Transcript Analog-to-Digital Converter and Multivibrators

Analog-to-Digital Converter
and 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 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
Comparator (analog)
Comparator (analog)
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
1-bit analog-digital converter
Voltage
here is half
reference
voltage of 5
volts
That extra 0.7 volts
1-bit analog-digital converter
Toward a 2-bit analog-digital
converter
3/4
1/2
1/4
Toward a 2-bit analog-digital
converter
Toward a 2-bit analog-digital
converter
Toward a 2-bit analog-digital
converter
Integrated circuit version
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, bistable 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
• In wave terminology, this provides one with a
single pulse.
Pulse
STABLE
UNSTABLE
STABLE
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 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.
Schmitt trigger
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
The lower trip point
Below the lower trip
and going down
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
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
ON
OFF
Charge building up
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 0.7v 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.
A-stable
low
Turns
off
ON
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.
555
555 as Monostable multivibrator
555 as Astable Multivibrator
555 Timer (WorkBench version)
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.