AnalogtoDigital Converter and Multivibrators PHY 202 Blum 1

Analog-to-Digital Converter and Multi-vibrators PHY 202 (Blum) 1

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 PHY 202 (Blum) 2

Comparator (analog) PHY 202 (Blum) 3

Comparator (analog) PHY 202 (Blum) 4

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 PHY 202 (Blum) 5

1 -bit analog-digital converter Voltage here is half reference voltage of 5 volts That extra 0. 7 volts PHY 202 (Blum) 6

1 -bit analog-digital converter PHY 202 (Blum) 7

Toward a 2 -bit analog-digital converter 3/4 1/2 1/4 PHY 202 (Blum) 8

Toward a 2 -bit analog-digital converter PHY 202 (Blum) 9

Toward a 2 -bit analog-digital converter PHY 202 (Blum) 10

Toward a 2 -bit analog-digital converter PHY 202 (Blum) 11

Integrated circuit version PHY 202 (Blum) 12

3. 7 / 5 PHY 202 (Blum) 13

* 2 x^y 8 = PHY 202 (Blum) 14

Binary Mode PHY 202 (Blum) 15

Compare PHY 202 (Blum) 16

Decimal Mode PHY 202 (Blum) 17

/ 2 x^y 8 = PHY 202 (Blum) 18

Multi-vibrators http: //www. ee. ed. ac. uk/~kap/Hard/555/node 1. html PHY 202 (Blum) 19

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. PHY 202 (Blum) 20

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. PHY 202 (Blum) 21

Pulse STABLE PHY 202 (Blum) UNSTABLE 22

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. ” PHY 202 (Blum) 23

Shapers • Another purpose of mono-stable multivibrators is to “shape” input signals. • Recall in digital circuits we want signals to be clearly high or low; a mono-stable multivibrator can take signals which are not of this form and create signals which are. PHY 202 (Blum) 24

Schmitt trigger PHY 202 (Blum) 25

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. PHY 202 (Blum) 26

Schmitt trigger The upper trip point Above the upper trip and going up The lower trip point PHY 202 (Blum) Below the lower trip and going down 27

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. PHY 202 (Blum) 28

A-stable multi-vibrator PHY 202 (Blum) 29

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 10 K resistor on the right PHY 202 (Blum) 30

A-stable high low ON OFF Charge building up PHY 202 (Blum) 31

A-stable • Charge builds up on the left capacitor, “pullingup” the voltage presented to the base of the transistor on the right. • When the base reaches about 0. 7 v the transistor on the right turns on. • Current now starts to flow through the 1 K 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. PHY 202 (Blum) 32

A-stable low Turns off PHY 202 (Blum) ON 33

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% PHY 202 (Blum) 34

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. PHY 202 (Blum) 35

555 PHY 202 (Blum) 36

555 as Monostable multivibrator PHY 202 (Blum) 37

555 as Astable Multivibrator PHY 202 (Blum) 38

555 Timer (Work. Bench version) PHY 202 (Blum) 39

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. PHY 202 (Blum) 40