Several types of wire based communication networks exist to provide communication among electronic devices. Many of these networks transmit a differential representation of the data over the network. A differential network uses a transmission cable that has a positive and a negative conductor, and positive and inverted representations of the data are sent on the conductors. A differential signal has the advantage of allowing faster data rates because the differential signals traverse lower voltage swings than single ended signals. Also, the data is less susceptible to noise in a differential signal bus because common mode signal noise picked up on the transmission cable is cancelled by sensing only the difference between the positive and negative conductors of the cable.
One critical parameter in differential signal wire based networks is the differential cross-over voltage of the signal transmitters. The differential cross-over voltage is the point where the voltage at the output of the positive signal transmitter crosses over with the voltage at the output of the negative signal transmitter. To minimize communication errors from power supply noise, electromagnetic interference (EMI), or signal ringing, the cross-over voltage should be at a point equidistant between the maximum and minimum voltages of the outputs. This point is often referred to as mid-rail.
If the network is a wire based serial network, transceivers are used to transmit and receive signals on the same transmission cable. Transmitters of wire based analog transceivers are generally designed with open-loop differential drivers. The drivers are open-loop in that they do not include a feedback mechanism in controlling their output. These transmitters are designed by tuning the cross-over voltage to an optimal mid-rail assuming a nominal process skew and nominal loading on the transmitter outputs. A problem with tuning is that when the transmitter is realized in silicon the cross-over voltage can deviate from the optimal mid-rail value due to undesired process variations or due to asymmetric parasitic off-chip loading. A deviation in the cross-over voltage from the mid-rail voltage value can result in low yield in semiconductor fabrication of the transmitters. A mask iteration may be needed to take into account the non-nominal conditions and to re-tune the cross-over voltage to the mid-rail value.
What is needed is a differential transmitter with a self adjusting cross-over voltage.