* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download No Slide Title
History of electric power transmission wikipedia , lookup
Power engineering wikipedia , lookup
Voltage optimisation wikipedia , lookup
Buck converter wikipedia , lookup
Pulse-width modulation wikipedia , lookup
Transmission line loudspeaker wikipedia , lookup
Electrical substation wikipedia , lookup
Electrical engineering wikipedia , lookup
Stray voltage wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Surge protector wikipedia , lookup
Electromagnetic compatibility wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Rectiverter wikipedia , lookup
Mains electricity wikipedia , lookup
Alternating current wikipedia , lookup
Electronic engineering wikipedia , lookup
Earthing system wikipedia , lookup
Ground loop (electricity) wikipedia , lookup
Practical Digital Design Considerations Part 2 Last Mod: January 2008 ©Paul R. Godin design 2.1 Circuit Interfaces ◊ A variety of electrical issues must be considered when interfacing between digital logic circuits, or between digital logic and analog circuits. ◊ This presentation addresses some specialized interfaces. design 2.2 Noise Issues design 2.3 Digital Signal ◊ Signal must be above or below a threshold voltage to represent a “1” or “0”. This voltage requires a reference voltage. ◊ Signal timing must be within a threshold for each bit to be recognized. Voltage or “1” Minimum Threshold Unknown Zone Maximum Threshold No Voltage or “0” Timing Ideal Signal design 2.4 Digital Signal Problems ◊ Noise ◊ Resistance ◊ Capacitance design 2.5 Induced Noise design 2.6 Induction ◊ Inductance ◊ Can be defined as the transference of electrical energy from one conductor to another. ◊ Faraday’s law (1831) ◊ “The voltage induced in a turn or coil of a conductor is proportional to the rate of change of magnetic lines of force that pass through the coil.” ◊ Faraday’s discovery of the relationship between electricity and magnetism is often cited as the most important discovery of the modern era. design 2.7 Magnetism ◊ Principle: ◊ Magnetic flux lines are established between the 2 poles of a magnet ◊ The quantity, density and length of these lines increases with: ◊ the characteristics of the medium ◊ the strength of the magnet N S design 2.8 Electromagnetism ◊ Principle: ◊ Magnetic flux lines are established when current flows in a conductor ◊ The quantity, density and length of these lines increases with current. ◊ The direction of the flux lines is determined by the direction of the current. design 2.9 Electromagnetism ◊ Principle: ◊ If conductors are coiled, the flux lines of the individual turns combine to create a bar magnet. ◊ If AC current is applied, the polarity of the magnet will reverse ◊ Note it takes time for the field to collapse and re-establish itself in the other direction. ◊ Inductance impedes current flow (XL). ◊ The effects of inductance increase with frequency. N S design 2.10 Total Impedance of a Bus ◊ Bus communications contain Resistance, Capacitance and Inductance. ◊ The effects of resistance increase with temperature. ◊ The effects of capacitance and inductance increase with frequency Electrical equivalent of a bus design 2.11 Electromagnetic Induction ◊ Principle: ◊ Magnetic flux lines crossing a conductor will induce current. ◊ There must be relative motion. ◊ The quantity of lines that cross the conductor over a period of time will determine the induced current. ◊ The direction of the current is based on the polarity and direction of the flux lines. Source Current Induced Current design 2.12 Induction: Source of Noise ◊ Noise is produced when a current is induced on a signalbearing conductor through electromagnetic induction. ◊ This noise may cause erratic operation of digital circuits. ◊ The noise may be picked up via connections on the bus or in the IC. design 2.13 EMF, EMR, EMI, RFI and EMP ◊ EMF and EMR: ◊ ◊ ◊ EMI and RFI: ◊ ◊ Electro-Magnetic Field and Electro-Magnetic Radiation, generally describes the strength of a magnetic field and its ability to induce current. May also refer to Electro-Motive Force (Voltage) Electro-Magnetic Interference and Radio Frequency Interference describe undesired, induced electrical current. RFI applied to higher frequencies. EMP: ◊ Electro-Motive Pulse which is a sudden but short-lived increase in EMI or RFI. Often destructive. EMP sources include electrical storms and