Protective earth wire does not protect?

Protective earth wires are often used in engineering circles for isolation, to suppress mutual interference between signals. It is true that a PE wire can sometimes improve isolation between signals, but a PE wire is not always effective and sometimes even worsens interference. The use of protective earth wire should be carefully analyzed according to actual situation and handled carefully.

A protective earth wire is an insertion of a ground wire between two signal wires that is used to isolate two signals. Show. Sometimes protective earth wires are placed on both sides of sensitive signals.

If you want to add a protective earth wire, you must first increase distance between two signal wires to leave room for protective earth wire. Reduce crosstalk. How useful would it be to insert a protective earth wire?

Low frequency analog ground

Let's look at size of crosstalk in case of a surface microstrip. Assume that trace is driven by an impedance of 50 ohms, a line width of 6 mils, a dielectric thickness of 3.6 mils, and a dielectric constant of 4.5. And suppose these two signals are analog signals with a carrier frequency of 30 MHz and a bandwidth of 2 MHz.

The figure below shows far-end crosstalk in three cases. When distance between lines is 6 mils, since two lines are closely connected, far-end crosstalk is relatively large. Increase distance to 18 mils and far-end crosstalk is greatly reduced. In addition, a protective ground wire is added between two wires, and both ends of ground wire are connected to ground through through holes, so that far-end crosstalk is further reduced.

A protective earth wire is very useful for isolating low-frequency analog signals. This is also reason for "grounding" that is often seen on many low frequency boards. But if isolated digital signals are required, situation is different. We discuss effect of PE wire insulation on digital signals in two cases: surface trace and internal stripline. In discussion that follows, I do not assume that all PCB traces are controlled by a 50 ohm impedance.

Surface Routing

However, using surface wiring stack structure described above, line width is 6 mils, dielectric thickness is 3.6 mils, and dielectric constant is 4.5. The attack signal is a stepped signal with rise time Tr=200 ps. Consider near and far end crosstalk in following three situations, as shown in figure below, where length of connecting section is 2000 mils.

Case 1: spacing=1w between two traces (w=6mil means line width)

Case 2: The distance between two lanes is gap=3w, just pulling avenue can shorten distance of guard line, but guard line is not applicable.

Case 3: The distance between two lines is gap=3w, a PE wire is used in middle, and GND through holes are drilled at both ends.

The figure below shows crosstalk waveforms in three cases: near-end or far-end crosstalk, when distance between tracks is increased from 1 to 3, crosstalk is greatly reduced. Based on this, protective earth wire is inserted between tracks, and crosstalk is shown in figure below in case 3. Compared to case 2, insertion of protective earth wire does not reduce crosstalk, but increases crosstalk. noise.

This example shows that increasing spacing between tracks is most effective way to reduce crosstalk. If protective earth wire is used incorrectly, it may worsen crosstalk, so when using protective earth wire, it must be carefully analyzed according to actual situation. In order for protective earth wire to play its insulation role, it is necessary to add many GND through holes on earth wire, and distance between through holes must be less than 1/10λ, as shown in figure. λ is wavelength corresponding to highest frequency component of signal.

Internal Routing

To route inner layer as shown below

Dielectric constant 4.5, impedance 50 ohms. Consider following three situations. The attack signal is a step waveform with a rise time Tr=200 ps, ​​an incident signal amplitude of 500 mV, and a link length of 2000 mils. The figure shows crosstalk at near end. crosstalk at the near end is further reduced from 3.44 mV to 0.5 mV. Signal isolation improved by 16V. For indoor tracks, adding a protective earth wire can provide more insulation.

For surface wiring, it is recommended to use tight GND vias to improve isolation effect. But for inner layer wiring, there is almost no added benefit of using tight GND vias. The following figure compares distance between GND vias with 2000mil (GND vias on both ends of PE wire) and distance between GND vias. crosstalk at near end is 400 mils, amount of crosstalk is almost unchanged.

What happens when interval increases to 5 w?

If you further increase spacing between traces and keep PE wire line width at 6 mils, effect of PE wire on surface layer traces is reduced. In figure below, when distance between two wires is increased to 5W, near and far end crosstalk in both cases is equivalent to that without PE wire, and there is no obvious improvement. Thus, for surface traces, when distance between traces is large, adding a protective earth wire in middle has little effect. If not handled properly, it will worsen crosstalk.

For laying inner layer, protective earth wire still plays a big role. As shown in figure below, distance between inner layers is 5 W, and waveforms of near-end crosstalk in two cases are shown in figure. A protective earth wire is added in middle, which can greatly reduce near-end crosstalk.

Conclusion

Protective ground is usually effective for isolating low-frequency analog signals. But guard traces between digital signals are not that useful and sometimes make things worse.

For surface wiring, if distance between GDN holes of protective earth wire is large, crosstalk may be more serious, and very tight GND holes must be used to achieve isolation effect. For internal traces, safety ground wire can reduce near-end crosstalk.