10. Appendix ... Module Design Considerations When Using DC To DC Converters 10.1. Introduction to the use of DC-DC Converters in Distributed Power Systems The proposed VME-P extensions encourage the use of a distributed power architecture. A 48 volt bulk power supply is distributed across the back plane. DC/DC converters may be used on the individual VME modules to provide locally generated voltages, derived from the bulk 48 volt supply as needed. The use of DC/DC converters requires careful attention to fundamental engineering practices. Noise and heat are the two most significant problems that must be solved. Improper application can result in high output noise, poor load regulation, excessive temperature rises and power bus instability. The requirements of the application must be well understood before selecting and implementing a converter. The variety of commercial converters is broad enough that a close match can be found for nearly any application. Inadequate engineering at the beginning of the design process will result in poor converter selection requiring eventual engineering compromises such as added input and output filtering, after-the-fact thermal management or reduced performance specifications. Some guidelines for implementing DC/DC converters are provided in this appendix. 10.1.1. Single Source for Different Voltages Many DC-DC converters are available with multiple output voltages. Careful choice of the correct converter may provide all the necessary voltages in a single package, minimizing layout, noise and thermal management problems. 10.1.2. Multiple Sources for Same Voltages Use of multiple sources for the same voltage introduces internal loop currents from the inevitable mismatches between various drivers of the same voltage. These loop currents flow from one source to the other through the very small resistance of the power supply connection traces, and can dissipate surprisingly large amounts of power. Recommendation Unless absolutely required by the application, do not use multiple sources for the same voltage. Specifically avoid the situation where a DC-DC converter is used in parallel with a bulk power supply directly taken from the backplane. If forced in to this situation, either implement multiple, independent power planes or use multiple DC-DC converters that are specifically designed to be slaved to each other to minimize output voltage differences. 10.1.3. Common Reference Points (Grounding) The concern here is to properly arrange the supply and return leads to the loads to minimize unwanted interactions between circuits. Incorrect management of the power supply and return paths is a common problem associated with using DC/DC converters. It has little to do with grounding, but grounding is the term most often (mis)used by engineers to describe the arrangement of the return path of the output of a power supply. Grounding, properly defined, is the connection of a local reference point within a circuit to an external reference point, which can withstand large currents without significant change in potential, which is common to all subsystems within a larger system. Tying the power supply return of a backplane to an iron stake driven four feet in to the earth is grounding; connecting an input of an op-amp to a local power return plane is not. Recommendation Keep loop areas small. Pay attention to where the currents will flow. A common error is to provide physically disparate paths for the flow of current from supply to load, and from load to supply. Noise picked up by the system from external interference, the noise floor of the circuit itself and the noise injected by the system in to adjacent systems are all directly proportional to the area enclosed by the complete loop. Recommendation Planes should be used for power distribution. They provide low inductance paths for high speed digital signals. Heavy loads should be placed near the converter and lighter loads placed farther away. Recommendation Provide separate analog and digital returns and tie the two together only once. The supply and return planes for analog and digital circuits should be kept separated except to connect the returns together at a single point as close to the DC/DC converter common as possible. Failure to do this will result in degraded performance by allowing DC voltage levels and noise from one circuit to interact with another circuit. Recommendation Analog circuits should be provided with a single analog reference. This arrangement is often called a star ground system. All analog signals should return to the reference via separate pc board tracks. If individual returns are impractical due to layout density, an analog return plane should be used which is separate from the digital return plane. Observation Designers should be aware that signal return currents will, whenever possible, flow along the same path (but, of course, upon their own layer) as the signal supply current. Return current planes should fully underlie the respective supply signals to insure that return currents have a direct and simple path back to the power supply to minimize noise. If the return current cannot follow the path of the supply current, a loop of increased inductance will be formed which will increase noise and decrease noise margins. Observation Further reduction in coupling between analog currents and digital currents can in some cases be achieved by placing a small series inductance between the analog return and the digital return such that digital noise sees a lower impedance to the backplane than to the adjacent analog return plane. 