1619 - How do I measure the common mode voltage of LatticeECP3 SERDES current-mode logic (CML) signaling ?
LatticeECP3 SERDES uses current-mode logic (CML) signaling for high-speed pins such as PCS_HDOUTP/N, PCS_HDINP/N, and PCS_REFCLKP/N. In order to successfully communicate between two devices with CML signaling, the common mode voltage of CML signaling must meet its specification, which can be found in LatticeECP3 Family Data Sheet. Please refer to Table 3-6 for PCS_HDOUTP/N, Table 3-9 for PCS_HDINP/N, and Table 3-12 for PCS_REFCLKP/N.
Two methods are often used to measure CML common mode voltage. One way is to use multimeter in DC voltage measuring mode. This method is very simple and easy to use. The other one is to use oscilloscope with high-impedance probe. It is important to use high-impedance probe. Otherwise, the DC bias mismatch between oscilloscope of low-impedance probe and device under test will give rise to wrong results. It is one of mistakes which people often make in measuring common mode voltage.