How to suppress interference on LVDS connections

December 11, 2015 // By EDN
A short paper from Langer EMV-Technik (Bannewitz, Germany) notes that LVDS drivers (and LVDS receivers) can be the source of significant interference emissions. The paper describes methods of measuring, analysing and rectifying the problem.

Disturbances that are caused by the LVDS drivers may penetrate to the outside through the defective shielding of cables and connectors and thus lead to emissions. Unwanted common-mode currents of the LVDS drivers are mostly responsible for these interference emissions. The common-mode currents largely depend on the type of LVDS module. Practice has shown that LVDS receivers can also output significant common-mode currents into the LVDS lines. Transmission system connectors are not usually able to withstand common-mode currents as well as differential-mode currents.

Symmetrical transmission systems couple out much greater common-mode currents: differential-mode currents cancel each other out due to the fact that the transmission system is symmetrical and are thus suppressed. If the connectors' symmetrical lines are not symmetrical in relation to the shielding, additional differential mode currents couple out from the connector.

Measurement technology may help clarify the common-mode current problem and enable the integration of expedient interference suppression measures in the right place.

High-resolution measurements have to be performed on the drivers' and receivers' LVDS pins. The MFA-R 0.2-6 near-field probe achieves the required resolution.

(see above) Figure 1. The measurement set-up. The LVDS modules are connected via RJ-45 sockets and patch cables. The MFA-R 0.2-6 near-field probe is used to carry out the measurements on the pins as shown in Figure 2 and 3.

Figure 2. Measurement with the MFA-R 0.2-6 between the LVDS receiver pin pair 1 and 2

Figure 3. Measurement with the MFA-R 0.2-6 outside the LVDS receiver pin pair 1 and 2 (pin 3 NC)