《模拟电路设计》（英文版）SECTION 9 HARDWARE DESIGN TECHNIQUES.pdf_大学文库

section 9 HARDWARE DESIGN TECHNIQUES Walt Kester, James Bryant, Walt Jung Adolfo garcia, John McDonald PROTOTYPING AND SIMULATING ANALOG CIRCUITS walt Kester, James Bryant the analog designer has acquired in the last decade or so, there is equally no doub While there is no doubt that computer analysis is one of the most valuable tools th initial test circuit or"bread board"is not correctly constructed, It may suffer from that analog circuit models are not perfect and must be verified with hardware. If the malfunctions which are not the fault of the design but of the physical structure of the breadboard itself. This section considers the art of successful breadboarding of high performance analog circuits Real electronic circuits contain many"components "which were not present in the circuit diagram, but which are there because of the physical properties of conductors, circuit boards, IC packages, etc. These components are difficult, if not impossible, to incorporate into computer modeling software, and yet they have substantial effects on circuit performance at high resolutions, or high frequencies, or It is therefore inadvisable to use SPice modeling or similar software to predict the ultimate performance of such high performance analog circuits. After modeling is complete, the performance must be verified by experiment. This is not to say that sPice modeling is valueless- far from it. Most modern high performance analog circuits could never have been developed without the aid of SPICE and similar programs, but it must be remembered that such simulations are only as good as the models used, and these models are not perfect. We have seen the effects of parasitic components arising from the conductors, insulators and components on the PCB, but it is also necessary to appreciate that the models used within SPice simulations are not perfect models Consider an operational amplifier. It contains some 20-40 transistors, almost as many resistors, and a few capacitors. A complete SPiCe model will contain all these components, and probably a few of the more important parasitic capacitances and spurious diodes formed by the diffusions in the op-amp chip This is the model that the designer will have used to evaluate the device during his design. In simulations such a model will behave very like the actual op-amp, but not exactly

2 SECTION 9 HARDWARE DESIGN TECHNIQUES Walt Kester, James Bryant, Walt Jung, Adolfo Garcia, John McDonald PROTOTYPING AND SIMULATING ANALOG CIRCUITS Walt Kester, James Bryant While there is no doubt that computer analysis is one of the most valuable tools that the analog designer has acquired in the last decade or so, there is equally no doubt that analog circuit models are not perfect and must be verified with hardware. If the initial test circuit or "breadboard" is not correctly constructed, it may suffer from malfunctions which are not the fault of the design but of the physical structure of the breadboard itself. This section considers the art of successful breadboarding of high performance analog circuits. Real electronic circuits contain many "components" which were not present in the circuit diagram, but which are there because of the physical properties of conductors, circuit boards, IC packages, etc. These components are difficult, if not impossible, to incorporate into computer modeling software, and yet they have substantial effects on circuit performance at high resolutions, or high frequencies, or both. It is therefore inadvisable to use SPICE modeling or similar software to predict the ultimate performance of such high performance analog circuits. After modeling is complete, the performance must be verified by experiment. This is not to say that SPICE modeling is valueless - far from it. Most modern high performance analog circuits could never have been developed without the aid of SPICE and similar programs, but it must be remembered that such simulations are only as good as the models used, and these models are not perfect. We have seen the effects of parasitic components arising from the conductors, insulators and components on the PCB, but it is also necessary to appreciate that the models used within SPICE simulations are not perfect models. Consider an operational amplifier. It contains some 20-40 transistors, almost as many resistors, and a few capacitors. A complete SPICE model will contain all these components, and probably a few of the more important parasitic capacitances and spurious diodes formed by the diffusions in the op-amp chip. This is the model that the designer will have used to evaluate the device during his design. In simulations, such a model will behave very like the actual op-amp, but not exactly

