CHAPTER 19: Data Conversion pptx

• The fact that real-world signals are analog requires microprocessor-based systems to have an A/D at one end and a D/A at the other end... Analog signals can have any value; digital sig

CHAPTER 19

Data Conversion

Describe and Analyze:

• Analog vs Digital Signals

• Resolution

• Digital-to-Analog Conversion

• Analog-to Digital Conversion

• Troubleshooting

• The low cost of microprocessors and the power of using software to carry out signal processing has revolutionized electronics

• The fact that real-world signals are analog requires microprocessor-based systems to have an A/D at one end and a D/A at the other end

A Typical System

<insert figure 19-1 here>

Analog signals can have any value; digital signals cannot

• Analog signals are continuous, meaning that between

any two values, there is always another value For

example, between 1.0000 Volts and 1.0001 Volts

there is 1.00005 Volts (and an infinite number of

other values)

• Digital signals are discrete, meaning that the

difference between any two digital values cannot be less than 1 For example, the next number after

binary 1010 is 1011 There is no value between 1010 and 1011

• Resolution is the smallest difference you can “see” in

a system In a digital system, it is always 1 bit, but you need to know how many bits are in a “word”

• Resolution is a percentage of the maximum binary value For example: suppose you have an 8-bit

converter The resolution would be:

Resolution = 0.39%

• Resolution is not the same as accuracy

For example:

6 / 3 = 2.00635

has 6 digits of resolution (that’s 1 ppm!)

but only 3 digits of accuracy (2.00)

More bits = finer resolution = less “graininess”

• To build an analog-to-digital, you first need a digital-to-analog converter (also called DAC

or D/A)

• The basic ingredients are a precise (or at

least stable) voltage reference, some

precision resistors, some digitally controlled switches, and an op-amp to sum it all up

• See figure on following slide

R-2R resistor network supplies binary-weighted current

A “Multiplying DAC” application

• The basic idea is to use a DAC and a

comparator, and something to generate

binary numbers

• The analog input is applied to one

comparator input The output of the DAC is applied to the other

• Binary values are tried, and the comparator tells the logic about the analog input vs the DAC output

• The simplest approach would be to use a binary

counter to drive the DAC As the count increases from zero, the output voltage of the DAC walks up a staircase At some value of DAC output, the

comparator “flips” and the counter stops counting Whatever number is in the counter is the answer

• The problem with that approach is that it takes too long to count up The more bits, the longer it takes

• Instead of just counting up from zero, the standard approach is to use a technique called

Successive Approximation

• It requires a digital logic circuit called a “successive approximation register” (SAR)

• Using SAR, the MSB is applied to the DAC first If the comparator flips, take it out; if not, leave it in

Analog-to-Digital

The basic process

The hardware

S&H: Sample-and-Hold

• Use a scope to examine waveforms Look for “missing codes” which appear as

“landings” on a staircase (sawtooth)

• Look for “stuck bits”

• Check the reference voltage (with a good meter)

• Look for DC offsets