Although the printed circuit board (PCB) layout has a key role in the high-speed circuit, but it is often one of the last steps of the circuit design process. High-speed PCB layout has many issues , it was written on this subject have been a lot of literature . In this paper, from a practical point of view to explore the high-speed circuit wiring problems . The main purpose is to help new users to design high-speed circuit PCB for a variety of different issues to consider when wiring attention . Another purpose is to have no contact with the PCB layout for some time to provide a review of customer data . Due to limited space , this can not be discussed in detail all the problems , but we will discuss the key part to improve circuit performance , shorten design time, save the modification time with the greatest effectiveness .
Although this is mainly for high-speed op-amp circuit related , but the issues and approaches discussed here most other wiring for high-speed analog circuits are generally applicable . When the operational amplifier operates at a very high frequency (RF) band circuit performance depends largely on the PCB layout . The " drawing " looks good , high-performance circuit design, wiring due to careless careless if affected, finally only get ordinary performance. Anticipate and pay attention to important details will help to ensure that the desired circuit performance throughout the routing process .
Schematics
Despite the excellent good wiring diagram is not guaranteed , but a good start in the fine wiring schematics . When the schematic drawing to deliberate , and must consider the signal flow of the entire circuit. If left to right with a normal stable signal flow diagram , then the PCB should have the same good signal stream. On the schematic given as much useful information . Because sometimes circuit design engineer is not, customers will ask us to help solve the problem of the circuit, the designer of this work is engaged , technicians and engineers will be very grateful , including us.
Besides the normal reference identifier , power and error tolerance, a schematic diagram is given of what information should it ? Here are some suggestions , general schematics can be turned into class diagram. Join waveforms, mechanical information about housing , printed wire length , blank areas ; indicate which components need to be placed above the PCB ; given adjustment information , component ranges , thermal information , controlled impedance traces , comments, brief circuit action Description ...... and others.
Do not believe anyone
If the wiring is not your own design , sure to set aside plenty of time to carefully check the wiring people 's designs. At this point very little prevention is worth a hundred times remedy. Do not expect people to understand the wiring of your thoughts. Early in the design process of wiring your advice and guidance is the most important . The more information you can provide more and more throughout the routing process your intervention, the better the results obtained PCB . Cabling Design Engineer to set a tentative completion point - Progress Report in accordance with the wiring you want a quick check . This " closed loop " approach to prevent wiring astray , thereby minimizing the possibility of rework .
Indicating the need to engineer the wiring comprising: a short description of the circuit function input and output position indicated sketch PCB , PCB lamination information ( e.g. , how thick board , the number of layers , the signal layers and the ground plane details - Function consumption , ground , analog, digital and RF signals ) ; layers need those signals ; requires the placement of important components ; exact location of the bypass element ; which traces very important ; what lines need to be controlled impedance traces ; what you need to match the length of the line ; dimension elements ; which tracks need to stay away from each other ( or near ) ; which lines need to stay away from each other ( or near ) ; which components need to stay away from each other ( or near ) ; What are the components to be placed above the PCB , which placed below. Never complain about other people's need to give too much information - too right ? Is ; too it? Do not .
A learning experience : About 10 years ago , I designed a multi-layer surface-mount circuit board - on both sides of the board has components. Many board is fixed with screws in a gold-plated aluminum housing ( because there are very strict earthquake indicators ) . Feedthrough pin provides bias across the board. This pin is connected by bonding wires to the PCB . This is a very complicated device . Some components on the board is set for testing (SAT) of . But I have clearly defined the location of these components. Can you guess what these components are installed in place? By the way, in the board below. When the product engineers and technicians had to disassemble the entire unit to complete the setting and then reassembled when they looked very unhappy. Since then I have never committed such a mistake.
Location
As in the PCB , the location is everything. What position on a circuit on the PCB , its specific circuit element is mounted on what position , as well as other circuits are adjacent , it's all very important.
Typically, the position of the input , output and power supply is pre-determined , but the circuit between them needs to " play to their creativity ," the. That is why attention to detail will generate huge returns wiring reasons . Starting from the position of key components , depending on the circuit and the entire PCB to consider. A predetermined path from the start position and the key element of the signal helps ensure that the design to achieve the desired objectives. Once to get the right design can reduce the cost and stress - will shorten the development cycle.
Bypass power
In the end of the amplifier 's power supply in order to reduce the noise bypass PCB design process is a very important aspect - including high-speed operational amplifier or other high-speed circuits. Bypass high-speed op amp configuration , there are two commonly used methods.
Power grounded : In most cases, this approach is the most effective use of multiple capacitors in parallel to the op-amp power supply pin directly to ground. Generally two parallel capacitor is sufficient - but increased in parallel with a capacitor may be beneficial to some circuits.
