CHAPTER 19 Data Conversion

Objectives Describe and Analyze: Analog vs. Digital Signals Resolution Digital-to-Analog Conversion Analog-to Digital Conversion Troubleshooting

Introduction 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

Resolution Analog signals can have any value; digital signals cannot.

Resolution Analog signals are continuous, meaning that between any two values, there is always another value. For example, between Volts and Volts there is 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 There is no value between 1010 and 1011.

Resolution 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 = (1 / 2 8 )  100% Resolution = (1 / 256)  100% Resolution = 0.39%

Resolution Resolution is not the same as accuracy. For example: 6 / 3 = has 6 digits of resolution (that’s 1 ppm!) but only 3 digits of accuracy (2.00)

Resolution More bits = finer resolution = less “graininess”.

Digital-to-Analog 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.

Digital-to-Analog R-2R resistor network supplies binary-weighted current.

Digital-to-Analog A “Multiplying DAC” application.

Analog-to-Digital 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.

Analog-to-Digital 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.

Analog-to-Digital 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. Repeat the process with each bit down to the LSB.

Analog-to-Digital

The basic process.

Analog-to-Digital The hardware.

S&H: Sample-and-Hold Holds V in steady while it is being converted.

Troubleshooting 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.