Abstract: it is an accepted principle that in all analog printed circuit boards, the signal line should be as short as possible. This is because the longer the signal line is, the more induction and capacitance will be donated in the circuit, which is undesirable. The reality is that it is impossible to make all the signal lines the shortest, so the first thing to consider when wiring is the most prone to interference signal lines.
In analog printed circuit board, signal line can fulfill various functions, such as signal input, feedback, output and provide reference signal. Therefore, signal lines must be optimized in various ways for different applications. However, it is generally accepted that in all analog printed circuit boards, the signal line should be as short as possible. This is because the longer the signal line is, the more sensors and capacitors will be added to the circuit, which is undesirable. The reality is that it is impossible to make all the signal lines the shortest, so the first thing to consider when wiring is the most prone to interference signal lines.
Particular attention should be paid to the wiring of signal lines in the following circuits:
1) high-frequency amplifier/oscillator;
2) multistage amplifiers, especially those with higher output power;
3) high-gain dc amplifier;
4) small signal amplifier;
5) differential amplifier.
1. High-frequency amplifier/oscillator
If the hf amplifier printed circuit board wiring is not reasonable, will lead to amplifier bandwidth reduction. This is because a large capacitor is formed between the two adjacent ground and signal lines, which together with the output resistance forms a low-pass filter. This low-pass filter lowers the amplifier bandwidth. At the same time, if the input signal line and the output signal line are close to each other, the feedback signal will cause oscillation. In order to avoid these problems, sufficient space should be left between the above wires (Lindsey, 1985).
Electronic circuit designers often have a common experience that if you design a high-frequency amplifier, it actually oscillates. A similar problem is encountered in the layout of the oscillator, which does not oscillate at the designed frequency. This problem is caused by the capacitor coupling between the signal lines. Therefore, it is important to reduce the capacitor coupling between the signal lines in the PCB layout.
2. Multi-stage amplifier with high power output
Multistage amplifiers are prone to low frequency oscillations if the power and ground wires are too long. Since the wires have their own resistivity, a large current due to the high power output will flow through the wires. This problem can be solved by adding a large enough capacitor between the power supply and the ground to form a circuit to remove the power supply. Alternatively, separate power and ground wires are provided for different amplifier stages, so there is no common power and ground access.
3. High gain dc amplifier
High-gain dc amplifiers are commonly used for small signal amplification. When a transistor or dc amplifier is welded to a printed circuit board, a thermocouple will be formed at the connection between the copper cylinder and the pin of the device, resulting in different alternating voltages and a disturbance signal to the amplifier. In order to minimize and maintain a constant rate of temperature change around the input stage of a dc amplifier, it is recommended to enclose the input stage with an isolation device to avoid the influence of air flow around it.
4. Tiny signal amplifier
The signal amplifier deals with small signals, including the following two types.
(1) high impedance (low current) amplifier
In this amplifier, there is a capacitor coupling between two adjacent signal lines, which can seriously affect the performance of the circuit and even cause the low-level signal to be overwritten. Capacitance coupling between two wires in a high impedance circuit. To minimize coupling, it is recommended to maintain sufficient distance between signal lines with high impedance and other interference signals. In general, the distance is at least 40 times the width of the signal line.
In any case, the ground capacitance of the low-level signal line should be high to reduce the coupling voltage. This means that the low-level signal wires should be close to the ground. If you cannot ensure sufficient width between low level signal lines, lay a ground line between them to reduce coupling.
When amplifiers are powered by photocells or chemical cells, the impedance can reach millions or even hundreds of millions of ohms. If the printed circuit board is not adequately cleaned after etching, the electrolyte remaining on the surface of the circuit board will create a large resistance between adjacent wires, and even if the circuit board is completely cleaned, there will still be no more than 10, 12, 0 leakage resistance. Furthermore, these resistances cannot be evenly distributed, so that the resistance between two adjacent wires may be higher than that between two distant wires. Therefore, the input of the low-level i1v (current/voltage) converter should be protected on both sides of the printed circuit board by a protective loop connected to a point of equal potential to the total connection point. If this is done, the exact value of the leakage resistance is not so important because the difference voltage applied to it is small.
High-impedance amplifier printed circuit boards should not be plated through holes. The bulk resistivity of printed circuit board material is lower than that of the surface, so it is difficult to install protective rings on the substrate. The best way to do this is to connect the end of the high-impedance amplifier to a teflon insulator instead of a printed circuit board wire.
(2) low impedance (low voltage) amplifier
In a low impedance circuit, it is possible to generate an induced voltage because of an inductive coupling or magnetic field. This interference can be reduced to a certain extent by:
1) keep enough distance between high level ac signal line and low level signal line;
2) lay ground wire near the signal line;
3) avoid forming a ground loop to prevent external magnetic field from interfering with low-level signals.
5. Differential amplifier
A differential amplifier amplifies only the difference between two signals, not the common voltage signal. If the differential amplifier and printed circuit board design is not reasonable, when the signal level is low, the common voltage will produce a small differential interference signal. The input impedance of differential amplifier is high, and the imbalance of any parameters at the input end will cause great disturbance to the circuit. Therefore, the design of printed circuit board must ensure that the amplifier is completely symmetrical in physical structure.
There is a defined leakage resistance at the input end of the differential amplifier that can cause an unbalanced voltage offset. This problem can be solved by adding a guard to the input circuit. The guard encloses the signal line. If it maintains the same voltage as the low end of the two input signal lines, it will cause an increase in effective resistance. This device ensures that the source terminal and guard body are at the same level as the low level end of the source. The guard wire forms a loop that encloses the signal wire from the input end to the input connection point of the amplifier and is connected to the guard device of the equipment. This is an effective way to deal with low level differential signals. In addition, the printed circuit board substrate of small signal differential amplifier is more suitable to use epoxy glass material, which helps to reduce leakage current.