Different capacitor in parallel with capacitance values help to ensure that the power pin on a very wide frequency band can only see a very low AC (AC) impedance. This op-amp power supply rejection ratio (PSR) attenuation frequency is especially important. The capacitor helps to compensate for the amplifier to reduce the PSR. Maintain low impedance in the range of many ground path decade will help to ensure that unwanted noise can not enter the op amp. Figure 1 illustrates the advantages of using a plurality of capacitors connected in parallel . At low frequencies , the large capacitor provides a low impedance path to ground . However, once the frequency of their own resonance frequency , the capacitor will be reduced capacitive and inductive gradually showing . This is why it is important to use a plurality of capacitors reason : When the frequency response of a capacitor begins to decrease , the frequency response of the capacitor start another function , it is possible to maintain a low AC impedance in the octave range for many decades .
Starting directly from the op-amp power supply pin ; capacitor has a minimum capacitance value and the minimum physical size and operational amplifier PCB should be placed on the same side - and as close to the amplifier. Ground terminal of the capacitor should be printed with the shortest pin or wire connected directly to the ground plane. Above ground connection should be as close to the load side of the power amplifier in order to reduce interference between the terminal and the ground terminal . Figure 2 shows such a connection method.
For the second large capacitor capacitance value should repeat the process. The best place to start from 0.01mF minimum capacitance value and close to placing a 2.2mF ( or a little larger ) with low equivalent series resistance (ESR) of the electrolytic capacitors. 0508 case size 0.01mF using capacitors have very low series inductance and excellent high frequency performance.
Power supply terminal end : Another method used to configure one or more bypass across the operational amplifier between the positive terminal and the negative power supply terminal capacitance. When it is difficult to configure four capacitors in the circuit usually use this method. Its disadvantage is that the capacitor case size may be increased , because the voltage across the capacitor is twice the single-supply voltage value bypass method . Increasing need to improve the rated voltage of the device breakdown voltage , which is to increase the housing size. However, this method can improve the PSR and distortion performance .
Because each circuit and wiring are different, so the configuration , the capacitance value of the capacitor , and the number of the request must be the actual circuit .
Parasitics
The so-called parasitic effects are those who slipped into your PCB and large facilities in the circuit damage, headaches , unexplained glitches . They are invisible high-speed circuits into the parasitic capacitance and parasitic inductance. Package parasitic inductance including the long pin and the printed line is formed ; ground pad to pad and pad to the power plane to the parasitic capacitance formed between the printed line ; interaction between the through -hole , and many other possible parasitics. Figure 3 (a) shows a typical schematic of the operational amplifier noninverting . However , considering the parasitic effects , it may become the same circuit in Figure 3 (b) that.
In high-speed circuits, small value will affect the performance of the circuit . Sometimes dozens of picofarads (pF) capacitance is sufficient. Related Example: If only the inverting input terminal of the additional parasitic capacitance of 1pF , it is about 2dB can cause spikes in the frequency domain ( see Figure 4 ) . If the parasitic capacitance is large enough , it can cause instability and oscillation circuit .
When looking for a problem of parasitic source , may need it a few basic formulas to calculate the size of the parasitic capacitance to those described above . Equation ( 1 ) is calculated as a parallel plate capacitor ( see Figure 5) .
C represents the capacitance , A is in units of cm2 plate area , k represents the relative dielectric constant of the PCB material , d between the plates represents the distance in cm .
Parasitic inductance of the strip is another need to be considered because it is too long or the printed lines due to the lack of a ground plane .
( 2 ) shows a calculation formula printed inductance (Inductance) formula. See Figure 6 .
W represents the width of the printed lines , L represents the length of the printed lines , H represents the thickness of the printed lines. All dimensions are in mm .
Figure 7 shows a high-speed oscillation -inverting input of the op amp length of 2.54 cm printed line impact. The equivalent parasitic inductance of 29 nH (10-9H), enough to cause persistent low pressure oscillations will continue until the entire transient response cycle. Figure 7 also shows how to use the ground plane to reduce the parasitic inductance.
Also the through hole is a parasitic source ; they can cause parasitic inductance and capacitance. Equation ( 3 ) is calculated as the parasitic inductance ( see Figure 8 ) .
T indicates the thickness of the PCB , d expressed in units of cm hole diameter .
Equation ( 4 ) shows how to calculate the value of the parasitic capacitance of the through -hole ( see Figure 8 ) caused .
er represents the relative permeability of PCB material. T represents the thickness of the PCB. D1 said pad around the through-hole diameter . D2 denotes the diameter of the ground plane isolation hole. All dimensions are in cm . On a 0.157 cm thick, a through-hole PCB can increase the parasitic inductance of 1.2 nH and 0.5 pF parasitic capacitance ; This is why the PCB layout to be sure to keep alert reason , the impact of reduced parasitic effects you want Min .
