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# Analog to Digital Converters - PowerPoint PPT Presentation

Analog to Digital Converters. Stu Godlasky Nikita Pak James 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

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### Analog to Digital Converters

Nikita Pak

James Potter

• What is an analog to digital converter (ADC)

• Going from analog to digital

• Types and properties of ADC

• Converts an analog signal to discrete time digital

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

• Two step process

• Sampling – Measuring analog signal at uniform time intervals

• Quantization – Assigning discrete measurements a binary code (each sample will have a binary number associated with it)

Example of digital signal from 3 bit ADC

010 010 011

T1 T2 T3 T4

• Every analog signal has a frequency

• Nyquist Frequency (half sampling frequency)

• Aliasing occurs when signal above Nyquist frequency

• 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 intervals

N = 2k where k is the number of bits

• 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

• Also referred to as an Integrating ADC

Integrator

• Converts in two phases (ramp up / ramp down )

• Input voltage measurement not dependant on integrator components

Pros

• Conversion result is insensitive to errors in the component values

• Fewer adverse affects from noise

• High accuracy

• Cons

• Slow

• Accuracy is dependant on the use of precision external components

• Cost

• 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

• 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

• Vin = 0.6 V

• Vref = 1V

N = 2n (n = number of bits)

N = 210 = 1024

Vref = 1V/ 1024

= 0.0009765625V (resolution)

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.

• Cons

• Higher resolution successive approximation ADCs will be slower

• 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

• Cons

• Expensive

• Prone to produce glitches in the output

• Each additional bit of resolution requires twice the comparators and resistors

• Input over sampled, goes to integrator

• Integration compared with ground

• Iteration drives integration of error to zero

• Output is a stream of serial bits

Pros

• High resolution

• No need for precision components

• Cons

• Slow due to over sampling

• Only good for low bandwidth

### Analog to Digital Converter Applications

Nikita Pak

• Music recording

• Data acquisition/measurement devices

• thermocouples

• digital multimeters

• strain gauges

• Consumer Products

• cell phones

• digital cameras

• 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 (Nyquistfrequency = 22.05 kHz is higher than human ear can detect)

• CD recording often done with flash A to D

• 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

• 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

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

• 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

Input Pins

MC9S12C32

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

Results of Successive Approximation

Reference Voltages

Source

V

source

“Holds” Source Voltage

James Potter

• 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

8 registers,

Each with

High and low byte

Result Registers:Left-Justified (Default)

• High Byte

• Low Byte

Result Registers:Right-Justified

• High Byte

• Low Byte

### Control Registers

Single Channel (MULT = 0)Single Conversion (SCAN = 0)

ATDDR7

ATDDR6

7

6

5

4

3

2

1

0

ATDDR5

ATDDR4

ATDDR3

Result

Register

Interface

ATDDR2

ATDDR1

ATD Converter

ATDDR0

Single Channel (MULT = 0)Continuous Conversion (SCAN = 1)

ATDDR7

ATDDR6

7

6

5

4

3

2

1

0

ATDDR5

ATDDR4

ATDDR3

Result

Register

Interface

ATDDR2

ATDDR1

ATD Converter

ATDDR0

Multiple Channel (MULT = 1)Single Conversion (SCAN = 0)

ATDDR7

ATDDR6

7

6

5

4

3

2

1

0

ATDDR5

ATDDR4

ATDDR3

Result

Register

Interface

ATDDR2

ATDDR1

ATD Converter

ATDDR0

Single Channel (MULT = 1)Continuous Conversion (SCAN = 1)

ATDDR7

ATDDR6

7

6

5

4

3

2

1

0

ATDDR5

ATDDR4

ATDDR3

Result

Register

Interface

ATDDR2

ATDDR1

ATD Converter

ATDDR0

### Status Registers

• 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 number of successive conversions in ATDCTL3

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

• Step 4: Configure resolution, sampling time, and ATD clock speed in ATDCTL4

PRS0, 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) 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)