weapons. design 2.14 Ambient Noise ◊ Ambient Noise: ◊ Background, steady, predictable noise. ◊ Measuring the frequency of the noise may help determine the source. ◊ 60Hz = lighting, motors, electrical appliances, electrical cabling, transformers, etc… ◊ 100Hz to 20 MHz = switching power supplies, electronic devices, telephone systems, video, etc… ◊ > 20MHz = radio transmitters, cellular telephones, etc… ◊ Often the source is the power supply. design 2.15 Transient Noise ◊ Transient Noise: ◊ Irregular and unpredictable noise. ◊ Transient noise is often difficult to pinpoint. ◊ Likely sources are switching circuits, such as ◊ Elevators, photocopiers, welders, electrical storms, static electricity, switching high-current devices, etc… ◊ Crosstalk: ◊ Noise where within a bus (or cable), one signal-bearing conductor induces a signal on an adjacent conductor. design 2.16 Reducing the Effect of Noise ◊ Distance: ◊ ◊ ◊ Shielding: ◊ ◊ ◊ Magnetic flux lines are less dense with distance from the source Reduce the length of conductors Add conductive shielding around a circuit to either protect it from EMR, or reducing its emissions of EMR. Leave trace material on a circuit board for ground. Other ◊ Employ circuit-based methods for reducing the effects of EMR, such as filters, improved noise margins, other practical design considerations, specialized trace techniques, etc… design 2.17 Grounding and Shielding Electromagnetic Field Metallic Shield Metallic Shield Induced Current Induced Current design 2.18 Effective Grounding design 2.19 Grounds ◊ An earth ground is a point or conductor that has zero electrical potential. ◊ A common ground is a reference point for signals. ◊ Grounds are important for circuit operation and for safety. They must be properly configured and used. ◊ Most systems maintain several levels of grounds. design 2.20 Ground Types ◊ There are 3 basic types of grounds in electrical systems: ◊ Earth ground: primary role is safety. It is also the absolute reference point for voltage. The potential of an earth ground is considered 0 Volts. ◊ Chassis ground: usually used for safety and for noise shielding. Meant to have a potential of 0 Volts. ◊ Common ground: a point of reference for signals and voltages on the circuit board. Often a relative reference point for voltage. Earth Ground Chassis Ground Common Ground design 2.21 Ground Impedance ◊ The primary consideration for an effective ground system is maintaining low impedance, especially low resistance. VCC 5 50 Load Power Supply Describe how ground resistance can cause problems with electrical or electronic circuits. design 2.22 Resistance of Conductors Conductor resistance in circular copper conductors AWG Ω per 100ft (30.5 meters) 12 0.16 14 0.26 16 0.41 18 0.65 20 1.04 22 1.65 24 2.62 26 4.16 28 6.62 30 10.52 design 2.23 Ground Loops ◊ Ground Loops are multiple paths to ground. ◊ Ground loops interfere with the effectiveness of a ground. ◊ The different grounds may be at different potential, drawing current through the shield. ◊ The voltage reference may not be at 0 Volts Metallic Shield Current design 2.24 Ground Potential Difference Ground potential differences cause data communications problems. Here is an example: System 1 System 2 Building frame Building frame ◊ ground resistance design 2.25 Reducing Ground Problems ◊ Maintain separate grounds for those devices that may create noise problems. ◊ Maintain low impedance ground connections and conductors. ◊ Maintain separation from sources of EMI/RFI and ESD. ◊ Do not create ground loops (connections between grounds) near or through your circuits. ◊ Ground cable at one end only. Ground the cable to chassis ground, not signal ground. ◊ Maintain proper power supply and circuit board techniques. design 2.26 Circuit Isolation design 2.27 Circuit Isolation ◊ Loads may affect other circuits. V Vcc Motor Noise design 2.28 Isolation ◊ Exposing digital circuits to unwanted electrical signals (noise) can cause erratic operation or damage the devices. ◊ Noise may be transferred between circuits. ◊ The source of these undesirable electrical signals can be: ◊ the input or output devices to which the digital circuit is attached ◊ the digital circuit itself provides unwanted signals to the input and output devices ◊ the voltage reference may be to blame. design 2.29 Electrical Isolation ◊ Electrical isolation is a means of preventing a direct electrical