Eight classical answers to PCB wiring tips
1. Question: In a small signal circuit, the resistance of a very short copper wire must not be important?
Answer: The conductive tape of printed PCB circuit board is wider, and the gain error will be reduced. Wider conductive bands are usually preferred in analog circuits, but many PCB designers (and PCB design programs) prefer to use the smallest width of conductive bands to facilitate the layout of signal lines. In a word, it is very important to calculate the resistance of conductive tape and analyze its function in all possible problems.
2. Question: The problem of simple resistance was introduced before. There must be some resistance. Its performance is in full accord with our expectation. What is the resistance of a wire?
Answer: The situation is different. You mean a conductor or a conductive band in a PCB circuit board that acts as a conductor. Since room temperature superconductors have not yet been invented, any metal conductor plays the role of a low resistance resistor (it also has the role of capacitance and inductance), so we must consider its impact on the circuit.
3. Question: Is there a problem with the capacitance of the conductive tape with excessive width and the metal layer on the back of the printed circuit board?
Answer: The question is very small. Although the capacitance of the conductive tape of printed circuit boards is very important, it should always be estimated first. If this is not the case, even if the wider conductive band forms a large capacitance, it will not cause problems. If the problem arises, a small area of the ground plane can be removed to reduce the capacitance to the ground.
4. Question: What is the ground plane?
Answer: If the copper foil of the whole side of a printed circuit board (or the whole sandwich of a multilayer printed circuit board) is used for grounding, then this is what we call the grounding plane. Any ground wire arrangement should have as little resistance and inductance as possible. If a system uses a grounding plane, it is less likely to be affected by grounding noise. And the ground plane has the function of shielding and heat dissipation.
5. Question: The ground plane mentioned here is very difficult for manufacturers, isn't it?
Answer: There were some problems in this respect 20 years ago. Nowadays, the fabrication of ground plane has become a routine operation for printed circuit boards due to the improvement of bonding agent, solder retardant and wave soldering technology in printed circuit boards.
6. Question: You say that a system using a ground plane is very unlikely to suffer from ground noise. What can't solve the problem of ground noise left behind?
Answer: Although there is a ground plane, its resistance and inductance are not zero. If the external current source is strong enough, it will affect the precise signal. By arranging printed circuit boards reasonably, the problem can be minimized if the high current can not flow to the area that affects the ground voltage generated by the precise signal. Sometimes disconnection or slit on the ground plane can change the flow direction of large grounding current from the sensitive area, but forcibly changing the ground plane can also make the signal bypass into the sensitive area, so such technology must be used carefully.
7. Question: How can we know the voltage drop on a ground plane?
Answer: Voltage drop can usually be measured, but sometimes it can be calculated according to the resistance of the ground plane material and the length of the conductive band through which the current passes, but the calculation may be very complicated. The voltage in the range of DC to low frequency (50kHz) can be used as an instrument amplifier. If the ground of the amplifier is separated from its power supply, the oscilloscope must be connected to the power supply ground of the power supply circuit used.
8. Question: How to measure high frequency grounding noise?
Answer: It is very difficult to measure high-frequency grounding noise with a suitable broadband instrument amplifier, so it is more appropriate to use high-frequency and very high-frequency passive probes. It consists of a ferrite magnetic ring with an outer diameter of 6-8 mm. There are two coils on the magnetic ring, each coil of 6-10 turns. In order to form a high frequency isolation transformer, one coil is connected to the input end of the spectrum analyzer and the other coil is connected to the probe. The test method is similar to that of low frequency, but the spectrum analyzer uses the amplitude-frequency characteristic curve to represent the noise. This is different from the time domain characteristics. Noise sources can be easily distinguished according to their frequency characteristics. In addition, the sensitivity of spectrum analyzer is at least 60 dB higher than that of broadband oscilloscope.
