What’s wrong with PCB wiring?

Q: Surely the resistance of a very short copper wire in a small signal circuit is not important?

A: When the conductive band of PCB board is made wider, the gain error will be reduced. In analog circuits, it is generally preferable to use a wider band, but many PCB designers (and PCB designers) prefer to use a minimum band width to facilitate signal line placement. In conclusion, it is important to calculate the resistance of the conductive band and analyze its role in all possible problems.


Q: As mentioned earlier about simple resistors, there must be some resistors whose performance is exactly what we expect. What happens to the resistance of a section of wire?

A: The situation is different. You are referring to a conductor or a conductive band in a PCB that acts as a conductor. Since room-temperature superconductors are not yet available, any length of metal wire acts as a low-resistance resistor (which also acts as a capacitor and inductor), and its effect on the circuit must be considered.

What’s wrong with PCB wiring

Q: Is there a problem with the capacitance of the conductive band with too large width and the metal layer on the back of the PRINTED circuit board?

A: It’s a small question. Although capacitance from the conductive band of the PRINTED circuit board is important, it should always be estimated first. If this is not the case, even a wide conductive band forming a large capacitance is not a problem. If problems arise, a small area of the ground plane can be removed to reduce the capacitance to earth.

Q: What is the grounding plane?

A: If 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 grounding plane. Any ground wire shall be arranged with the smallest possible resistance and inductance. If a system uses an earthing plane, it is less likely to be affected by earthing noise. And the grounding plane has the function of shielding and heat dissipation.

Q: The grounding plane mentioned here is difficult for the manufacturer, isn’t it?

A: There were some problems 20 years ago. Today, due to the improvement of binder, solder resistance and wave soldering technology in printed circuit boards, the manufacture of grounding plane has become a routine operation of printed circuit boards.

Q: You said that it is very unlikely for a system to be exposed to ground noise by using a ground plane. What remains of the ground noise problem cannot be solved?

A: 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 properly arranging the printed circuit boards so that high current does not flow to areas that affect the grounding voltage of precision signals. Sometimes a break or slit in the ground plane can divert a large grounding current from the sensitive area, but forcibly changing the ground plane can also divert the signal into the sensitive area, so such a technique must be used with care.

Q: How do I know the voltage drop generated in a grounded plane?

A: Usually the voltage drop can be measured, but sometimes it can be calculated based on the resistance of the grounded plane material and the length of the conductive band through which the current travels, although the calculation can be complicated. Instrument amplifiers can be used for voltages in the dc to low frequency (50kHz) range. If the amplifier ground is separate from its power base, the oscilloscope must be connected to the power base 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 the two points. The combination of amplifier gain and oscilloscope sensitivity enables the measurement sensitivity to reach 5μV/div. Noise from the amplifier will increase the width of the oscilloscope waveform curve by about 3μV, but it is still possible to achieve a resolution of about 1μV, which is sufficient to distinguish most ground noise with up to 80% confidence.

Q: How to measure the high frequency grounding noise?

A: It is difficult to measure hf ground noise with a suitable wideband instrumentation amplifier, so hf and VHF passive probes are appropriate. It consists of a ferrite magnetic ring (outer diameter of 6 ~ 8mm) with two coils of 6 ~ 10 turns each. To form a high-frequency isolation transformer, one coil is connected to the spectrum analyzer input and the other to the probe. The test method is similar to the low frequency case, but the spectrum analyzer uses amplitude-frequency characteristic curves to represent noise. Unlike time domain properties, noise sources can 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.