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Analog-to-Digital Converter and Multi-vibrators

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Analog-to-Digital Converter and Multi-vibrators

.111 corresponds to 7/8

7/8 of 5 is 4.375

.100 corresponds to 1/2

1/2 of 5 is 2.5

- 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

+ Input higher than – input, output is high

+ Input lower than – input, output is low

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

Reference Voltage

Input voltage

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

Reference Voltage

Input voltage

Doesn’t occur

Warning: may need to flip switch back and forth.

Multi-vibrators

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

- 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.

- 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.

STABLE

STABLE

UNSTABLE

- 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.”

- 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.

- 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.

The upper trip point

Above the upper trip and going up

Below the lower trip and going down

The lower trip point

- 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.

- 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

high

low

OFF

ON

Charge building up

- 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.

low

ON

Turns off

- 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%

- 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.

- 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.

- 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