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

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

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

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

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

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. 1.5 min

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 011010010101010100101 101010101011111100101 000010101010111110011 010101010101010101010 111010101011110011000 100101010101010001111 Filter nth bit

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.

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)

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

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. http://www.msbtech.com/support/How_DACs_Work.php

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

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.

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.

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.

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). http://www.msbtech.com/support/How_DACs_Work.php

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

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

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

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

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 8 - 16 bit resolution Resolution corresponds to the voltage of the LSB Finer resolution = smoother voltage change

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

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.

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.

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

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

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

Minh Vo Full Scale Error Combination of gain and offset error

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

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

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

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)

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

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.

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.

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.

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

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.

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

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

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

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.

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. 10/31/00 ME 6405: Introduction to Mechatronics43

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…

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

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)

Thyristors cont’d. Wye-Chi Chok

…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.

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

Triac Characteristic Curve Wye-Chi Chok

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

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)

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

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

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

QUESTIONS?