PCB News - Precautions for analog circuit PCB design

There is a recognized rule that in all analog circuit printed circuit boards, the signal line should be as short as possible. This is because the longer the signal line, the more inductive and capacitive coupling in the circuit, which is undesirable. . The reality is that it is impossible to make all signal lines the shortest. Therefore, the first thing to consider when wiring is the signal line that is most prone to interference.

In the analog circuit printed circuit board, the signal line can complete various functions, such as signal input, feedback, output, and provide reference signals. Therefore, for different applications, signal lines must be optimized in various ways. However, there is an accepted rule that in all analog circuit printed circuit boards, the signal line should be as short as possible. This is because the longer the signal line, the more inductive and capacitive coupling in the circuit. This is not desirable. Arrived. The reality is that it is impossible to make all signal lines the shortest. Therefore, the first thing to consider when wiring is the signal line that is most prone to interference.

In particular, the wiring of signal lines in the following circuits requires special attention:

1) High frequency amplifier/oscillator;

2) Multi-stage amplifiers, especially amplifiers with higher output power;

3) High gain DC amplifier;

4) Small signal amplifier;

5) Differential amplifier.

1. High frequency amplifier/oscillator

If the high-frequency amplifier printed PCB board wiring is unreasonable, it will lead to a reduction in the bandwidth of the amplifier. This is because a large capacitor is formed between the two close ground wires and the signal wire, and this capacitor and the output resistance together form a low-pass filter. This low-pass filter reduces 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-mentioned wires (Lindsey, 1985).

Electronic circuit designers usually have such a common experience, that is, if a high-frequency amplifier is designed, it will actually oscillate. Similar problems can also be encountered in the layout of the oscillator, which does not oscillate at the designed frequency. This problem is caused by the capacitive coupling between the signal lines. Therefore, it is very important to reduce the coupling capacitance between the signal lines when the printed circuit board is laid out.

2. Multi-stage amplifier with high power output

If the power line and the ground line are too long, the multi-stage amplifier is prone to low-frequency oscillation. Because the wires themselves have resistivity, large currents caused by high power output will flow through these wires. Adding a large enough capacitor between the power supply and the ground to form a power supply decoupling circuit can solve this problem. Alternatively, separate power and ground wires are provided for different amplifier stages, so that there will be no common power and ground wires.

3. High gain DC amplifier

High-gain DC amplifiers are usually used for small signal amplification. When a device such as a transistor or a DC amplifier is soldered on the printed circuit board, a thermocouple is formed at the junction of the copper tube and the device pin, thereby generating different alternating voltages and forming an interference signal to the amplifier. In order to minimize the temperature change rate around the input stage of the DC amplifier and keep it constant, it is recommended to surround the input stage with an isolation device to avoid the influence caused by the air flow around it.

4. Small signal amplifier

The signal amplifier deals with tiny signals, including the following two types.

(1) High impedance (low current) amplifier

In this type of amplifier, there is a capacitive coupling between two adjacent signal lines, which will seriously affect the performance of the circuit and even cause low-level signals to be covered. In a high-impedance circuit, the capacitance between two wires is coupled. In order to reduce coupling, it is recommended to keep a sufficient distance between high-impedance signal lines and other interference signals. Generally, 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 in order to reduce the coupling voltage. That is to say, the low-level signal wire should be close to the ground wire. If you cannot guarantee sufficient width between the low-level signal lines, you can lay a ground wire between them to reduce coupling.

When the amplifier uses photocells or chemical batteries as a power source, the power source impedance can reach millions or even hundreds of millions of ohms. If the printed circuit board is not cleaned sufficiently after etching, the electrolyte remaining on the surface of the circuit board will generate a large resistance between adjacent wires, even if the circuit board is completely cleaned, it is still no more than 10 12 0 The leakage resistance exists. Moreover, these resistances cannot be equally distributed, so that the resistance between two adjacent wires may be higher than the resistance between two wires that are farther apart. Therefore, the input of the low-level I1 V (current/voltage) converter should be protected by a guard loop on both sides of the printed circuit board, and the guard loop should be connected to a point that is equipotential with the general connection point. If this is done, the precise value of the leakage resistance is not so important, because the difference voltage applied to it is already very small.

High-impedance amplifier printed circuit boards cannot use plated through holes. The volume resistivity of the printed circuit board material is lower than the surface resistivity, and it is difficult to install the guard ring on the substrate. The best way is to connect the terminals of the high-impedance amplifier to the PTFE insulator instead of the printed circuit board wires.

(2) Low impedance (low voltage) amplifier

In a low-impedance circuit, it is possible to produce induced voltages because of the inductive coupling or the presence of magnetic fields in the circuit. This interference can be reduced to a certain extent by the following methods:

1) Keep a sufficient distance between the high-level AC signal line and the low-level signal line;

2) Lay a ground wire near the signal wire;

3) Avoid forming ground loops to prevent external magnetic fields from interfering with low-level signals.

5. Differential Amplifier

The differential amplifier only amplifies the difference between the two signals, and does not amplify the common voltage signal. If the design of the differential amplifier and the printed circuit board is unreasonable, when the signal level is low, the common voltage will produce a small differential interference signal. The input impedance of the differential amplifier is high, and any unbalance of the input parameters will cause great interference to the circuit. Therefore, it is necessary to ensure that the amplifier is completely symmetrical in physical structure when designing the printed circuit board.

There is a certain leakage resistance at the input of the differential amplifier, which can cause an unbalanced voltage offset. This problem can be solved by adding a protective device to the input circuit. The protective device surrounds the signal line. If it can maintain the same voltage as the low-level ends of the two input signal lines, it will cause the effective resistance to increase. This device can ensure that the signal source terminal and the shield are the same level as the signal source's low-level end. The protective line should form a circle to enclose the signal line from the input end to the input connection point of the amplifier, and be connected to the protective device of the equipment. This is an effective method for processing low-level differential signals. In addition, the printed PCB board substrate of the small signal differential amplifier is more suitable for the use of epoxy glass material, which helps reduce leakage current.

The above is an introduction to the precautions for PCB design of analog circuits. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology