Transcript Analog-to-Digital and Multivibrators

Analog-to-Digital Converter
and Multi-vibrators
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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
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Comparator (analog)
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Comparator (analog)
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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
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1-bit analog-digital converter
Voltage
here is half
reference
voltage of 5
volts
That extra 0.7 volts
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1-bit analog-digital converter
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Toward a 2-bit analog-digital
converter
3/4
1/2
1/4
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Toward a 2-bit analog-digital
converter
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Toward a 2-bit analog-digital
converter
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Toward a 2-bit analog-digital
converter
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Integrated circuit version
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3.7 / 5
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* 2 x^y 8 =
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Binary Mode
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Compare
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Decimal Mode
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/ 2 x^y 8 =
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Multi-vibrators
http://www.ee.ed.ac.uk/~kap/Hard/555/node1.html
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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.
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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.
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Pulse
STABLE
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UNSTABLE
STABLE
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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.”
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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.
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Schmitt trigger
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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.
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Schmitt trigger
The upper trip point
Above the upper
trip and going up
The lower trip point
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Below the lower trip
and going down
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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.
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A-stable multi-vibrator
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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
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A-stable
high
low
ON
OFF
Charge building up
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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.
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A-stable
low
Turns
off
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ON
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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%
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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.
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555
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555 as Monostable multivibrator
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555 as Astable Multivibrator
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555 Timer (WorkBench version)
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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.
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