Presentation on theme: "EET 252 Unit 6 Analog-to-Digital Conversion Read Floyd, Section 12-1 and 12-2. Study Unit 6 e-Lesson. Do Lab #6. Homework #6 and Lab #6 due next."— Presentation transcript:
EET 252 Unit 6 Analog-to-Digital Conversion Read Floyd, Section 12-1 and 12-2. Study Unit 6 e-Lesson. Do Lab #6. Homework #6 and Lab #6 due next week. Quiz next week.
Most physical quantities (temperature, pressure, light intensity, etc.) are analog quantities. Transducers are devices that convert one of these physical quantities to an analog voltage or current. Example, a temperature sensor might produce a voltage in mV that is proportional to the temperature in degrees Fahrenheit. Analog Quantities
To use a computer to process analog information, we must first use an analog-to- digital converter (ADC) to transform the analog values into digital binary values. Conversely, we use a digital-to-analog converter (DAC) to transform digital values from the computer into analog values that can be used to control analog devices. Interfacing to the Analog World
A Typical Application Transducer Physical variable ADCDAC Actuator Computer Control physical variable............ Analog input (voltage or current) Digital inputs Digital outputs Analog output (voltage or current)
ADC: A Three-Step Process On the previous slide, the box labeled ADC actually represents three steps: 1.Anti-aliasing Filter 2.Sample and Hold 3.Analog-to-Digital Conversion (Quantization) The circuits that perform these steps may be on separate chips or may be combined onto a single ADC chip.
Aliasing on the Digital Oscilloscope To see an example of aliasing, use the oscilloscope to display a 10 kHz sine wave. For this frequency, what is a reasonable value for the SEC/DIV setting? Try setting the SEC/DIV to a much higher value, and you’ll see an alias of the original sine wave. Se discussion on page 20 of oscilloscope’s manual.oscilloscope’s manual
Anti-Aliasing Filters The job of an anti-aliasing filter is to remove frequencies from the input signal that are higher than our sampling circuit can handle. This prevents the system from being fooled into thinking that the input signal contains frequencies that it doesn’t really contain.
During the quantization process, the ADC converts each sampled value of the analog signal into a binary code. The more bits that are used in this code, the more accurate is the representation of the original signal. The following slides show an example of how using 2 bits (Figures 12.7 and 12.8) results in much less accuracy than using 4 bits (Figures 12.9 and 12.10). Number of Bits and Accuracy
Several common ways of specifying an ADC’s resolution: Number of bits, n Number of output codes, = 2 n, or number of steps in the output, = 2 n − 1 Percentage resolution, = 1 / (2 n − 1), expressed as a percentage Step size, = V ref / 2 n Resolution
Resolution: Examples Formula4-bit ADC10-bit ADC Number of bits n 4 Number of output codes 2n2n 16 Number of steps in the output 2 n −1 15 Percentage resolution 1 / (2 n −1) 6.67% Step size (assuming 5 V reference voltage) V ref / 2 n 312.5 mV
There are several standard designs: 1.Digital-Ramp ADC 2.Successive Approximation ADC* 3.Flash ADC* 4.Dual-Slope ADC* 5.Sigma-Delta ADC* 6.Up/Down Digital-Ramp ADC 7.Voltage-to-Frequency ADC *Discussed in the textbook How to Build an ADC
Many ADCs and DACs contain one or more operational amplifiers (op amps). Op amps are extremely versatile devices that you’ll study in EET 207. We just need to know a little bit about op amps…. Operational Amplifiers
Op amps are often used as comparators, in which case there is no feedback between the op amp’s output and either input: Op Amp with No Feedback V out is HIGH when V in2 > V in1. V out is LOW when V in2 < V in1.
ADC0804 (Datasheet on course website)ADC0804 Note separate analog and digital grounds, series RC network to control timing, and “handshaking lines” that a microprocessor uses to communicate with the ADC. A Popular ADC Chip