10.2. Protection of Input Circuits 10.2.1. Overcurrent Protection (Use of Fuses) The concern with input fusing is to provide protection of the PC board and connectors in the event of a catastrophic failure of the converter resulting in excessive current draw and overheating. The concern with output short circuit protection is to protect the DC-DC converter and the rest of the printed circuit board from physical damage in the event of a sustained over current condition on the output of the DC-DC converter. Some converters are already provided with inherent short circuit protection and some are not. Consult the manufacturer's data sheet for individual device specifications. Recommendation Use devices with internal output short circuit protection wherever possible, rather than fusing the output. Output fusing results in unecessary voltage drops. Do not use converters with excess output current capacity. Recommendation Inputs to DC/DC converters should be fused with a fast-blow type fuse rated at no less than 150% and no more than 200% of the full load input current to the converter. Observation Fusing at 150% of rated load is usually necessary to prevent fuse action due to normal inrush currents at the time power is applied. Fusing in excess of 200% is overkill and may result in a safety hazard. The use of slow-blow fuses may result in the device being deemed unacceptable by UL or other agencies. In any event, the I2T rating of the fuse must be selected to support the inrush current of the selected DC-DC converter. 10.2.2. Transient Voltage Protection The concern with transient protection is to protect the DC-DC converter in the event of an over voltage condition on the bulk power distribution line. Some converters are already provided with inherent transient protection and some are not. Consult the manufacturer's data sheet for individual device specifications. Recommendation An avalanche zener type transient suppresser should be used across the inputs of DC/DC converters unless already built into the device. The suppresser clamps the input to a safe level in the event of a power line transient. The suppresser should be between the converter and the fuse such that a sustained over voltage condition will open the fuse. The voltage rating of the suppresser should be above the nominal rating of the bulk power supply and below the maximum input voltage of the converter as specified in the manufacturer's data. 10.3. Conducted Emissions & Conducted Susceptibility The concern is to avoid interfering with nearby electronics by conducting noise generated inside the converter back into the bulk power supply. Conducted interference will affect nearby circuitry. A decision must be made as to how much interference the application will be allowed to produce. Any circuit which emits noise by conduction is probably also susceptable to noise which arrives by conduction. Vendor's data sheets usually provide information on emissions and immunity; often plotting emissions vs frequency on a graph against international standards curves. 10.3.1. Applicable Conducted Emission and Susceptibilty Standards There are several standards that may be consulted for information. The Federal Communications Commission (FCC) in the United States and Verband Deutscher Elektrotechniker (VDE) in Europe set standards for commercial products. The applicable regulations are Title 47 CFR1 Part 15 Subpart J; and VDE 0871, 0875 and 0878. In addition, the U.S. Department of Defense issues very comprehensive standards (MIL-STD-461B); and guidelines (MIL-STD-462) for making measurements. MIL-STD-461B establishes limits for conducted and radiated emissions well below any of the commercial standards referred to above. The military standards document may be used as a guideline for designing interference-free, nonmilitary equiment. The VXIbus specification provides limits and test guidelines for conducted emissions and susceptibility applicable to instrumentation buses and VME form factors. Thus it may be an acceptable guideline to use. Other regulatory agencies whose standards may need to be consulted include the Canadian Standards Association (CSA), the British Standards Institution (BSI), the British Approvals Board for Telecommunications (BABT), the International Special Committee on Radio Interference (CISPR), or the International Electrotechnical Commission (IEC). The following graph shows noise level standards issued by the FCC and VDE. The CE03 curve, which is the MIL-STD-461B conducted emissions, power and interconnecting leads, 0.015 to 50 MHz limit, should not be directly compared to the FCC and VDE curves since the test procedures are quite different; but is included here for general information purposes. 10.3.2. Use of Input Filter Networks to Reduce Conducted Emissions & Susceptibility to Conducted Noise Input filters can be used to reduce the emission and susceptibility of conducted noise by a converter. The concern here is for suppressing conducted emissions between the bulk power supply lines and the converter. Input filtering to the DC-DC converter is useful not only to provide cleaner inputs to the converter itself but, more importantly, to limit the amount of noise fed back to other boards in the system through the backplane. Most converters are internally provided with input filters to reduce reflected ripple current. If a converter can not be found with sufficient input filtering, then external filter components may be added. Each manufacturer provides guidelines suitable for their products. Some manufacturers supply EMI filter modules which can be added to reduce input noise to well below MIL Spec standards. Recommendation VME-P boards which use DC-DC converter modules to supply alternative voltages should be filtered on the input side of the converter in order to reduce conducted noise. The filter may be built into the converter or added on externally. Observation Input filters may be as simple as ceramic capacitors across the inputs and/or to the input return[JTA1][JTA2]. Refer to manufacturer's recommendations. Observation An input balun filter may be required in the input of DC/DC converters to suppress common mode noise generated by the filter and conducted back into the power line. The balun presents a high inductance to the common mode noise but virtually none to the differential mode current. 