A weird problem of high frequency low voltage signal fed into my machine

[udir]

Hi There
I am experiencing a weird problem with my newly design.
Its basically a simple carrier board with SOM that operates a touch screen and 2 other applications.
This device is Fed from simple 12 V DC power adaptor and is said to be "floating".
When I started testings and an external lab, they used a probe that generates a low voltage signal in a high frequency and touched either the power supply cable when its connected to the machine or the touch screen itself, and it suddenly froze! when i touch the screen nothing happens.....only restart helps.
I have no idea why it happens as the device is suppose to be floating.any idea how to attack this?
Ron

 

[jiripolivka]

To introduce any signal to a "floating" circuit, one must define a local ground to which the control signl is referred as well as the output. Find a "ground" of the circuit, sometimes the negative end of an electrolytic capacitor blocking the DC input can be used. If possible, couple the control signal through a 1 to 10 nF capacitor.
Touching any point without the ground reference causes the freezing.