Analog-to-Digital Conversion
Terminology
Analog : continuously valued signal, such as temperature or speed, with infinite possible values in between
Digital : discretely valued signal, such as integers, encoded in binary
analog-to-digital converter : ADC, A/D, A2D; converts an analog signal to a digital signal
Analog Signals
Analog signals – directly measurable quantities in terms of some other quantity
Examples:
Thermometer – mercury height rises as temperature rises
Car Speedometer – Needle moves farther right as you accelerate
Digital Signals
Digital Signals – have only two states. For digital computers, we refer to binary states, 0 and 1. “1” can be on, “0” can be off.
Examples:
Light switch can be either on or off
Door to a room is either open or closed
ADC Basic Principle
- The basic principle of operation is to use the comparator principle to determine whether or not to turn on a particular bit of the binary number output.
- It is typical for an ADC to use a digital-to-analog converter (DAC) to determine one of the inputs to the comparator.
Sampling
- It is a process of taking a sufficient number of discrete values at point on a waveform that will define the shape of waveform.
- The more samples you take, the more accurately you will define the waveform.
- It converts analog signal into series of impulses, each representing amplitude of the signal at given point…….
3 Basic Types
• Flash ADC
• Digital-Ramp/Dual slope/Counter slope ADC
• Successive Approximation ADC
Flash ADC
- Consists of a series of comparators, each one comparing the input signal to a unique reference voltage.
- The comparator outputs connect to the inputs of a priority encoder circuit, which produces a binary output
How Flash Works
- As the analog input voltage exceeds the reference voltage at each comparator, the comparator outputs will sequentially saturate to a high state.
- The priority encoder generates a binary number based on the highest-order active input, ignoring all other active inputs.
Flash
Advantages
- Simplest in terms of operational theory
- Most efficient in terms of speed, very fast
(limited only in terms of comparator and gate propagation delays)
Disadvantages
- Lower resolution
- Expensive
- For each additional output bit, the number of comparators is doubled
Dual Slope ADC
- Also known as Counter-Ramp or Digital Ramp ADC
- A dual slope ADC is commonly used in measurement instruments (such as DVM’s).
Dual Slope Function
- The Dual Slope ADC functions in this manner:
o When an analog value is applied the capacitor begins to charge in a linear manner and the oscillator passes to the counter.
o The counter continues to count until it reaches a predetermined value. Once this value is reached the count stops and the counter is reset. The control logic switches the input to the first comparator to a reference voltage, providing a discharge path for the capacitor.
o As the capacitor discharges the counter counts.
o When the capacitor voltage reaches the reference voltage the count stops and the value is stored in the register.
Successive approximation ADC
- Much faster than the digital ramp ADC because it uses digital logic to converge on the value closest to the input voltage.
- A comparator and a DAC are used in the process.
Successive Approximation ADC
- A Successive Approximation Register (SAR) is added to the circuit
- Instead of counting up in binary sequence, this register counts by trying all values of bits starting with the MSB and finishing at the LSB.
- The register monitors the comparators output to see if the binary count is greater or less than the analog signal input and adjusts the bits accordingly
Examples of A/D Applications
- Microphones - take your voice varying pressure waves in the air and convert them into varying electrical signals
- Strain Gages - determines the amount of strain (change in dimensions) when a stress is applied
- Thermocouple – temperature measuring device converts thermal energy to electric energy
- Voltmeters
- Digital Multimeters