10.4. Radiated Emissions & Radiated Susceptibility The concern here is for suppressing radiated emissions and susceptibility to radiated emissions. Radiated emissions can be propogated as magnetic coupling, electric field coupling or electromagnetic radiation. Emissions may have differential-mode or common-mode origins. Differential-mode emissions result from current flowing around loops formed by signal and return conductors within circuits. Common-mode emissions result from voltage drops in circuits that cause some parts of the circuits to be at common-mode potentials above "ground". For example, voltage drops in the digital logic ground system. Electromagnetic radiation is generally not a factor. Magnetic and electric field coupling, however, are issues that require attention. Most converters are designed with careful attention to magnetic design to minimize loop area; and utilize shielded packaging to block electric field coupling. Low cost, low power converters of less than 5 watts may have plastic cases and thus must be carefully evaluated for electric field emissions. Higher power converters are designed with metal cases both for thermal performance and shielding. Metal cases may be either five sided or six sided. Five sided cases require a power return plane beneath the converter to complete the shield. All converters require connection of the shield to the input power return or as designated by the manufacturer. Manufacturer's data must be examined to ascertain effectiveness of their shielding. 10.4.1. Applicable Emission and Susceptibility Standards The same organizations that provide standards for conducted emissions and susceptibility, also provide standards for radiated emissions and susceptibility. Each vendor provides technical specifications regarding their products. As in the case of conducted emissions, the MIL-STD provides the most comprehensive standards and test procedures; and can be used to design interference free non-military equipment. The VXIbus specification provides limits and test guidelines for radiated emissions and susceptibility applicable to instrumentation buses and VME form factors. Thus it may be an acceptable guideline to use. 10.4.2. Use of Shielding to Reduce Radiated Emissions & Susceptibility to Radiated Noise Recommendation DC-DC converters used in VME boards with analog circuitry should have fully enclosing metal cases (six sided). These cases should be connected, where possible, to the return of the input power supply at the point where the return is connected to the converter (not to ground) to minimize radiated noise. Observation Further reductions in radiated noise may be achieved by designing a side panel for the entire circuit board which is itself conductive and connected to ground. In the ideal case, a full Faraday cage shield is made from the board return plane, the side panel, the front panel, plus top & bottom ventilated shields, as is described in the NIM (DOE/ER-0457T) and VXI standards. Observation IEEE Specification 1101-1987 states: "Clarify board specifications to allow and designate areas on the board outline that would provide connection to the chassis earth ground point on three edges of the board. A board that has grounding only on one edge can act as a quarter wave antenna when it is radiating. A board that is grounded at two opposite edges would radiate as a half wave antenna, tuned to twice the frequency of the quarter wave mode. The mounting pads that attach the front panels can be used as grounding points. The edges of the board that go into the board guide could be used as additional grounding points. The circuit board manufacturer may either directly connect logic return to chassis ground, or may connect it through a capacitor, which would allow a dc potential between ground and logic return." However, this approach to minimizing radiated emissions may in fact increase noise due to magnetic emissions by creating new ground loops by producing multiple return paths to ground. The designer must be aware of the type of emissions critical to system performance in order to select the correct set of noise reduction techniques. In many high-precision analog systems the magnetic and electrostatic forms of noise coupling far outweigh the radiative coupling between printed circuit boards. 10.5. Noise and Ripple in Output Circuits The concern here is to minimize both differential and common mode noise on the output of the converter in order to avoid interference with circuitry being powered by the converter. Output filtering may be necessary in addition to proper decoupling [JTA3]in order to reduce harmonic noise coming from the DC-DC converter, which can severely degrade analog circuit performance. Local decoupling capacitance, correctly applied, can minimize the amount of noise that a particular digital or analog component feeds back in to the board by providing a source of small, high-frequency current that responds instantaneously to local changes in demand - which a converter located some inches away from the load cannot do. Conversely, filtering is used to eliminate unwanted harmonics such as the switching frequency of the DC-DC converter from entering the board in the first place. Analog components typically have a power supply noise rejection ratio which is inversely proportional to the frequency of the noise. An OP Amp that may have 100 dB of power supply rejection ratio at DC, may degrade to 20 dB at 100 KHz. Manufacturer's recommendations should be closely followed with respect to reducing output noise. 