Common-Mode Voltage Control for Mixed-Voltage Applications
Original Publication Date: 2003-Feb-20
Included in the Prior Art Database: 2003-Jun-21
Disclosed is a circuit topology that translates common-mode voltage between circuits. A common-mode translation between voltage domains alleviates common-mode mismatch between circuits and consequently common-mode noise. Controlling common-mode mismatch is important in Infiniband applications since the Infiniband standard requires a low common-mode noise voltage and allows the common-mode voltage of the output driver to driver to vary. The topology consists of a differential amplifier that has an easily controllable common-mode voltage and a bias circuit that sets the common-mode voltage of the amplifier. The desired common-mode voltage of the amplifier is a weighted average of the common-mode of the circuit driving the amplifier and the circuit being driven by the amplifier. Directly measuring the common-mode voltage of the amplifier’s output and subsequently adjusting the bias current of the amplifier sets the common-mode voltage of the amplifier. The bias current is adjusted until the amplifier's common-mode voltage is the desired weighted average. The common-mode voltage of the amplifier’s output is available on an internal node of the amplifier. Common-mode voltage is defined in terms of a differential signal. A differential signal is “two nodes that have equal but opposite signal excursions around a fixed potential Fairchild Semiconductor, http://www.fairchildsemi.com/products/interface/techfeature.html ). The fixed potential is the common-mode voltage of the differential signal, and a differential signal’s nodes are equal only when the nodes are at their common-mode voltage under normal operating conditions. Generally, the common-mode voltage of an amplifier’s differential output is the voltage at each output node when the differential input nodes are both at their common-mode voltage. Furthermore, the instantaneous common-mode voltage of a differential signal is the average of the two nodes of the differential signal.