When to grounding to Earth(PE).

I met a strange problem on my dspboard.

I connect one capacitor (103/400V) between GND pin and Earth pin in my dsp board.

Sometimes, if I connect the Earth pin to the PE of the AC power. The dspboard works well, for example, if I just use the ground of probe of osco to touch the GND quichly, the dspboard will still works well. If I just let Earth pin impending ( unconnected), the dspboard will not works well if i use the ground of probe of osco to touch the GND quichly.

But today, if I connect the Earth pin to the PE of the AC power. The dspboard will not work well, for example, if I just use the ground of probe of osco to touch the GND quichly, the dspboard will not work well. If I just let Earth pin impending ( unconnected), the dspboard will work well even I use the ground of probe of osco to touch the GND quichly.

By the way, I USE TDS2012 osco. Somebody told me ever the ground of the probe is connected to PE in the osco. I am not so sure with that.

So, could you tell me when to connect the Earth pin to PE of AC power?

In article , Simon wrote: [....]

First, I'd look for a floating pin in the design based on what you've said.

You may want there to be a connection to the earth ground but you don't want the ground currents of other devices flowing through the sensitive parts of the PCB. You need to expand your thinking to enclose the entire system.

For safety reasons, the external metal has to be well grounded. For EMI reasons, you are likely to want to enclose your completed product in a conductive housing. This may mean metal or metal coated plastic.

The equipment you are connecting to may have its own ground connections. The cables may force the issue somewhat. You want to first figure out where the low frequency and DC currents will flow. This will determine some of the direct connections. In other places, you may want to connect RF currents, there you have the choice between a direct connection and a capacitor.

One thing you want to avoid is making flaky connections for RF. Two bits of sheet metal touching lightly can cause trouble. A little bit of, lets say 100MHz, leaking into some analog circuits may not cause any problems. If that 100MHz gets strongly AM modulated by the fan vibrations and a couple of bits of metal, it is far more likely to cause trouble.