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This article discusses the concept of analog-to-digital conversion using comparators, as well as the operation and applications of multi-vibrators. It also explores the use of 555 timers and crystals in electronic circuits.
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Analog-to-Digital • We have seen a simple digital-to-analog converter, now we consider the reverse process • For this purpose we introduce a new circuit element — the comparator • We have seen considered last semester a digital comparator, a logic circuit that determined whether the input word A is larger than the input word B • Now we look at an analog comparator, it determines whether voltage A is larger than voltage B
Almost what we want • What we want: if the voltage into – input exceeds the voltage into the + input, then the output is low; otherwise it is high • What we have: if the voltage into – input exceeds the voltage into + input by 0.7, then the output is low; otherwise it is high
1-bit analog-digital converter Voltage here is half reference voltage of 5 volts That extra 0.7 volts
Toward a 2-bit analog-digital converter 3/4 1/2 1/4
Multi-vibrators http://www.ee.ed.ac.uk/~kap/Hard/555/node1.html
Multi-vibrator • A multi-vibrator is an electronic circuit that can exist in a number of “states” (voltage and/or current outputs). • A flip-flop is a bi-stable multi-vibrator, bi-stable means it has two stable states. • A state is stable if it is robust against the fluctuations (noise) that are always occurring.
Mono-stable multi-vibrator • A mono-stable multi-vibrator has one stable output (usually zero). • It also has an unstable state. Certain input will put the circuit into its unstable state, which lasts for a set length of time before returning to the stable state. • Unstable states are still robust to noise but do not last indefinitely long. • In wave terminology, this provides one with a single pulse.
Pulse STABLE STABLE UNSTABLE
One shots • One purpose of a mono-stable multi-vibrator is to output a signal of a specified duration. • The input (trigger) may be short (or unknown) in duration, but the output pulse has a predictable duration (can be controlled by the time constant of an RC circuit). • = RC • The time constant and duration are not equal but are proportional. • Such a circuit is called a “one shot.”
Shapers • Another purpose of mono-stable multi-vibrators is to “shape” input signals. • Recall in digital circuits we want signals to be clearly high or low; a mono-stable multi-vibrator can take signals which are not of this form and create signals which are.
Schmitt trigger • If the voltage is above a certain value (the upper trip point) and rising, the output is high. • If the voltage is below another value (the lower trip point) and falling, the output is low. • Otherwise, it remains whatever it was.
Schmitt trigger The upper trip point Above the upper trip and going up Below the lower trip and going down The lower trip point
A-stable multi-vibrator • In an a-stable multi-vibrator, there are typically two states, neither of which is stable. • The circuit repeatedly flips back and forth between the states.
A-stable Multi-vibrator • Assume a state where the transistor on left is ON and transistor on right is OFF and the capacitor on the left has no charge. • Since the left transistor is on (hard) it is not dropping much voltage, therefore “all” the voltage is being dropped by the resistors • The capacitor on the left begins to charge through the 10K resistor on the right
A-stable high low OFF ON Charge building up
A-stable • Charge builds up on the left capacitor, “pulling-up” the voltage presented to the base of the transistor on the right. • When the base reaches about 0.7v the transistor on the right turns on. • Current now starts to flow through the 1K resistor on the far right, thus dropping the voltage level at the collector. • That low voltage makes its way to the base of the transistor on the left turning it off. • The cycle repeats itself.
A-stable low ON Turns off
Duty cycle • In a square wave (e.g. a computer’s clock), the wave is characterized by its frequency, its amplitude and its duty cycle. • The duty cycle is the percent of time that the signal is high. • Duty cycle = thigh/(thigh+tlow)*100%
555 Timer • A similar circuit uses the 555 chip (Integrated circuit) • The resistors and capacitors are external to the chip so that the period and duty cycle of the circuit can be controlled.
Crystals • The very high frequency square wave used for the CPU clocks are not generated in the manner described on the previous slides. • The high frequency signal is supplied by crystals subjected to an electric field.
