A Pre-compensation Scheme with constant output common-mode voltage for current mode logic (CML) Driver
Original Publication Date: 2005-Oct-21
Included in the Prior Art Database: 2005-Oct-21
This dislosure presents a precompensation scheme for CML driver, in which two tail currents are used to adjust the output differential swing while keeping the output common mode constant
A Pre-compensation Scheme with constant output common -mode voltage for current mode logic (CML) Driver
A Precompensation Scheme with constant output common mode voltage for Current Mode Logic (CML) Driver
High-Speed interconnects are widely used as demand for high data exchange rate increases. To overcome the low-pass nature of interconnection media, precompensation is often used at the transmitter side to minimize Inter Symbol Interference (ISI). Current Mode Logic Driver (CML) is ofter used because of low switching noise.
A typical current mode logic (CML) driver is shown in figure 1 and figure 2. G2 and G2N are driven by predrivers and are complementary signals. In figure 1 there are two termination to Vdd at far end. In figure 2 there is a differential termination at far end. The way to do precompensation with those structure is to adjust the current I to get different output swings. As indicated by the equations in figure 1 and 2, the output common-mode voltage are also affected.
We propose a new structure shown in figure 3. It uses a differential termination at far end. From the equation in figure 3 we can see that the commond mode voltage is related to the sum of I1 and I2 and the differential swing is related to the difference between I1 and I2. We can adjust the difference between I1 and I2 while keeping the sum of them constant. By doing that we change the differential swing of the output while keeping the common mode constant.