The RocketIO Users Guide specifies that the power consumption for the VTRX pins is zero when the receiver is AC coupled. Why is this?
No power is dissipated because there is no DC path to ground.
The following circuit illustrates an AC coupled receiver:
For power to be dissipated from the VTRX source, a path to ground would need to exist.
- DC current cannot flow from VTRX back through the transmission line due to the AC coupling capacitor blocking DC current.
- The only current that can flow is the leakage current through the receiver gate to ground. This is a very high impedance path, which sinks little current.
- The current sourced from VTRX in an AC coupling link has been measured to be less than 50 nA (nanoamps) per transceiver during 3.125 Gbps operation on the ML321 characterization board.
- Note that this number might vary depending on your transmission line, coupling capacitors, termination resistance, and VTRX voltage.