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Analog-to-Digital Converter and Multi-vibrators

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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

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Comparator (analog)

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Comparator (analog)

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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

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1-bit analog-digital converter

That extra 0.7 volts

Voltage here is half reference voltage of 5 volts

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1-bit analog-digital converter

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Toward a 2-bit analog-digital converter

3/4

1/2

1/4

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Toward a 2-bit analog-digital converter

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Toward a 2-bit analog-digital converter

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Toward a 2-bit analog-digital converter

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Integrated circuit version

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3.7 / 5

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* 2 x^y 8 =

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Binary Mode

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Compare

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Decimal Mode

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/ 2 x^y 8 =

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Multi-vibrators

http://www.ee.ed.ac.uk/~kap/Hard/555/node1.html

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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.

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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.

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Pulse

STABLE UNSTABLESTABLE

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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.”

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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.

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Schmitt trigger

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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.

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Schmitt trigger

The upper trip point

The lower trip point

Above the upper trip and going up

Below the lower trip and going down

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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.

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A-stable multi-vibrator

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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

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A-stable

ON OFF

low

high

Charge building up

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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.

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A-stable

ON

low

Turns off

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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%

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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.

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555

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555 as Monostable multivibrator

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555 as Astable Multivibrator

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555 Timer (WorkBench version)

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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.