD
• Step 1: Power-up the ATD and define settings in ATDCTL2
ADPU = 1 powers up the ATD
ASCIE = 1 enables interrupt
• Step 2: Wait for ATD recovery time (~ 20μs) before
proceeding
• Step 3: Set the number of successive conversions in
ATDCTL3
S1C, S2C, S4C, S8C determine the number of conversions (see Table 8-4)
Setting Up the ATD
• Step 4: Configure the resolution, sampling time, and ATD
clock speed in ATDCTL4
PRS0, PRS1, PRS2, PRS3, PRS4 set the sampling rate (see Table 8-6)
SRES8 sets the resolution to 8-bit (= 1) or 10-bit (= 0)
• Step 5: Configure the starting channel, single/multiple
channel, SCAN setting and whether result data should be
signed or unsigned in ATDCTL5
CC, CB, CA determine input channel (see Table 8-12)
MULT sets single (= 0) or multiple (= 1) inputs
SCAN sets single (= 0) or continuous (= 1) sampling
DJM sets output format as left-justified (=0) or right-justified (=1)
DSGN sets output data as unsigned (=0) or signed (=1)