The OpenLDI chipset implements a simple DC balancing scheme that reduces intersymbol interference (ISI). This demonstrates integrating digital functions onto the same chip as the LVDS interface. Without DC balance, a long cable can result in ISI for a single-bit transition and cause a bit error. This happens because a single-bit transition, after a long string of no transitions, may not contain the energy necessary to change the stored charge through the entire cable. The term disparity describes the stored charge on the cable. If the disparity magnitude is large, then the single-bit transition cannot overcome the intersymbol interference at the end of the cable.

The OpenLDI part provides DC balance on a frame-by-frame basis. During the frame, the transmitter monitors the input signal for transitions. If no transitions occur, the transmitter inverts the next frame to maintain balanced cable charge, thus keeping the disparity between plus 10 and minus 9. The seventh LVDS data bit indicates whether the data in the payload is true or inverted.

This simple DC balance scheme keeps the signal eye diagram wide open at the receiver end. In addition, it provides enough DC balance to satisfy fiber-optical interconnect requirements, allowing the OpenLDI chipset to interface with standard parallel fiber-optical products.

Another integrated enhancement to the OpenLDI chipset is the transmitter pre-emphasis feature. Without pre-emphasis, the signal coming out of a cable loses the sharp transition edges due to the cable’s high-frequency filter effect. With pre-emphasis, the driver accentuates the transitions to compensate for the filter effect at the end of the cable.

The pre-emphasis feature is user-selectable. When pre-emphasis is selected, the transmitter has two current drive levels. It delivers additional dynamic current during transitions to overcome the cable’s filtering and supplies a lower drive current after the transition. It opens the signal eye diagram by overcoming cable distortion of the signal.

LVDS is now spawning follow-on technologies that expand its applications. The first follow-on is Bus LVDS, which allows the low-voltage differential signals to work in bidirectional and multidrop configurations. Another LVDS derivative, ground-referenced LVDS (GLVDS), is progressing through the standardization process. GLVDS moves the differential signal’s common-mode voltage close to ground, which allows chips operating from very low supply voltages to communicate over a high-speed standard interface.