10.5.1. Differential and Common Mode Noise Effects The effects of noise on digital circuits is decreased noise margins and possibly false switching. The effects on analog circuits is decreased resolution and the addition of offsets to the signals. Differential noise (noise on the power or signal lead with respect to the return lead) can be readily filtered and controlled. However, common mode noise (noise common to the power and return leads with respect to earth ground) is difficult to control. For example, an analog circuit may have a sensitive transducer providing input from a remote location. If the analog circuit itself is bouncing around on a common noise platform with respect to the earth ground, then precision and sensitivity may be difficult to obtain. 10.5.2. Determining Acceptable Noise Specifications The amount of noise which a circuit will tolerate depends on the noise margins of the digital circuits and the required resolution of the analog circuits. Most of the common mode noise in a crate is due to capacitive coupling in the power supplies and backplane. 10.5.3. Considerations for Sensitive Analog Circuits Significant amounts of common mode noise will be found at higher frequencies where the common mode rejection of analog amplifiers is inadequate. In addition, high frequency noise is often rectified by active and passive components resulting in signal offsets. Common mode noise can be reduced by the use of differential amplifiers, DC-DC converters and filtering. However, the circuits eventually must be connected to a common reference. They can not be left floating. There is no clean earth ground on the VME backplane. The analog power returns are eventually connected to a common power return which is bouncing with capacitively coupled noise finding its way back to its source. The best solution is to reduce the noise at its source. Prevent it in the first place. Pay carefull attention to reducing loop areas and lowering the inductance of power return paths. Pay carefull attention to lowering the system noise by proper implementation of the VME crate power supplies and harnesses; and grounding of the rack. Observation The total noise level should be kept below 1 LSB to preserve the accuracy of the system. Recommendation Input signals should, whenever possible, be differential. Noise can be either differential or common mode. Iinput filtering networks such as LC circuits and differential receivers may be used to obtain a higher S/N ratio for differential inputs. In some cases, common mode noise may be reduced with simple baluns. Recommendation Sensitive analog circuits should not be placed in the upper area of the printed circuit board near P1 as this is the most likely location of the DC-DC converter. Recommendation Place all circuits susceptible to radiated emissions away from any converters (and backplanes and board guides). The preferred position is near the front panel. 10.5.4. Filtering of Differential and Common Mode Noise in DC-DC Converter Output Circuits Observation All vendors provide applications literature which extensively discusses the control of output noise. The applications literature is far more comprehensive than these guidelines. Observation Converter manufacturers frequently offer external filters for customers who require very low noise. These filters are typically better than one can design in-house. Observation Differential mode output noise filtering may often be obtained by the simple addition of an external capacitor across the output. Only high quality, low equivalent series resistance (ESR) ceramic capacitors should be used. They should be connected from as near the output pin as possible to as near the common pin as possible. The capacitor should be of low impedance at the frequency of interest. Different capacitor types become self-resonant at different frequencies due to lead inductance. At the switching frequency of most DC-DC converters, most electrolytic and tantalum capacitors are outside of their useful range. Observation Additional differential mode output noise filtering may be obtained by the addition of a 2nd order LC filter in the output. This filter is more predictable than the simple capacitor discussed above and will work better at high frequencies. However, the inductor must carry full load ampere turns without core saturation. Care must be taken to ensure that the resonant frequency of the filter is outside of the converter's control loop bandwidth. Use the minimum inductance possible in order to have negligible effect on the load regulation due to effective series resistance (ESR). Observation A balun type filter may be used for common mode noise, and the leakage inductance of the balun used with differential mode capacitors as a low impedance pi filter to attenuate differential mode noise. By properly phasing the inductors in the balun, the net differential mode core flux will always be zero in the balun but the common mode flux will be additive presenting a high impedance to common mode currents. Make sure the common mode resonant frequencies are 1/2 or less of the fundamental noise component frequency.