1. Kevin Johnson Minh Vo Lam Duong Wye-Chi Chok
DAC, Diodes, Triacs
ME 6405 – Intro to Mechatronics
Student Lecture
Kevin Johnson
Minh Vo
Lam Duong
Wye-Chi Chok

2. Outline DAC Diodes Triacs What is a DAC? Types of DAC Specifications
Kevin Johnson
Outline
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

3. Principal components of DAC
Kevin Johnson
Principal components of DAC
Explain picture in detail. 1 min

4. 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

5. Kevin Johnson
DAC ideal output.
Each binary number sampled by the DAC corresponds to a different output level.
Digital Input Signal
Analog Output Signal
Explain picture in detail. 1 min

6. Ideally Sampled Signal
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.
Output typical of a real, practical DAC due to sample & hold
Ideally Sampled Signal
DAC
Explain graphs in detail min

7. Smoothing Used when a continuous analog signal is required.
Kevin Johnson
Smoothing
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
Filter
nth bit

8. Applications. Audio/Video Signal Generators Motor, valve, actuator
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.

9. Types of DAC implementations
Kevin Johnson
Types of DAC implementations
Binary Weighted Resistor
R-2R Ladder
Pulse Width Modulator (not covered)
Oversampling DAC, aka Delta Sigma (used internally in HCS12)

10. Binary Weighted Resistor
Kevin Johnson
Binary Weighted Resistor
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

11. Kevin Johnson
Binary weight theory
Need to fill jars to a specific level using set of measuring cups.
Cups are ½, ¼, 1/8, 1/16, etc.

12. BWR Pros and Cons Advantages Simple Fast Disadvantages
Kevin Johnson
BWR Pros and Cons
Advantages
Simple
Fast
Disadvantages
Need large range of resistor values (2048:1 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

13. R-2R ladder basic circuit
Kevin Johnson
R-2R ladder basic circuit
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.

14. R-2R Ladder results Final result is:
Kevin Johnson
R-2R Ladder results
Final result is:
Assuming Rf = R (and ignoring negative)
Resolution is smallest step: i.e. B=1 in above equation.

15. R-2R Ladder Advantages: Disadvantages Only 2 resistor values
Kevin Johnson
R-2R Ladder
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.

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

17. Delta-sigma Pros and Cons
Kevin Johnson
Delta-sigma Pros and Cons
Pros:
Very accurate
High bit-depth possible
Reduced aliasing
Cons:
Requires very fast oversampling clock.
At least 2^n times faster than sampling rate
Complicated
Sensitive to clock jitter

18. General comments Circuits as shown produce only unipolar output
Kevin Johnson
General comments
Circuits as shown produce only unipolar output
Replacing ground with –Vref will allow Vout to be positive or negative

19. Specifications of a DAC
Minh Vo
Specifications of a DAC
Reference Voltage
Resolution
Sampling Rate
Settling Time
Linearity
Errors
Important in selecting a DAC

20. Reference Voltage Vref
Minh Vo
Reference Voltage Vref
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 1’s
Max voltage if not amplified
VREF also defines the voltage step by which the output changes in response to a 1-LSB transition at the input.
Multiplying DAC voltage can be changed during operation
Full-scale voltage – slightly smaller than the reference voltage

21. Minh Vo
Resolution
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 bit resolution
Resolution corresponds to the voltage of the LSB
Finer resolution = smoother voltage change

22. Sampling Rate fsampling
Minh Vo
Sampling Rate fsampling
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
Typical computer sound cards 48kHz sampling freq

23. Minh Vo
Settling Time
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
Ideally a DAC would instantaneously change its output value when the digital input would change. However, in a real DAC it takes time for the DAC to reach the actual expected output value.

24. Minh Vo
Linearity
The difference between the desired analog output and the actual output over the full range of expected values
Linear (Ideal)
Non-Linear
The linearity is the relationship between the output voltage and the digital signal input.

25. Errors Gain Error Offset Error Full Scale Error Non Linearity
Minh Vo
Errors
Gain Error
Offset Error
Full Scale Error
Non Linearity
Non-Monotonic
Resolution Errors
Settling Time and Overshoot

26. Minh Vo
Gain Error
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
Digital Input
Desired/Ideal Output
Analog Output Voltage
Low Gain
High Gain
Low Gain Error: Step amplitude is lower than the desired output
Gain Error is adjustable to zero using an external potentiometer

27. Minh Vo
Offset Error
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).
Digital Input
Desired/Ideal Output
Output Voltage
Positive Offset
Negative Offset

28. Minh Vo
Full Scale Error
Combination of gain and offset error

29. Differential Non-Linearity
Minh Vo
Differential Non-Linearity
Voltage step size changes vary with as digital input increases. Ideally each step should be equivalent.
Digital Input
Ideal Output
Analog Output Voltage
VLSB
2VLSB
Diff. Non-Linearity = 2VLSB

30. Integral Non-Linearity
Minh Vo
Integral Non-Linearity
Occurs when the output voltage is non linear. Basically an inability to adhere to the ideal slope.
Digital Input
Ideal Output
1VLSB
Int. Non-Linearity = 1VLSB
Analog Output Voltage

31. Minh Vo
Non-Monotonic
Occurs when the an increase in digital input results in a lower output voltage.
Analog Output Voltage
Digital Input
Desired Output
Monotonic
Non-Monotonic

32. Minh Vo
Resolution Errors
Does not accurately approximate the desired output due large voltage divisions.
Vout
Desired Analog signal
Approximate output
2 Volt. Levels
Digital Input 1
Poor Resolution(1 bit)

33. Settling Time and Overshoot
Minh Vo
Settling Time and Overshoot
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.
Settling time generally determines maximum operating frequency of the DAC

34. 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.

35. Lam Duong
A bit of history
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.

36. Lam Duong
A bit of history
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.

37. Lam Duong
P-N Junction Diode
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. n p
Depletion Region
Majority carriers

38. Lam Duong
P-N Junction Diode
Forward Biased n p if
Depletion Region
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.

39. P-N Junction Diode
Lam Duong
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 won’t be able to cross the depletion region and thus are unable to induce a current. n p
Reverse Biased
Depletion Region ir V

40. 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. V I
conduction
region
non-conduction
Ideal Curve

41. Lam Duong
Schottky Diode
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

42. Lam Duong
Flyback Diode
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.
Lenz’s 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.

43. Lam Duong
Other Types of Diodes
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.
10/31/ ME 6405: Introduction to Mechatronics43

44. What are TRIACS? In order to know, we must first look at thyristors…
Wye-Chi Chok
In order to know, we must first look at thyristors…

45. 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

46. How do Thyristors work? PNPN (4-layer) device:
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)

47. Thyristors cont’d.
Wye-Chi Chok

48. …now then, what are TRIACS?
Wye-Chi Chok
…now then, what are TRIACS?
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.

49. 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

50. Triac Characteristic Curve
Wye-Chi Chok

51. 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

52. 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)

53. Triac Applications High Power TRIACS
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

54. Triac Applications Simple Triac Switch Small control current/voltage
Wye-Chi Chok
Simple Triac Switch
Small control current/voltage
Eliminates Mechanical wear in a Relay
Much Cheaper

55. Real World Triacs Come in various shapes and sizes
Wye-Chi Chok
Real World Triacs
Come in various shapes and sizes
Essentially all the same operationally
Different mounting schemes

56. QUESTIONS?