DAC, Diodes, Triacs

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DAC, Diodes, Triacs
ME 6405 Intro to Mechatronics Student Lecture

• Kevin Johnson
• Minh Vo
• Lam Duong
• Wye-Chi Chok

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Outline
Kevin Johnson

• DAC
• What is a DAC?
• Types of DAC
• Specifications
• Diodes
• What are diodes?
• P-N Junction Diode
• Real vs. Ideal
• Types of Diodes Applications
• Triacs
• What are thyristors?
• What are triacs?
• Applications

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Principal components of DAC
Kevin Johnson
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What is a DAC?
Kevin Johnson

• Convert digital signal (number) to analog
  signal (voltage or current)
• Either multiplying or non-multiplying
• Non-multiplying contains its own reference
• Multiplying takes external reference.
• Two main types ladder and delta-sigma

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DAC ideal output.
Kevin Johnson

• Each binary number sampled by the DAC corresponds
  to a different output level.

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Kevin Johnson
DAC real output.
DACs capture a number and hold that value for a
given sample interval. This is known as a
zero-order hold and results in a piecewise
constant output.
DAC
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Smoothing
Kevin Johnson

• Used when a continuous analog signal is required.
• Signal from DAC can be smoothed by a Low pass
  filter

Piece-wise Continuous Output
Analog Continuous Output
Digital Input
n bit DAC
0 bit
011010010101010100101 101010101011111100101 000010
101010111110011 010101010101010101010 111010101011
110011000 100101010101010001111
Filter
nth bit
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Kevin Johnson
Applications.

• Audio/Video
• MP3 players
• Cellphones
• Television
• (well, old ones)

• Signal Generators
• Sine wave generation
• Square wave generation
• Triangle wave generation
• Random noise generation

• Motor, valve, actuator
• Rarely usually PWM.

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Types of DAC implementations
Kevin Johnson

• Binary Weighted Resistor
• R-2R Ladder
• Pulse Width Modulator (not covered)
• Oversampling DAC, aka Delta Sigma (used
  internally in HCS12)

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Binary Weighted Resistor
Kevin Johnson

• Assume binary inputs B0 (LSB) to Bn-1 (MSB)
• Each Bi is 1 or 0 and is multiplied by Vref to
  get input voltage

B5
B4
B3
B2
B1
B0
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Binary weight theory
Kevin Johnson

• Need to fill jars to a specific level using set
  of measuring cups.
• Cups are ½, ¼, 1/8, 1/16, etc.

http//www.msbtech.com/support/How_DACs_Work.php
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BWR Pros and Cons
Kevin Johnson

• Advantages
• Simple
• Fast
• Disadvantages
• Need large range of resistor values (20481 for
  12-bit) with high precision in low resistor
  values
• Need very small switch resistances
• Op-amp may have trouble producing low currents at
  the low range of a high precision DAC

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R-2R ladder basic circuit
Kevin Johnson

• Equivalent resistance to ground at each top node
  is R.
• At each node, current gets split in two.
• Since nodes are cascaded, currents are ½, ¼, 1/8,
  etc.

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R-2R Ladder results
Kevin Johnson

• Final result is
• Assuming Rf R (and ignoring negative)
• Resolution is smallest step i.e. B1 in above
  equation.

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R-2R Ladder
Kevin Johnson

• Advantages
• Only 2 resistor values
• Lower precision resistors acceptable
• Disadvantages
• Slightly slower conversion rate
• Op-amp must still handle very small currents at
  high bit numbers.

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Delta-sigma DAC
Kevin Johnson

• Now all cups are the same size (or more
  precisely, he uses the same cup over and over).
• Cup size is
• 1/(2n).
• He must add this amount the proper number of
  times
• (pulse-count modulation).

http//www.msbtech.com/support/How_DACs_Work.php
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Delta-sigma Pros and Cons
Kevin Johnson

• Pros
• Very accurate
• High bit-depth possible
• Reduced aliasing
• Cons
• Requires very fast oversampling clock.
• At least 2n times faster than sampling rate
• Complicated
• Sensitive to clock jitter

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General comments
Kevin Johnson

• Circuits as shown produce only unipolar output
• Replacing ground with Vref will allow Vout to be
  positive or negative

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Specifications of a DAC
Minh Vo

• Reference Voltage
• Resolution
• Sampling Rate
• Settling Time
• Linearity
• Errors

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Reference Voltage Vref
Minh Vo

• Determines the output voltage range
• Non-multiplying DAC
• Fixed Vref set internally by manufacturer
• Multiplying DAC
• Vref is set externally and can be vary during
  operation
• Full-scale voltage Vfs
• Voltage when all digital inputs are 1s

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Resolution
Minh Vo

• The resolution is the amount of output voltage
  change in response to a least significant bit
  (LSB) transition.
• Smaller resolution results in a smoother output
• A common DAC has a 8 - 16 bit resolution