[JTA4] Observation Adequate common mode noise attenuation can be often obtained by merely connecting capacitors from the output leads to the case. This requires that the case be correctly connected to the power input return and the lead length of the capacitors kept as short as possible. 10.6. Load Decoupling Capacitors are placed across integrated circuits for both decoupling and bypassing purposes. High speed digital circuits draw current impulses from the power supply leads when they switch. The coupling of this noise into the power supply plane must be minimized or it will affect the operation of other circuits. Decoupling capacitors can supply the current for these impulses locally at the IC. Thus the impulse currents do not flow through the inductance of the power supply planes and do not contribute to the noise. Op amp circuits may have power supply rejection ratios of 100 dB at DC; but may degrade to 20 dB at 100 KHz (and to 0 dB at 1 MHz!). The noise on the power supply leads needs to be bypassed around the analog ICs to avoid degrading the operation of the circuits. Bypassing capacitors provide this function. 10.6.1. Reduction of Load Generated Noise Recommendation Designers should provide decoupling capacitors at each load (integrated circuit) to minimize noise spikes generated across the inductance of the power leads by rapidly changing currents in high speed analog and digital circuits. Care must be taken to choose proper components for decoupling. 10.6.2. Considerations for High-Speed Digital Circuits Observation Decoupling capacitors must be as close to the power/ground pins as possible with absolutely minimal lead length. Even the shortest of lead length or PCB trace will provide inductance that lowers the effective frequency of the capacitor - perhaps negating the capacitance completely. 10.6.3. Considerations for Sensitive Analog Circuits Observation The correct placement of bypassing capacitors across analog circuits is not always immediately apparent. The idea is to provide the shortest return path for noise currents to their sources. Recommendation Analog signals should be buffered upon input to the board to reduce noise feedback from the board back to the sensor. 10.6.4. Component Selection for Proper Decoupling Over Wide Frequency Ranges Observation Monolithic ceramic capacitors have very low series inductance. They are ideal for high frequency decoupling. Disc ceramic capacitors, although less expensive, are sometimes quite inductive. To ensure that an analog circuit is adequately decoupled and bypassed at both high and low frequencies is to use a tantalum bead capacitor in parallel with a monolithic ceramic one. The combination will have high capacitance and will remain capacitive at VHF frequencies. 10.7. PC Board Trace Design Observation The minimum width and thickness of printed circuit board (PCB) traces are normally chosen on the basis of current carrying capacity and allowable temperature rise. However, traces to and from DC/DC converters must also be chosen to have adequate cross sectional area to minimize voltage drops. Voltage drops in the traces can have a substantial effect on regulation, output voltage accuracy and proper start up. Recommendation As a guideline for designing PCB traces to supply power to and from DC/DC converters used on 9U X 400mm VME-P size boards, design the load conductors to contribute less than 0.1% voltage drop at rated current. A simple calculation can be made to determine minimum PCB trace width (W) by combining the equations for cross sectional area (A), resistance of copper (R) and voltage drop across the trace (V): Where I = Current in through the trace in amps R = Resistance of the trace in ohms = Resistivity of the copper in ohms/cm (1.72 X 10-6 ohms/cm at 25C) L = Length of the trace in cm = Temperature coefficient of resistivity (0.0039/C for copper) V = Voltage drop across the trace in volts. A = Cross-sectional area of trace in cm2 T = Temperature rise above 25 C W = Width of the trace in cm H = Height of the trace (thickness) in cm (1 oz/ft2=.0036cm.; 2oz/ft2=.0071cm.) For example, a DC/DC converter must supply 5 volts at 8 amps to a load located 5 cm from the converter. The trace is thus 10 cm long (5cm supply and 5cm return). The maximum voltage drop across the trace should be 0.1% of 5 volts which is .005 volts. If 2 oz/ft2 copper is used on the PCB and the maximum operating temperature is expected to be 50 C, then the minimum trace width would be as follows. A power plane is obviously needed in this case: 10.7.1. Design Considerations of Power and Return Planes Power planes and return planes should completely underlie all components which connect to that power source. Planes must be designed such that no internal blockages exist to the flow of current which would result in internal current loops. Component layout in the plane should provide unrestricted current flow to those devices which consume the greatest amount of instantaneous current to minimize the trace resistance and inductance from the power source to the load. 10.7.2. Solid Planes Versus Screen Planes Solid planes are preferable to screen planes. Return signals always follow the path of least impedance back to their source. The path of least impedance at high frequencies is the path of minimum loop area. Thus the return signal prefers to stay as close as possible to its outgoing signal. 