path between devices. ◊ Isolating the grounds prevents ground line noise from interfering with digital devices. ◊ Ground line noise can be caused by differences in grounding, and by switching high current devices. ◊ Electrical isolation may be used for safety reasons, or if the reference voltages are different. design 2.30 Isolation ◊ One method for isolation is to use common grounds instead of earth or chassis ground for signal reference. ◊ Another is to use independent power supplies. design 2.31 Optical Isolator ◊ Optical isolator utilizes light as the transmission medium between 2 digital devices. ◊ The device usually consists of a circuit that converts digital signals to light pulses, and a circuit that converts the light received back to a digital signal. TX RX design 2.32 Optical Isolator Opto-isolators frequently contain both the transmitter and receiver in the same unit. design 2.33 Decoupling design 2.34 Decoupling ◊ Coupling happens when two devices share the same power source. This can present a problem, as noise on the power supply may affect digital logic function. ◊ Decoupling describes the act of removing the problems associated with noise on the power source. design 2.35 Power Supply Noise ◊ Power supply noise is often created by the logic switching of devices where current requirements may change abruptly. ◊ This noise appears on both the VCC/VDD and the ground conductor buses. ◊ The noise may cause some devices to fault on their logic. Typical Power Supply Noise design 2.36 Power Supply Noise ◊ Solutions for power supply noise: ◊ Add decoupling or bypass capacitors between VCC/VDD and ground. Place these capacitors as close to the offending ICs as possible. Recommended is one 0.01F for each IC. ◊ Add a 0.1F capacitor for every 5 ICs. ◊ Maintain short power supply buses. ◊ Ensure the power supply buses have low resistance. ◊ Maintain a separate power supply for high-current devices. ◊ Use a properly designed power supply with input and output capacitors. design 2.37 Other sources for Noise design 2.38 Transmission Line Noise ◊ If the transmission line are not properly impedance-matched to the system, some signal reflection will occur. ◊ On some cables, if a terminator is not present, the signal will reflect from the end of the conductor, resulting in ringing. design 2.39 Switching Loads design 2.40 Transistors ◊ Transistors are frequently used to switch devices with higher current or voltage requirements. ◊ In digital electronics, transistors are used as switches. For the example below, a logic high will turn it on; a low will turn it off. V Load design 2.41 Relays ◊ Relays are used to isolate one electrical circuit from another and also allow circuit ground isolation. ◊ Typically, relay are used to switch relatively higher voltages and currents with relatively smaller signals. High V or High I Circuit N Low V, Low I Circuit S design 2.42 Relays ◊ Relays used in digital electronics vary considerably in physical construction, electrical specifications and application. ◊ Types include: ◊ Solid State Relays (SSR’s) that do not rely on mechanical motion but utilize semiconductor components to make electrical connection. These are becoming increasingly popular. ◊ Electro-Magnetic Relays (EMR’s) that utilize an energized coil to create motion and physically pull contacts together. Very common device that has many applications. design 2.43 Relays ◊ Special consideration must be taken into account when using relays: ◊ EMR coils produce a high voltage at the moment they are de-energized. Diodes often prevent these voltages from appearing on the circuit. ◊ Typically, a transistor or other semiconductor switch is used to isolate the digital circuit from the relay. Most digital circuits cannot be directly connected to the coil of a relay without a driver. ◊ Relays are much slower than semiconductor components and are not particularly well suited to a high degree of switching. Relays often bounce. ◊ Mechanical relays do wear and may be susceptible to dust, humidity and temperature effects. Resistance may be present at the contact points. design 2.44 Signal Conditioning design 2.45 Signal Requirements ◊ Digital devices rely on digital signals that: ◊ ◊ ◊ ◊ ◊ have proper logic voltage levels are able to source or sink the appropriate current have the correct timing have fast rising and falling edges are free from noise design 2.46 