PCB Technical - Eight classic questions and answers about multilayer circuit board/PCB wiring

Eight classic questions and answers about multilayer circuit board/PCB wiring

1. Question: In the design of small data signal circuits, the resistance of a short section of copper core wire must not be critical, right?

Answer: The conductive band of the printed PCB board is made wider, and the gain measurement error will be reduced. Generally speaking, it is better to use a wider conductive tape in analog circuits, but many designers of printed PCB circuit boards (and printed PCB circuit board structure design applications) prefer to use a small width conductive tape to facilitate the signal The layout of the lines. In other words, in all places where problems will occur, it is very important to measure the resistance of the conductive tape and analyze its function in depth.

2. Question: The question about simple resistance is described in detail above. There must be some internal resistance, and its characteristics are in line with actual people's expectations. I want to ask what will happen to the resistance of a piece of wire?

Answer: The situation is different. What you are referring to is that a piece of wire may be a piece of conductive tape in a PCB that acts as a wire. Because room temperature superconductors have not yet come out, all metal wires have the function of a low-resistance resistor (it also has the function of a capacitor and a reactor), and it may be necessary to consider its adverse effects on the circuit board.

3. Question: Will the capacitor formed by the conductive tape with too large width and the metal layer on the reverse side of the printed PCB circuit board be okay?

Answer: The problem is very small. Although the capacitance formed by the conductive tape of the printed PCB circuit board is very important, it should often be estimated first. If you can't find the above situation, even if the wider conductive tape forms a large capacitance, it will not cause a problem. If there is a problem, you can remove a small piece of ground plane area to reduce the capacitance to ground.

4. Q: What is a ground plane?

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

Answer: There were indeed some problems in this industry 20 years ago. Today, because of the improvement of the adhesive, solder resist and wave soldering technology in printed circuit diagrams, the manufacture of ground planes has become a routine operation for printed PCB 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 adversely affect the precision signal. This problem can be minimized by effectively arranging printed PCB circuit boards so that large currents cannot flow to areas that adversely affect the generation of ground voltages for precision signals. Sometimes breaking or slitting on the ground plane can change the direction of the large ground current from the sensitive area, but forcibly changing the ground plane can also make the signal detour into the sensitive area, so the only process technology must be used carefully.

7. Question: How can we know the voltage drop caused on a ground plane?

Answer: Usually the short-circuit current can be measured, but sometimes it can be calculated based on the resistance of the ground plane material and the length of the conductive strip that the current passes through. Only 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. LED lighting The resistance between any two points on the ground plane can be measured by adding a probe to these two points. The combination of amplifier gain and oscilloscope sensitivity allows the measurement sensitivity to reach 5μV/div. The noise of the amplifier will expand the width of the oscilloscope waveform curve to about 3μV, but it will still make the screen resolution of the measurement reach the level of about 1μV, which can distinguish most ground noise, and the confidence level can reach 80%.

8. Question: How to accurately measure the noise of high-frequency grounding devices?

Answer: It is very difficult to accurately measure the noise of high-frequency grounding devices with a suitable wideband instrumentation amplifier. The reason why it is more appropriate to use high-frequency and VHF passive probes. It is composed of a ferrite magnetic ring (outer diameter of 6-8mm), with two coils on the magnetic ring, each with 6-10 turns. In order to form a high-frequency isolation transformer, one electromagnetic coil is connected to the input end of the spectrum analyzer, and the other electromagnetic 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 not the same as the time domain characteristics. Noise sources can also be easily distinguished based on their frequency characteristics. In addition, the sensitivity of the spectrum analyzer is at least 60dB higher than that of the broadband oscilloscope.