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I reused a design that was done on another product in my company that used 49.9 Ohm 1% .05W resistors for the PMD I/O terminations. As part of a derating analysis of the design I'm trying to figure out what the current is through these resistors which are pulled up to 3.3V. It appears that the one resistor draws current though 1/2 of the primary winding of the isolation transformer and the other resistor draws through the other 1/2 , with the center tap of the transformer connected to 3.3V. In this arrangement it seems that each resistor should draw 33 mA. Does this sound right? What are your recommendations for the power rating of the 49.9 Ohm termination resistors?
In 10M mode, the single-ended signal swing of each pin will be +/- 1.25V, i.e. each 50 Ohm resistor will see 1.25V across it. Based on this, the current will be 25mA (1.25V / 50 Ohms) and the wattage will be 31.25mW (25mA * 1.25V). For our evaluation boards, we often use 1/16 W or 1/10 W termination resistors, but based on the actual calculations, this level of power rating is not actually required.Patrick
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