SPICE MODELING SPICE modeling is a powerful tool for predicting the performance of analog circuits Analog Devices provides macromodels for over 450 ICs HOWEVER Models omit real-life effects No model can simulate all the parasitic effects of discrete components and a PcB layout THEREFORE Prototypes must be built and proven before production. Figure 9.1 However, this model is not published, as it contains too much information which would be of use to other semiconductor companies who might wish to copy or improve on the design. It would also take far too long for a simulation of a system containing such models of a number of op-amps to reach a useful result. For these and other reasons, the SPiCe models of analog circuits published by manufacturers or software companies are"macro"models, which simulate the major features of the component, but lack some of the fine detail. Consequently, SPicE modeling does not always reproduce the exact performance of a circuit and should always be verified experimentally. The basic principle of a breadboard is that it is a temporary structure, designed to test the performance of a circuit or system, and must therefore be easy to modify. There are many commercial breadboarding systems, but almost all of them are designed to facilitate the breadboarding of digital systems, where noise immunities are hundreds of millivolts or more. (We shall discuss the exception to this generality later. Non copper-clad Matrix board (Vectorboard, etc ) wire-wrap and plug-in breadboard systems(Bimboard, etc. ) are, without exception, unsuitable for high performance or high frequency analog breadboarding. They have too high resistance inductance, and capacitance. Even the use of standard IC sockets is inadvisable PRACTICAL BREADBOARDING TECHNIQUES The most practical technique for analog breadboarding uses a copper-clad board as a ground plane. The ground pins of the components are soldered directly to the plane and the other components are wired together above it. This allows hf decoupling paths to be very short indeed All lead lengths should be as short as possible, and signal routing should separate high-level and low-level signals. Ideally the layout should be similar to the layout to be used on the final PCB. This approach is often referred to as"deadbug"because the I Cs are often mounted upside down with their

3 SPICE MODELING SPICE modeling is a powerful tool for predicting the performance of analog circuits. Analog Devices provides macromodels for over 450 ICs HOWEVER Models omit real-life effects No model can simulate all the parasitic effects of discrete components and a PCB layout THEREFORE Prototypes must be built and proven before production. Figure 9.1 However, this model is not published, as it contains too much information which would be of use to other semiconductor companies who might wish to copy or improve on the design. It would also take far too long for a simulation of a system containing such models of a number of op-amps to reach a useful result. For these, and other reasons, the SPICE models of analog circuits published by manufacturers or software companies are "macro" models, which simulate the major features of the component, but lack some of the fine detail. Consequently, SPICE modeling does not always reproduce the exact performance of a circuit and should always be verified experimentally. The basic principle of a breadboard is that it is a temporary structure, designed to test the performance of a circuit or system, and must therefore be easy to modify. There are many commercial breadboarding systems, but almost all of them are designed to facilitate the breadboarding of digital systems, where noise immunities are hundreds of millivolts or more. (We shall discuss the exception to this generality later.) Non copper-clad Matrix board (Vectorboard, etc.), wire-wrap, and plug-in breadboard systems (Bimboard, etc.) are, without exception, unsuitable for high performance or high frequency analog breadboarding. They have too high resistance, inductance, and capacitance. Even the use of standard IC sockets is inadvisable. PRACTICAL BREADBOARDING TECHNIQUES The most practical technique for analog breadboarding uses a copper-clad board as a ground plane. The ground pins of the components are soldered directly to the plane and the other components are wired together above it. This allows HF decoupling paths to be very short indeed. All lead lengths should be as short as possible, and signal routing should separate high-level and low-level signals. Ideally the layout should be similar to the layout to be used on the final PCB. This approach is often referred to as "deadbug" because the ICs are often mounted upside down with their

leads up in the air(with the exception of the ground pins, which are bent over and soldered directly to the ground plane). The upside-down ICs look liked deceased insects, hence the name Figure 9.2 shows a hand-wired breadboard based around two high speed op amps which gives excellent performance in spite of its lack of esthetic appeal. The IC op amps are mounted upside down on the copper board with the leads bent over. The ignals are connected with short point-to-point wiring. The characteristic impedance of a wire over a ground plane is about 120ohms, although this may vary as much as +40% depending on the distance from the plane. The decoupling capacitors are connected directly from the op amp power pins to the copper-clad ground. when working at frequencies of several hundred MHz, it is a good idea to use only one side together with short pieces of wire soldered to both sides of the board. If care Is lor to of the board for ground. Many people drill holes in the board and connect both sides taken, however, this may result in unexpected ground loops between the two sides of the board, especially at rf frequencies. DEADBUG PROTOTYPE TECHNIQUE Figure 9.2 Pieces of copper-clad may be soldered at right angles to the main ground plane to provide screening, or circuitry may be constructed on both sides of the board(with connections through holes) with the board itself providing screening. In this case, the board will need legs to protect the components on the underside from being crushed When the components of a breadboard of this type are wired point-to-point in the air (a type of construction strongly advocated by robert A. Pease of National Semiconductor(Reference 1)and sometimes known as"bird s nest"construction) there is always the risk of the circuitry being crushed and resulting short-circuits Also if the circuitry rises high above the ground plane, the screening effect of the ground plane is diminished, and interaction between different parts of the circuit is more likely. Nevertheless the technique is very practical and widely used because the circuit may so easily be modified

