# Which side is the shield's drain cable connected and why?

I have 2 PCBAs connected using a shielded cable. The cable is transmitting an SPI signal from master PCBA to the slave at 30MHz. I have previously been advised to shield on the driving circuit (master.)

Could some one explain the reasoning for this?

I have sensitive circuitry on the master board, why would I want to couple any noise picked up on the shield back to my master ground?

The right thing to get the best shielding is to connect it on both sides and normally do it with a resistor and a capacitor in parallel. the high resistance resistors will make it dc free in average and the capacitor allows high frequency currents to flow, so the shield is effective. (The resistor is not really needed on both sides.)

Also you want to connect the shield not do GND but functional earth. The last thing you want to do is provide a path for the EMI into your circuit.

Anyway, normally transmitters are less sensitive to noise than receivers, so if you must decide which side to connected a shield, the side with the transmitter is the one.

As SPI as single ended bus sends and receives on both sides and you go pretty fast for single ended signals. I would opt for both sides.

If you have a spectrum analyser and a coupling clamp with a burst generator, you can measure that a only one side connected shield is just less effective as a both side connected shield. (It must be connected also with functional earth on both sides of course.)

Of course you also can try the way Americans do Ethernet traditionally: not connecting a shield on either side. it's not as effective, but nobody can "forget" the earthing.

Here some basic examples

• In this example what would be a functional earth? Also why is every application note online suggesting not to connect on both sides (you could get a current from the voltage difference in the 2 gnds and this current through the shield could magnetically couple into the sensitive signal circuit). Sep 1, 2020 at 8:03
• Well, in an office, @home, functional earth is the earthing pin of your power plug, or anything which is also connected to that potential, like metal waterpipes. In an industrial context - if done correctly - almost anything made of metal should be connected to functional earth. Also to have low resistance, in some production floors special rails are installed for functional earth connection. Sep 1, 2020 at 15:48
• Its a matter of philosophy. in the US often unshielded cables are used to avoid wrong connection while here in europe/germany we demand propper connection. sometimes even the cable coating is removed every meter or so to provide additional connections of the shield to functional earth. The ideal is a tight mesh of connections. As said, a only one side connected shield is much less effective. In fact you might even cause EMI at the not connected end. If you connect shield with capacitors and at least one resistor, its DC free and only currents caused by EMI will flow. Sep 1, 2020 at 15:51
• here is a document from the profibus consortium about propper shielding and earthing, profibus.com/…, it also mentions an active single side connected shield. Some more: belden.com/blog/smart-building/… , cablinginstall.com/home/article/16467511/… Sep 1, 2020 at 16:13