Signal Conditioning ◊ Signal conditioning is a means of ensuring that all digital signal problems may be corrected. ◊ Digital signals may need to be re-constructed, filtered or amplified in order to satisfy the requirements of the digital logic devices. design 2.47 Signal Conditioning and Passive Devices ◊ A passive device is one that requires no external power source. ◊ Passive devices include RC circuits. These circuits may: ◊ ◊ ◊ ◊ filter noise add a delay create an edge create a set or reset state on power-up design 2.48 Filters ◊ Which of these circuits will filter high frequencies, and which will filter low frequencies? Use EWB to determine the operation of these circuits. design 2.49 Filters ◊ What is the operational response of this circuit? How can these filters be used to condition signals? design 2.50 Delay ◊ Every electronic device and digital IC adds delay to the signal. ◊ In some instances, it may be necessary to add delay to a circuit to ensure signals arrive with the proper timing. ◊ An example of a circuit delay requirement is the oscilloscope. design 2.51 Example: Oscilloscope Delay ◊ The oscilloscope processes the incoming signal with the vertical section. This circuit provides an output to the horizontal section and to the CRT. ◊ The horizontal section has extra circuitry and therefore adds delay to the signal. A matching delay circuit is required for the vertical section so that both the horizontal and the vertical components of the signal are rejoined at the CRT with the same timing. design 2.52 Delay circuit ◊ A digital delay circuit may make use of an external RC network, as in the figure below. ◊ Short-delay circuits utilize an array of selectable, cascaded gates. Time Delay Circuit design 2.53 Power-up state ◊ When power is initially applied to an IC, sometimes the output isn’t in a predictable or desired state. A set or reset may need to be applied immediately on power-up. The RC circuit below accomplishes a reset: Describe how this circuit operates. design 2.54 Signal Conditioning (active) ◊ Active devices can also accomplish signal conditioning by regenerating a digital signal. Regenerating VO Regenerating VO and edges design 2.55 Unused I/O design 2.56 Handling Unused I/O’s ◊ Unused inputs and outputs must be handled correctly. Problems with incorrectly handling input or outputs may result in: ◊ Faulty logic ◊ Damage to the IC ◊ Higher power supply requirements design 2.57 TTL ◊ Open inputs to most TTL devices float high, however this is not a certainty. ◊ logic gates may see a low input if Vcc abruptly changes its voltage level, even just fractions of a volt, as may happen when other devices on the circuit are switching. design 2.58 TTL ◊ Tie the inputs so that the device will utilize the least amount of power. ◊ IIH usually requires less current than IIL tie inputs high. ◊ ICCH is usually less than ICCL tie the inputs so the output is a logic level high. design 2.59 CMOS ◊ CMOS devices are sensitive to voltage levels. Unlike TTL, floating CMOS inputs are neither high nor low. ◊ Inputs to CMOS require very little current to provide a logic level. The leads can pick up static charges from the environment, or have voltages induced from adjacent conductors and circuits. These charges can then adversely affect the output of a device. design 2.60 CMOS ◊ Feedback (output being fed back to the input) can cause the device to begin oscillating, causing the IC to overheat and the power requirements to increase. ◊ CMOS devices are static sensitive. A static charge on one of the open inputs can create breakdown of the entire device. ◊ Tie any unused inputs to VDD or VSS. design 2.61 Electro-Static Discharge (ESD) design 2.62 ESD ◊ Electro-Static Discharge may cause damage to digital components, especially to CMOS-type devices. ◊ Proper handling techniques are discussed by the manufacturers. These include: ◊ ◊ ◊ ◊ ◊ Grounded work area (ground mats) Grounded personnel (ground straps) Proper handling techniques (circuit cards) Proper packaging (static-free foam, bags, etc) Proper procedures (power-up phases, unused inputs) design 2.63 Review Questions ◊ Explain inductance. ◊ What is the most important rule in regards to grounding? ◊ What problems does noise cause with electrical signals? ◊ How can we help isolate the source of noise? ◊ More review questions in class! design 2.64 END ©Paul R. Godin prgodin @ gmail.com design 2.65