Analog to Digital Converters

ANALOG TO DIGITAL CONVERTERS

Stu GodlaskyNikita PakJames Potter

INTRODUCTION

What is an analog to digital converter (ADC)

Going from analog to digital

Types and properties of ADC

WHAT IS AN ANALOG TO DIGITAL CONVERTER

Converts an analog signal to discrete time digital

Computers need digital. (On / Off , High / Low , 1/0)

GOING FROM ANALOG TO DIGITAL

Two step process1) Sampling – Measuring analog signal at

uniform time intervals2) Quantization – Assigning discrete

measurements a binary code (each sample will have a binary number associated with it)

T1 T2 T3 T4

Example of digital signal from 3 bit ADC010 010 011

ALIASING

Every analog signal has a frequency Nyquist Frequency (half sampling frequency) Aliasing occurs when signal above Nyquist

frequency

http://upload.wikimedia.org/wikipedia/commons/2/28/AliasingSines.svg

QUANTIZATION ERROR

Analog (infinite values) – Digital (finite values)

Upon reconstruction of analog signal Increases as resolution decreases

Resolution - Q

EFSR - full scale voltage range

N = Number of discrete voltage intervalsN = 2k where k is the number of bits

QUANTIZATION ERROR

• Quantized signal only has values at midpoint of voltage band

TYPES OF ANALOG TO DIGITAL CONVERTERS

Dual Slope A/D Converter Successive Approximation A/D Converter Flash A/D Converter Delta – Sigma A/D Converter

DUAL SLOPE ANALOG TO DIGITAL CONVERTER

Also referred to as an Integrating ADC

Integrator


Converts in two phases (ramp up / ramp down )

Input voltage measurement not dependant on integrator components

http://en.wikipedia.org/wiki/File:Dual_slope_integrator_graph.svg


Pros Conversion result is

insensitive to errors in the component values

Fewer adverse affects from noise

High accuracy

Conso Slowo Accuracy is

dependant on the use of precision external components

o Cost

SUCCESSIVE APPROXIMATION ANALOG TO DIGITAL CONVERTER DAC = Digital to Analog Converter EOC = End of Conversion SAR = Successive Approximation Register S/H = Sample and Hold Circuit Vin = Input Voltage

Vref = Reference Voltage

http://en.wikipedia.org/wiki/File:SA_ADC_block_diagram.png

SUCCESSIVE APPROXIMATION ANALOG TO DIGITAL CONVERTER

Uses an n-bit DAC and original analog results Performs a bit by bit comparison of VDAC and

Vin

If Vin > VREF / 2 MSB set to 1 otherwise 0

If Vin > VDAC Successive Bits set to 1 otherwise 0

SUCCESSIVE APPROXIMATION ADC EXAMPLE

10 bit ADC Vin = 0.6 V

Vref = 1V

N = 2n (n = number of bits)N = 210 = 1024Vref = 1V/ 1024

= 0.0009765625V (resolution)

SUCCESSIVE APPROXIMATION DIGITAL TO ANALOG CONVERTER

Pros Capable of high

speed and reliable Medium accuracy

compared to other ADC types

Good tradeoff between speed and cost

Capable of outputting the binary number in serial (one bit at a time) format.

Conso Higher resolution

successive approximation ADCs will be slower

FLASH ANALOG TO DIGITAL CONVERTER

Also called a parallel ADC 2N – 1 Comparators 2N Resistors Control Logic (encoder)


Uses the resistors to divide reference voltage into intervals

Uses comparators to compare Vin and the reference voltages

Encoder takes the output of comparators and uses control logic to generate binary digital output


Pros Very Fast (Fastest) Very simple operational

theory Speed is only limited

by gate and comparator propagation delay

Conso Expensiveo Prone to produce

glitches in the outputo Each additional bit of

resolution requires twice the comparators and resistors

SIGMA-DELTA ANALOG TO DIGITAL CONVERTER

Input over sampled, goes to integrator Integration compared with ground Iteration drives integration of error to

zeroOutput is a stream of serial bits

SIGMA-DELTA ANALOG TO DIGITAL CONVERTER

Pros High resolution No need for precision

components

ConsoSlow due to over

sampling o Only good for low

bandwidth

COMPARISON OF ADCS

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

ANALOG TO DIGITAL CONVERTER APPLICATIONSNikita Pak

ANALOG TO DIGITAL CONVERTER APPLICATIONS

Music recording Data acquisition/measurement devices

thermocouples digital multimeters strain gauges

Consumer Products cell phones digital cameras

MUSIC RECORDING

A to D used to convert sound pressure waves into discrete digital signal (later, D to A used to convert back to an electrical signal for a speaker)

Saves a tremendous amount of space

Ex. CD samples at 44.1 kHz (Nyquist frequency = 22.05 kHz is higher than human ear can detect)

