Analog-to-Digital Converter and Multi-vibrators

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Analog-to-Digital Converter and Multi-vibrators. Simple Digital to Analog Converter. .111 corresponds to 7/8 7/8 of 5 is 4.375. Simple Digital to Analog Converter. .100 corresponds to 1/2 1/2 of 5 is 2.5. Analog-to-Digital.

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### Analog-to-Digital Converter and Multi-vibrators

Simple Digital to Analog Converter

.111 corresponds to 7/8

7/8 of 5 is 4.375

Simple Digital to Analog Converter

.100 corresponds to 1/2

1/2 of 5 is 2.5

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 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
Comparator (analog)

+ Input higher than – input, output is high

Comparator (analog)

+ Input lower than – input, output is low

1-bit analog-digital converter

Input voltage is less than half of reference voltage, result is low.

Reference Voltage

Input voltage

1-bit analog-digital converter

Input voltage is more than half of reference voltage, result is high.

Reference Voltage

Input voltage

Integrated circuit version

Warning: may need to flip switch back and forth.

### 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.
References
• http://www.ee.ed.ac.uk/~kap/Hard/555/node2.html#modes
• http://en.wikipedia.org/wiki/555_timer_IC
• http://www.kpsec.freeuk.com/555timer.htm