4 leads up in the air (with the exception of the ground pins, which are bent over and soldered directly to the ground plane). The upside-down ICs look liked deceased insects, hence the name. Figure 9.2 shows a hand-wired breadboard based around two high speed op amps which gives excellent performance in spite of its lack of esthetic appeal. The IC op amps are mounted upside down on the copper board with the leads bent over. The signals are connected with short point-to-point wiring. The characteristic impedance of a wire over a ground plane is about 120ohms, although this may vary as much as ±40% depending on the distance from the plane. The decoupling capacitors are connected directly from the op amp power pins to the copper-clad ground. When working at frequencies of several hundred MHz, it is a good idea to use only one side of the board for ground. Many people drill holes in the board and connect both sides together with short pieces of wire soldered to both sides of the board. If care is not taken, however, this may result in unexpected ground loops between the two sides of the board, especially at RF frequencies. "DEADBUG" PROTOTYPE TECHNIQUE Figure 9.2 Pieces of copper-clad may be soldered at right angles to the main ground plane to provide screening, or circuitry may be constructed on both sides of the board (with connections through holes) with the board itself providing screening. In this case, the board will need legs to protect the components on the underside from being crushed. When the components of a breadboard of this type are wired point-to-point in the air (a type of construction strongly advocated by Robert A. Pease of National Semiconductor (Reference 1) and sometimes known as "bird's nest" construction) there is always the risk of the circuitry being crushed and resulting short-circuits. Also if the circuitry rises high above the ground plane, the screening effect of the ground plane is diminished, and interaction between different parts of the circuit is more likely. Nevertheless the technique is very practical and widely used because the circuit may so easily be modified

SOLDER-MOUNT PROTOTYPE 图 Figure 9.6 If a conductor runs over a ground plane it forms a microstrip transmission line Solder- Mount has strips which form microstrip lines when mounted on a ground plane(they are available with impedances of 50ohms, 60ohms, 75ohms, and 100ohms). These strips may be used as transmission lines, for impedance matching or simply as power buses. Glass fiber/epoxy PCB is somewhat lossy at VhF and UHF, but the losses will probably be tolerable if microstrip runs are short. Both the"deadbug "and the"Solder -Mount"breadboarding techniques become tedious for complex circuits. Larger circuits are often better prototyped using more formal layout techniques An approach to prototyping more complex analog circuits is to actually lay out a double-sided board using CAD techniques. PC-based software layout packages offer ease of layout as well as schematic capture to verify connections. Although most layout software has some amount of auto-routing capability, this feature is best left to digital designs. After the components are placed in their approximate position, the interconnections should be routed manually following good analog layout guidelines. After the layout is complete, the software verifies the connections per the schematic diagram net list Many design engineers find that they can use Cad techniques(reference 3) to lay out simple boards themselves, or work closely with a layout person who has experience in analog circuit boards. The result is a pattern generation tape(or Gerber file)which would normally be sent to a PCb manufacturing facility where the final board is made. Rather than use a pc board manufacturer. however automatic drilling and milling machines are available which accept the PG tas ctly by drilling all holes and using a milling technique to remove copper and create t (Reference ese systems produce single and double-sided circuit boards dir insulation paths and finally, the finished board. The result is a board very similar to the final manufactured double-sided Pc board, the chief exception being that there

8 "SOLDER-MOUNT" PROTOTYPE Figure 9.6 If a conductor runs over a ground plane it forms a microstrip transmission line. Solder-Mount has strips which form microstrip lines when mounted on a ground plane (they are available with impedances of 50ohms, 60ohms, 75ohms, and 100ohms). These strips may be used as transmission lines, for impedance matching, or simply as power buses. (Glass fiber/epoxy PCB is somewhat lossy at VHF and UHF, but the losses will probably be tolerable if microstrip runs are short.) Both the "deadbug" and the "Solder-Mount" breadboarding techniques become tedious for complex circuits. Larger circuits are often better prototyped using more formal layout techniques. An approach to prototyping more complex analog circuits is to actually lay out a double-sided board using CAD techniques. PC-based software layout packages offer ease of layout as well as schematic capture to verify connections. Although most layout software has some amount of auto-routing capability, this feature is best left to digital designs. After the components are placed in their approximate position, the interconnections should be routed manually following good analog layout guidelines. After the layout is complete, the software verifies the connections per the schematic diagram net list. Many design engineers find that they can use CAD techniques (Reference 3) to lay out simple boards themselves, or work closely with a layout person who has experience in analog circuit boards. The result is a pattern-generation tape (or Gerber file) which would normally be sent to a PCB manufacturing facility where the final board is made. Rather than use a PC board manufacturer, however, automatic drilling and milling machines are available which accept the PG tape (Reference 4). These systems produce single and double-sided circuit boards directly by drilling all holes and using a milling technique to remove copper and create insulation paths and finally, the finished board. The result is a board very similar to the final manufactured double-sided PC board, the chief exception being that there