DAC Diodes and Triacs Siri Belton Jeremy Hill

DAC, Diodes, and Triacs Siri Belton Jeremy Hill and Brandon Whitt

• What is DAC? • Applications • Types of DAC – Binary Weighted Resistor – R-2 R • Specifications – – – Reference Voltage Resolution Sampling Rate Settling Time Linearity Errors • Diodes – Overview – Real v. Ideal – Types: Zener, LED • Triacs Outline

Siri Belton What is DAC? • Device that converts digital numbers into an analog output. • Output can be a voltage or a current 1 0 0 1 0 1 0 0 1 1 1 1 0 0 1 1 0 1 0 1 1 DAC

Siri Belton What is DAC? • Each binary number corresponds to a specific voltage output

Siri Belton Reference Voltage • Use input reference voltages to determine the analog output. • Output voltage, unipolar 0 to V • Output voltage, bipolar -V to V ref ref N-Bit Binary Word Analog Reference Voltage (Vref) Digital to Analog Converter Analog Output (Vout)

Siri Belton Applications • Used anywhere a digital signal is used to create an analog output • Audio/Video – CDs, Cell Phones, Speakers, DVDs, Youtube • Signal Generator

Siri Belton Types of DAC • • • Binary-weighted resistor R-2 R ladder Pulse width modulation Oversampling (Delta Sigma) Cyclic Hybrid DAC

Siri Belton Binary-Weighted Resistors • Adds resistors in parallel to divide voltage on each branch by a power of two • Transistors act as switches

Siri Belton Binary-Weighted Resistors Equivalent Circuit: A B Where A, B, C, and D are 1 or 0 C D

Siri Belton Binary-Weighted Resistors • Advantages: – Useful for conversions up to 8 -bit – Simple – Fast • Disadvantages – Need large range of resistor values (2048: 1 for a 12 -bit conversion) with high precision resistor values – Need small switch resistances – Op-amp can have trouble producing low currents at the lower range of a high precision DAC

Siri Belton R-2 R Ladder • Each bit controls the switch to the op-amp or ground (grounded if zero)

Siri Belton R-2 R Ladder

Siri Belton R-2 R Ladder • Advantages – Need only 2 resistor values – Lower precision is acceptable • Disadvantages – Slower conversion rate

Jeremy Hill Specifications of a DAC • • • Reference Voltage Resolution Sampling Rate Settling Time Linearity Errors

Jeremy Hill Reference Voltage (Vref) • The reference voltage determines the output voltage range. • For Non-multiplying DAC: – Vref is set internally by the manufacturer – Constant Value • For Multiplying DAC: – Vref is set externally – Can be varied during operation • Full Scale Voltage (Vfs) – Voltage when all digital inputs are 1’s

Jeremy Hill Resolution • 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

Jeremy Hill Sampling Rate (fs) • Sampling rate is the rate at which the DAC can convert the digital input to an output voltage • The Nyquist Criterion is used to ensure the output correctly represents the digital input • fmax is the max frequency of the analog signal to be reconstructed • fs is limited by the clock speed of the input signal and the settling time of the DAC

Jeremy Hill 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

Jeremy Hill Linearity • The difference between the desired analog output and the actual output over the full range of expected values Linear (Ideal) Non-Linear

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

Jeremy Hill Gain Error • Deviation in the slope of the actual transfer function from the ideal transfer function – Can be determined by measuring the output voltage for a digital input of all 1’s

Jeremy Hill Offset Error • Occurs when there is an offset in the actual output voltage from the ideal output – Can be determined by measuring the output voltage for a digital input of zero

Jeremy Hill Full Scale Error • Combination of gain and offset error

Jeremy Hill Differential Non-Linearity (DNL) • The difference between two successive digital output codes is ideally 1 VLSB • DNL error is the deviation from a step of 1 VLSB • Manufacturers will specify a maximum DNL error

Jeremy Hill Integral Non-Linearity (INL) • The difference in the ideal linear voltage and the actual output voltage for a given digital code – Manufacturers will specify the max INL error

Jeremy Hill Non-Monotonic • Occurs when an increase in digital input results in a lower output voltage – If the DNL error is less than +/- 1 LSB the DAC is guaranteed to be monotonic

Jeremy Hill Resolution Errors • Resolution will determine how close the output voltage matches the desired signal 1 Bit Resolution 3 Bit Resolution

Jeremy Hill 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

Diodes What are they? Brandon Whitt • 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.

Diodes How do they work? Brandon Whitt • A diode is created when a p-type semiconductor is joined with and n-type semiconductor. • At the boundary a depletion region will form within the diode. Here the p-carriers will diffuse into the ntype region and vice versa. Majority carriers p n Depletion Region

Brandon Whitt Diodes Real vs Ideal I conduction region non-conduction region V Ideal Curve Ideal Diode – no resistance to current flow in the forward direction and infinite resistance in the reverse direction.

Zener Diode Brandon Whitt • Every p-n junction (i. e. diode) will break down in reverse bias if enough voltage is applied. Zener diodes are designed to operate in this breakdown region. • Zener diodes have a specified voltage drop when they are used in reverse bias. They are able to maintain a nearly constant voltage under conditions of widely varying current.

Brandon Whitt Other Diodes • Light Emitting Diodes (LEDs): Photons are emitted when the carriers pass through the junction and recombine with the doped region. • Photodiode: Photons hitting the doped regions cause charged carriers to form. These can be used to sense light in and Optoisolator.

TRIAC Triode for AC current Brandon Whitt • The TRIAC is an electronic component that can allow current to flow in EITHER direction when triggered (bidirectional). • TRIACs make good switches for AC current. • They can handle hundreds of amps and thousands of watts of power.

Brandon Whitt TRIACs They’re made of smaller components • TRIACs are composed of Transistors and Thyristors. • Two Transistors (PNP and NPN back-to-back) are combined to make a Thyristor. Current can only go one direction (Unidirectional). • With forward voltage, small gate current pulse turns on the device. Once on, each transistor supplies gate current for the other so the device stays on.

Brandon Whitt TRIACs They’re made from two Thyristors • A TRIAC is a 3 -terminal switch composed of 2 thyristors facing opposite directions • It can conduct current bidirectionally • MT 1 and MT 2 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 – known as holding current

TRIACs Circuit Example • Simple Triac Switch • Small control current/voltage • Eliminates Mechanical wear in a Relay • Much Cheaper Brandon Whitt

TRIACs Summary Brandon Whitt • TRIACs start conducting when a minimum current (gate threshold current) flows into or out of its gate sufficient to turn on relevant junctions in that quadrant of operation • Device remains in “on” state even after gate current is removed so long as current through the device remains above holding current • Once current falls below holding current for an appropriate time period, device switches “off”

TRIACs Pros and Cons Brandon Whitt Pros: • Can handle much more current than a transistor • Much cheaper than relays Cons: • Can not stop the current from flowing by using the gate. The current must be stopped at the terminal.

TRIACs Applications Brandon Whitt High Power: Switches in AC circuits using milliamp control currents to turn on kilowatt power flows. Low Power: Dimmers for light bulbs, speed controls for electric fan motors, control circuits in appliances Specs to consider when purchasing a TRIAC: • Gate signal requirements • Voltage drop • Steady-state/holding/peak current specifications