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PCB Tech
8 classic questions and answers about PCB wiring
PCB Tech
8 classic questions and answers about PCB wiring

8 classic questions and answers about PCB wiring

2021-11-07
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Author:Downs

  1. Q: In the small signal PCB circuit, the resistance of a short copper wire must not be important, right?

  Answer: The conductive strip of the printed PCB circuit board is made wider, and the gain error will be reduced. In analog circuits, it is usually better to use a relatively wide conductive tape, but many printed circuit board designers (and printed circuit board design programs) prefer to use the smallest width conductive tape to facilitate the layout of signal lines. In short, it is very important to calculate the resistance of the conductive tape and analyze its role in all possible problems.

  2. Question: I have introduced the problem of simple resistance. There must be some resistances, and their performance is in full compliance with our expectations. What is the resistance of a piece of wire?

Answer: The situation is different. What you are referring to is a piece of wire or a piece of conductive tape in a PCB circuit board that acts as a wire. Since room temperature superconductors have not yet come out, any piece of metal wire acts as a low-resistance resistor (it also has capacitance and inductance), so it is necessary to consider its impact on the circuit.

  3. Question: Is there a problem with the capacitor formed by the conductive tape with too large width and the metal layer on the back of the printed circuit board?

  Answer: The problem is very small. Although the capacitance formed by the conductive tape of the printed circuit board is important, it should always be estimated first. If the above situation does not exist, even if a wider conductive band forms a large capacitance, it will not cause a problem. If it causes problems, remove a small piece of ground plane area to reduce the capacitance to ground.

 

pcb board

   4. Q: What is a ground plane?

  Answer: If the copper foil on the entire side of a printed circuit board (or the entire interlayer of a multilayer printed circuit board) is used for grounding, then this is what we call a ground plane. The arrangement of any ground wire must make it have as little resistance and inductance as possible. If a system uses a ground plane, it is less likely to be affected by ground 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, right?

  A: There were indeed some problems in this regard 20 years ago. Today, due to the improvement of adhesives, solder resists and wave soldering techniques in printed circuits, the manufacture of ground planes has become a routine operation for printed circuit boards.

  6. Question: You said that a system uses a ground plane to make it less likely to suffer from ground noise. What else can't be solved for the remaining ground noise problem?

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. This problem can be minimized by arranging the printed circuit board reasonably so that large currents cannot flow to the area that affects the ground voltage generated by the precision signal. Sometimes a break or slit on the ground plane can change the direction of a large ground current from the sensitive area, but forcibly changing the ground plane can also make the signal detour into the sensitive area, so this process technology must be used carefully.

  7. Question: How can I know the voltage drop generated on a ground plane?

  Answer: Generally, the voltage drop can be measured, but sometimes it can be calculated based on the resistance of the ground plane material and the length of the conductive tape that the current passes through, but the calculation may be complicated. Instrumentation amplifiers can be used for voltages in the DC to low frequency (50kHz) range. If the amplifier ground is separated from its power ground, the oscilloscope must be connected to the power ground of the power circuit used. The resistance between any two points on the ground plane of the led lighting can be measured by adding a probe to these two points. The combination of amplifier gain and oscilloscope sensitivity can make the measurement sensitivity reach 5μV/div. The noise of the amplifier will increase the width of the oscilloscope waveform curve to about 3μV, but it is still possible to make the measurement resolution reach about 1μV level, which is enough to distinguish most ground noises, and the confidence level can reach 80%.

  8. Q: How to measure high frequency ground noise?

  Answer: It is very difficult to measure high frequency ground noise with a suitable wideband instrumentation amplifier, so it is more appropriate to use high frequency and VHF passive probes. It is composed of a ferrite magnetic ring (with an outer diameter of 6-8mm). There are two coils on the magnetic ring, each with 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 the low-frequency case, but the spectrum analyzer uses the amplitude-frequency characteristic curve to express the noise. This is different from time-domain characteristics, where noise sources can be easily distinguished based on their frequency characteristics. In addition, the sensitivity of using a spectrum analyzer is at least 60dB higher than using a broadband oscilloscope. How do you feel after reading the classic questions and answers about PCB wiring?