THVD2450: Shield As Ground

Hi Mark,

Shielded cable increases noise immunity of RS-485 cabling and reduce radiated emissions. Shield connections are generally made to a solid ground (chassis ground or metal casing) with low impedance at one end and a series RC network at the other to prevent ground-loop currents. Cable shielding can also offer significant protection from induced burst and surge transience. If you are worried about induced noise on the shielding coupling to the signal ground of you device, it would be a good idea to isolate the chassis ground of the system to the signal ground through some resistance. 

This topic is briefly described in Section 9 this application report:

AN-1057 Ten Ways to Bulletproof RS-485 Interfaces

Let me know if this makes sense or if you have any questions about this topic.

Regards,
Eric

Hi Mark,

I will contribute to the discussion as well and expand on some of the topics mentioned by Eric because this topic is somewhat Grey and the answer will vary depending on the application. There really isn’t one right way to implement this for all applications and I will try to offer some information on a deeper level that may be helpful in other applications you may encounter.

First off, is it ok to use the shield as a ground?

Generally yes, but there some caveats to consider. First, you called it a Shield Wire, which I interpret as more of a Shield than a Wire. I would consider a Wire along the lines of any regular conductor. There are different types of ground shielding in cables. Foil shields are better from an EMI shielding perspective assuming the cable will not be in a continually flexing environment that can weaken and tear the foil shielding over time. Connecting the foil shield to the connectors can also be difficult, and sometimes there is a Drain Wire added to the cable that is an un-coated wire in physical contact with the shield that allows electrical connections to the foil shield easier. Depending on the frequency content, these drain wires can become inductive, form a current loop and radiate EMI themselves. It is important that they be kept as short as possible inside the connecdtor and preferably the entire foil shield should be connected to connectors that carry this shielding through the connector and onto the PCB. Some foil shielding may be very thin and some applications may cause the current carrying capability of the foil to be an important factor if it is the only ground connection in the cable.

Braided wires are also commonly used as a shielding but they are not as effective as foil and can have small gaps between the braided wires for radiation to enter or exit the cable. But if used they should also be connected as close to the PCB as possible. A drain wire may or may not be included in a cable with this type of shielding as well but the same factors apply as with the foil.

If a drain wire is used, it is only good for Analog applications with low frequencies. These drain wires do not make very good ground connections for high-speed digital signals. Your RS-485 application most likely is not considered a high-speed digital application, but there are some systems with fairly high data rates and signals with fast slew rates that behave a lot more like a high-speed digital signal than an low frequency analog signal. The use of a connector with a metal shell that will allow a complete connection to the cable shield may be required for high-speed applications.

Eric mentioned the use of ground-loop currents. The approach to have a low impedance connection between the shield and the equipment chassis or board only at one end, with a higher impedance connection at the other, is only good for low-speed applications. Higher speed applications are sensitive to higher frequencies and the ground shield needs to be low impedance at both ends for the frequency range it is operating as a shield. This is otherwise known as the Ground Transfer Impedance. Since impedance is sensitive to the ratio of capacitance and inductance in the system, this impedance will vary with frequency. If the shield only has a low impedance connection at one end, the return current will see an inductive ground and will be a poor shield for higher frequencies. If multiple ground paths care created between the systems such as a dedicated ground conductor wire within the cable and also a ground shield of higher impedance, a ground loop will be formed and create more problems. Some current will flow through the low impedance ground conductor and some will flow through the higher impedance shield. This can setup a current loop which can cause a low frequency noise to couple into the signals and be detectable by the system and is to be avoided. If the shield is your only ground connection then using it as your ground is fine. However, there is another ground wire, then you need to be careful to avoid creating a loop.

However, there is a valid reason to implement a low impedance connection at one end, and a higher impedance or RC connection at the other end of the cable for low-speed applications. Most of the near-field energy around the conductors is in the electric field mode and not the magnetic field mode for low-speed high-impedance circuitry. The shield acts as a Faraday cage surrounding the conductors preventing electric fields from entering or exiting the shield and only need a single ground connection.

However, for high-speed applications with low-impedance circuitry, most of the near-field energy around the signal conductors is in the magnetic field mode and only a magnetic shield will work in this application. The magnetic coupling creates high-frequency currents to circulate in the shield and move relative to the signal’s current acting as a counteracting force. This helps cancel the magnetic fields to provide a magnetic shielding effect. But this requires a low impedance connection at both ends of the shield so that the shield currents can enter and exit the cable easily.

I will note also, that a high-frequency system is not only found with high data rates but rather is a product of the signal’s rise and fall times. High frequency content is needed to create fast slew rates in a switching signal. If a signal has a fast slew rate it is can be considered a high-speed signal even if it is used for a slow data rate.

Your question implies a low frequency application with a single ground shield connection at one end. If you wish implement this type of connection, you should use both a resistor and capacitor and not simply a capacitor. Using only a capacitor will act as a DC block for the flow of DC current between the two systems and really what you want to do is pass DC and filter higher frequency content. Using a resistor and a capacitor allow you to pass DC through the resistor and absorb high frequency in the capacitor.

There is just one other aspect that may or may not be relevant to your application I would like to mention in this informative post. This is the question of how many signals are contained in the shield. You mentioned that you are using shielded twisted pair cable. This could be a cable with a single twisted pair that is shielded, a cable with multiple twisted pairs that are shielded individually, or a cable with multiple twisted pairs that are all contained in a common shield for the entire cable. Ideally a shield such a foil shield that is completely connected at both ends will form a perfectly symmetric shield and will radiate nothing. This prevents crosstalk between multiple shielded twisted pair cables.

However, if multiple wires within a common shield can suffer from crosstalk due to the common ground shared amongst them if not also directly. Noise coupled onto the ground from one signal conductor can induce a current in the other conductor through the ground connection indirectly. This is where I think you are thinking it is a bad idea to use the shield as a ground connection. In this regard you are correct, but there are other factors to consider that I have previously mentioned. Ideally, you will have individually shielded twisted pairs, either with a single pair per cable as is common with RS-485, or with multiple individually shielded pairs.

I know that was a lot if information at a pretty high level which may or may not be relevant for your application depending on the frequency content. But I hope it was informative.

Regards,

Jonathan