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Sampling Rate fsampling
Minh Vo

• Rate of conversion of a single digital input to
  its analog equivalent
• When the input changes rapidly, fmax, the DAC
  conversion speed must be high
• Nyquist Criterion
• Limited by the clock speed of the input signal
  and the settling time of the DAC

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Settling Time
Minh Vo

• DAC needs time to reach the actual expected
  analog output voltage
• The time required for the output voltage to
  settle within /- ½ of VLSB of the expected
  voltage

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Linearity
Minh Vo

• The difference between the desired analog output
  and the actual output over the full range of
  expected values

Linear (Ideal)
Non-Linear
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Errors
Minh Vo

• Gain Error
• Offset Error
• Full Scale Error
• Non Linearity
• Non-Monotonic
• Resolution Errors
• Settling Time and Overshoot

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Gain Error
Minh Vo

• Deviation in the slope of the ideal curve and
  with respect to the actual DAC output

High Gain Error Step amplitude is higher than
the desired output
Low Gain Error Step amplitude is lower than the
desired output
Gain Error is adjustable to zero using an
external potentiometer
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Offset Error
Minh Vo

• Occurs when there is an offset in the output
  voltage in reference to the ideal output

This error may be detected when all input bits
are low (i.e. 0).
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Full Scale Error
Minh Vo

• Combination of gain and offset error

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Differential Non-Linearity
Minh Vo

• Voltage step size changes vary with as digital
  input increases. Ideally each step should be
  equivalent.

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Integral Non-Linearity
Minh Vo

• Occurs when the output voltage is non linear.
  Basically an inability to adhere to the ideal
  slope.

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Non-Monotonic
Minh Vo

• Occurs when the an increase in digital input
  results in a lower output voltage.

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Resolution Errors
Minh Vo

• Does not accurately approximate the desired
  output due large voltage divisions.

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Settling Time and Overshoot
Minh Vo

• Any change in the input time will not be
  reflected immediately due to the lag time.
• Overshoot occurs when the output voltage
  overshoots the desired analog output voltage.

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What is a Diode?
Lam Duong

• A diode is a two terminal electric component
  which conducts current more easily in one
  direction than in the opposite direction.
• The most common usage of a diode is as an
  electronic valve which allows current to flow in
  one direction but not the opposite direction.

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A bit of history
Lam Duong

• Diodes were known as rectifiers until 1919, when
  a physicist by the name of William Eccles coined
  the term diode, which from its Greek roots means
  through-path.
• In 1873 Fredrick Guthrie discovered thermionic
  diodes (vacuum tube diodes) . Heating the cathode
  in forward bias permitted electrons to be
  transmitted into the vacuum, but in reverse bias
  the electrons were not easily release from the
  unheated anode.

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A bit of history
Lam Duong

• In 1874 Karl Braun discovered the first solid
  state diode (crystal diode). It consists of using
  Galena crystals as the semiconducting material.
• In 1939 Russell Ohl discovered the first P-N
  junction at Bell Labs.
• Today, the majority of diodes are made of
  semiconductor silicon P-N junctions.

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P-N Junction Diode
Lam Duong

• A P-N junction diode consists of a p-type
  semiconductor (silicon) joined with an n-type
  semiconductor.
• P-type A semiconductor doped with impurities to
  create positive charge carriers (holes).
• N-type A semiconductor doped with impurities to
  create negative charged carriers.
• A depletion region is created when negative
  charge carriers from the N-type region diffuse
  into the P-type region, and vice versa.

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P-N Junction Diode
Lam Duong

• The behavior of a diode depends upon the polarity
  of the supply voltage.
• Under forward bias the depletion region is
  reduced in size and less energy is required for
  the charged majority carriers to cross the
  depletion region.
• This decrease in energy requirement results in
  more charged majority carriers to cross the
  depletion region which induces a current.

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P-N Junction Diode
Lam Duong
Depletion Region

• Under reverse bias the depletion region is
  greatly increased in size and requires
  significantly more energy from the majority
  carriers in order to cross.
• Most majority carriers wont be able to cross the
  depletion region and thus are unable to induce a
  current.

ir
V
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Real vs. Ideal
Lam Duong

• Ideal P-N Diode no resistance to current in
  forward bias and infinite resistance in reverse
  bias. (Similar to a switch)
• In reality there is resistance to current flow in
  forward bias. It requires a certain voltage to be
  reached before the depletion region is eliminated
  and full current flow is permitted.
• Likewise, in reverse bias there is a small
  reverse (leakage) current induced by the flow of
  minority carriers. At a certain voltage (break
  down voltage) the reverse current will increase
  significantly. This is called the Avalanche
  current.

