This presentation is the property of its rightful owner.
1 / 47

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

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.

Analog-to-Digital Converter and Multi-vibrators

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

## 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

Doesn’t occur

### 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.

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.

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