[time-nuts] SMD TADD-1 distribution amplifier

Charles Steinmetz csteinmetz at yandex.com
Fri Dec 18 12:27:33 EST 2015


Poul-Henning wrote:

>A significant reason for the TADD-1 existing in the first place was to
>break groundloops.  This is incompatible with tying all the BNC's together.

It is perfectly possible to ground coax connector bodies at RF 
without grounding them at power-line frequencies (and expected 
harmonics) or DC.  Indeed, that is what the TADD-1 attempts to 
do.  The BNC bodies are connected to the enclosure with 10nF capacitors.

However, the implementation in the TADD-1 is not as ideal as I'd 
like.  The bypass isn't directly to the enclosure, but rather to the 
ground plane of the PC card, which appears to be connected to the 
enclosure only by fasteners at the corners of the PCB -- some inches 
away from the BNCs.  That is why I will almost never approve a design 
that uses isolated, PC-mount coax connectors.  The loop from each BNC 
body, through the capacitor, to the enclosure should be no more than 
1/2" (~ 1cm) long, preferably less.

I like to add a resistor in parallel with each capacitor, to shunt 
any unexpected DC or LF leakage current.  This resistor needs to be 
large enough (in value) to limit any circulating ground currents to 
low values that will not cause significant voltage drop in the 
shields (i.e., ground loops), but at the same time will effectively 
terminate any expected leakage currents.  Generally, this will be 
from 10 to 100 ohms, depending on the sensitivity of the most 
sensitive inputs in the system and whether guarding is employed.

Best regards,

Charles




More information about the time-nuts mailing list