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Schottky Diode
Lam Duong

• Unlike P-N junction diodes, Schottky diodes are
  based on a metal and semiconductor junction.
• An advantage of Schottky diodes over P-N junction
  diodes is that Schottky diodes have no recovery
  time when switching from conducting to
  non-conducting state and vice versa.
• The main disadvantage of Schottky diodes are that
  they operate in low voltage compare to P-N
  junction diodes (up to 50V).
• Another significant difference is that the
  on-voltage for a Schottky diode is around .3V
  while it is .7V for a P-N junction diode.

Metal
N-Type
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Flyback Diode
Lam Duong

• Schottky diodes are often used as Flyback diodes
  due to their quick recovery and low forward
  voltage drop.
• A Flyback diode is a diode used to eliminate the
  sudden voltage spike that occurs across an
  indicutive load when voltage is abruptly reduced
  or removed.
• Lenzs law - if the current through an inductance
  changes, this inductance induces a voltage so the
  current will go on flowing as long as there is
  energy in the magnetic field.
• Flyback diodes are important in mechatronics
  applications where one may want to vary the
  voltage of an inductive load to control its
  operation.

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Other Types of Diodes
Lam Duong

• Light Emitting Diodes (LEDs) - A diode formed
  from a semiconductor such as gallium arsenide,
  carriers that cross the junction emit photons
  when they recombine with the majority carrier on
  the other side.
• Photodiode Exploits the fact that all
  semiconductors are subject to charged carrier
  generation when they are exposed to light.
  Photodiodes are often used to sense light such as
  in an Opto-isolator.
• Zener Diode Allows current in forward bias like
  a regular diode, but also in reverse bias if the
  voltage is larger than designed voltage, called
  the Breakdown voltage.

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What are TRIACS?
Wye-Chi Chok

• In order to know, we must first look at
  thyristors

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What are Thyristors?
Wye-Chi Chok

• Class of semiconductor components that can only
  go in 1 direction.
• Wide range of devices, SCR (silicon controlled
  rectifier), SCS (silicon controlled switch),
  Diacs, Triacs, and Shockley diodes
• Used in high power switching applications
• i.e. hundreds of amps / thousands of watts

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How do Thyristors work?
Wye-Chi Chok

• PNPN (4-layer) device
• PNP and NPN transistor back-to-back.
• With forward voltage, small gate current pulse
  turns on device.
• once on, each transistor supplies gate current
  for the other, so no need for gate input
• only way to turn it off is to stop current (i.e.
  bring voltage to zero)

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Thyristors contd.
Wye-Chi Chok
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now then, what are TRIACS?
Wye-Chi Chok

• A TRIAC (TRIode for Alternating Current) is a
  3-terminal AC semiconductor switch.
• Composed of 2 thyristors facing opposite
  directions such that it can conduct current in
  either direction.
• MT1 and MT2 are current carrying terminals while
  the Gate terminal is used for triggering by
  applying a small voltage signal.
• Once triggered, it continues to conduct current
  until the current falls below a threshold value.

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Triac Operation
Wye-Chi Chok

• 5 layer device
• Region between MT1 and MT2 are parallel switches
  (PNPN and NPNP)
• Allows for positive or negative gate triggering

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Triac Characteristic Curve
Wye-Chi Chok
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Triac Characteristic Curve
Wye-Chi Chok

• 1st quadrant - MT2 is () with respect to MT1
• VDRM is the break-over voltage of the Triac and
  the highest voltage that can be blocked
• IRDM is the leakage current of the Triac when
  VDRM is applied to MT1 and MT2
• IRDM is several orders of magnitude smaller than
  the on rating

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Triacs
Wye-Chi Chok

• Pros
• Better than a transistor as it has much better
  current surge rating it can handle more current
  as it simply turns on more
• Inexpensive compared to relays
• Cons
• Can't manually control turn-off with the gate
  must turn off by stopping current through the
  device via the terminals.
• Specs to buy one
• Gate signal requirements
• Voltage drop
• Steady-state/holding current (continuously
  handle)
• Peak current (maximum amount to handle surge)

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Triac Applications
Wye-Chi Chok

• High Power TRIACS
• Switching for AC circuits, allowing the control
  of very large power flows with milliampere-scale
  control currents
• Can eliminate mechanical wear in a relay
• Low Power TRIACS
• Light bulb dimmers (done by applying power
  later in the AC cycle aka PWM of AC wave)
• Motor speed controls for electric fans and
  other AC motors, and heaters
• Modern computerized control circuits in
  household appliances

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Triac Applications
Wye-Chi Chok

• Simple Triac Switch
• Small control current/voltage
• Eliminates Mechanical wear in a Relay
• Much Cheaper

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Real World Triacs
Wye-Chi Chok

• Come in various shapes and sizes
• Essentially all the same operationally
• Different mounting schemes

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QUESTIONS?