10.7.3. Use of Stripline Layout Techniques to Control Impedance and Noise Stripline layout techniques are one of the most powerful tools available to the printed circuit board designer, providing the dual benefit of carefully controlled impedance and increased noise immunity. Most circuit boards are designed with the power and return planes on adjacent inner layers of the board to maximize the distributed capacitance between the two plane layers. Although a laudable practice, this places the signal traces on the outer layers of the board where they are more susceptible to externally induced noise. The electric field of the outer layer conductors is also nonuniform as the relative permittivity of the printed circuit board is different from that of air. The permittivity difference, plus the geometric effects of 'wire over plane' construction, lead to increased trace-to-trace coupling of signals. In boards with more than four layers, the outer signal layers also have a different characteristic impedance than the inner circuit layers, resulting in impedance discontinuities at each via which distort analog signals. A better technique is to use stripline layout where the signal carrying layer is sandwiched between the power and return planes. In this geometry the field lines are much more uniform and trace-to-trace coupling is greatly reduced. Since the traces are well covered by planes that present a low impedance to external interference, electrostatically coupled noise is also reduced. Recommendation Stripline printed circuit board techniques, where the conducting trace is on an inner layer between two planes, is recommended to control trace impedance and reduce noise emissions. 10.8. Thermal Considerations Observation The concern here is to understand the build up of heat in the converter due to it's less than perfect efficiency and to properly design for its removal. Thermal considerations can well be the most important aspect of using DC/DC converters. The wide range of application possibilities precludes a simplistic set of guidelines. Each situation requires thermal analysis. A converter in still air with no other conductive cooling paths will have a thermal resistance as stated by the manufacturer. In situations where this will result in exceeding the allowable base plate temperature of the converter or will cause excessive localized temperatures on the PCB, additional measures to remove heat will be required. The simplest measure is to thermally attach the converter to the PCB to provide a conductive thermal path. Next, the contribution of airflow over the converter to the thermal resistance must be analyzed. If the situation is still not acceptable, the final measure is to provide additional heat sinking to the converter as determined by manufacturer's recommendations. The use of heat sinks may, but not always, increase the component height of the converter to the point where a double width module must be used. Observation Manufacturer's data sheets and application notes should be carefully considered with regard to thermal characteristics. Recommendation Efficiency curves should be carefully examined and a converter selected to be as efficient as possible at the expected operating current. This reduces losses in the converter and radically modifies the approach required for cooling. This not only improves output voltage regulation but also minimizes the total power available in the short-circuit condition. 10.8.1. Cooling DC-DC Converters The most important advice here is to follow the manufacturer's recommendations. Recommendation Air flow past the DC/DC converter should not be restricted. Recommendation High heat generating devices should be placed toward the top of the module. Recommendation Fan outputs should be derated by 60% to 80% when calculating airflow across a DC/DC converter to compensate for the back pressure and turbulence encountered in the typical application. Observation Derating curves supplied by DC/DC converter manufacturers specify velocity of air in linear feet per minute (LFM) of air flow across the converter. Fans are specified to move a volume of air in cubic feet per minute (CFM). To convert CFM to LFM, the equation is: LFM = CFM/Area; where Area is the cross sectional area through which the air will flow. 10.8.2. Using Heatsinks Observation Practical application of high density DC/DC converters larger than about 5 watts may require the addition of a heat sink or forced air flow. Recommendation If heat sinks are used, they should be oriented such that the fins are parallel to the airflow (e.g., vertical). Recommendation Since the majority of the heat is transferred through the bottom mounting plate and leads of the DC-DC converter, a copper plane of the printed circuit board may be used as a heat sink provided care is exercised to provide required electrical isolation. 10.8.3. Considerations Regarding Adjacent Circuitry Much of the heat dissipated by a converter will be via the mounting pins and bottom plate. The surrounding area will be heated. Consider this elevated temperature when placing other circuitry in the area around the converter. For example, consider the temperature coefficients of discreet components and the offset voltages and current of active components. 1Code of Federal Regulations Page: 5 [JTA1]Do you mean ground here or do you mean bulk power return? Page: 5 [JTA2]Shouldn't we also mention things like series inductors or pi filters? Page: 7 [JTA3]I would suggest that since people in general don't know the difference between return and ground, they probably also don't know the difference between filtering and decoupling. We should probably educate them. Page: 9 [JTA4]Obviously I'm not clever enough, as I need a picture for this to be perfectly clear. H950519B.DOC 1