CD recording often done with flash A to D

DATA ACQUISITION

Data acquisition: the process of obtaining signals from sensors that measure physical conditions

Sensors give analog voltage that must be converted to work on a computer

Most National Instruments DAQ’s use successive approximation A to D

MEASUREMENT DEVICES

Thermocouple: a junction of dissimilar metals creates a voltage difference that is temperature dependent

Digital multimeter: converts signal to a voltage and amplifies it for measurement

More accurate than analog counterparts

MEASUREMENT DEVICES

Strain gauge: most common type measures the change in resistance as a metal pattern is deformed

€

R =ρL

A

CONSUMER PRODUCTS

Cell phones: convert your voice into a digital signal so it can be more efficiently transmitted by compressing the signal

Digital camera ccd: absorbed photons create charges that are converted into a sequence of voltages

These voltages are converted to a digital signal

Both often use flash A to D

ADC ON YOUR MICROCONTROLLER

Input PinsADC Built-into

MC9S12C32

ADC IN BLOCK DIAGRAMATD 10B8C

Port AD

DETAILS OF ATD 10B8C

Analog-To-Digital Resolution: 8 or 10 Bits (manually chosen) 8-Channel multiplexed inputs

Conversion time: 7 µs (for 10 bit mode) Optional external trigger “Successive approximation” type ADC

ATD 10B8C BLOCK DIAGRAM

ATD 10B8C BLOCK DIAGRAM

Reference Voltages

SourceVsource

Results of Successive Approximation

“Holds” Source Voltage

REGISTERSANDSETTING UP YOUR ATD10B8CJames Potter

ADC REGISTERS

All information about registers found in Chapter 8 of MC9S12C Family Reference

Manual

8 Result Registers 6 Control Registers 2 Status Registers 2 Test Registers 1 Digital Input Enable Register 1 Digital Port Data Register

RESULT REGISTERS

RESULT REGISTERS

8 registers,Each withHigh and low byte

RESULT REGISTERS:LEFT-JUSTIFIED (DEFAULT)

High Byte

Low Byte

RESULT REGISTERS:RIGHT-JUSTIFIED

High Byte

Low Byte

CONTROL REGISTERS

CONTROL REGISTERS:ATDCTL2









SINGLE CHANNEL (MULT = 0)SINGLE CONVERSION (SCAN = 0)

7 6 5 4 3 2 1 0

Port AD

ATD Converter

ResultRegisterInterface

ATDDR0

ATDDR1

ATDDR2

ATDDR3

ATDDR4

ATDDR5

ATDDR6

ATDDR7

SINGLE CHANNEL (MULT = 0)CONTINUOUS CONVERSION (SCAN = 1)

7 6 5 4 3 2 1 0

Port AD

ATD Converter


ATDDR0

ATDDR1

ATDDR2

ATDDR3

ATDDR4

ATDDR5

ATDDR6

ATDDR7

MULTIPLE CHANNEL (MULT = 1)SINGLE CONVERSION (SCAN = 0)

7 6 5 4 3 2 1 0

Port AD

ATD Converter


ATDDR0

ATDDR1

ATDDR2

ATDDR3

ATDDR4

ATDDR5

ATDDR6

ATDDR7

SINGLE CHANNEL (MULT = 1)CONTINUOUS CONVERSION (SCAN = 1)

7 6 5 4 3 2 1 0

Port AD

ATD Converter


ATDDR0

ATDDR1

ATDDR2

ATDDR3

ATDDR4

ATDDR5

ATDDR6

ATDDR7

STATUS REGISTERS

STATUS REGISTER 0:ATDSTAT0



SETTING UP YOUR ATD10B8C

SETTING UP THE ATD Step 1: Power-up the ATD and define settings in

ATDCTL2ADPU = 1 powers up the ATDASCIE = 1 enables interrupt

Step 2: Wait for ATD recovery time (~ 20μs) before proceeding

Step 3: Set number of successive conversions in ATDCTL3

S1C, S2C, S4C, S8C determine number of conversions (see Table 8-4)

SETTING UP THE ATD Step 4: Configure resolution, sampling time, and ATD

clock speed in ATDCTL4PRS0, PRS1, PRS2, PRS3, PRS4 set sampling rate (see Table 8-6) SRES8 sets resolution to 8-bit (= 1) or 10-bit (= 0)

Step 5: Configure starting channel, single/multiple channel, SCAN and result data signed or unsigned in ATDCTL5

CC, CB, CA determine input channel (see Table 8-12)MULT sets single (= 0) or multiple (= 1) inputsSCAN sets single (= 0) or continuous (= 1) samplingDJM sets output format as left-justified (=0) or right-justified (=1)DSGN sets output data as unsigned (=0) or signed (=1)

THANK YOU

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Analog to Digital Converters