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Analog-to-Digital Converter and Multi-vibrators PowerPoint Presentation

Analog-to-Digital Converter and Multi-vibrators

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

### Multi-vibrators

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

Finish this truth table

Doesn’t occur

Integrated circuit version

Warning: may need to flip switch back and forth.

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.

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

- 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.

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%

Duty cycle example: thigh = 1.407 ms

Duty cycle example: thigh + tlow = 2.111 msDuty cycle = (1.407/2.111) = 66.65%

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

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