Solution
In a single-core shielded BNC cable, the capacitance between the center conductor and the shield is much greater than the air capacitance between an external noise source and the shield. At low frequencies, noise cannot couple into AI− (the BNC shield). At higher frequencies, if noise couples into AI− through the small air capacitance, it will also couple into AI+ (the BNC center conductor) because of the significantly larger capacitance within the cable. Consequently, the induced noise presents itself as common-mode noise and is effectively rejected by the products’ CMRR to earth.
A shielded twisted-pair cable can also be used, but it requires additional attention. The shield must be grounded at both the amplifier end and the BNC end (differential BNC module). If the shield is connected to ground only at the amplifier end, grounding performance decreases at higher frequencies because the extended return path to earth has high impedance. In this situation, noise may not return through that path, reducing shielding effectiveness. For this reason, grounding the shield at both ends is recommended. Furthermore, there is greater variability within the cable itself, since maintaining closely matched capacitance from AI+ to shield and AI− to shield is difficult, and imbalance may increase with longer cable lengths. This imbalance can reduce the overall system CMRR to earth. For the differential BNC module (such as NI-9223 and NI-9239 BNC), 1M coaxial BNC(M)-to-BNC(M) cables have been tested in EMI environments within specified qualification limits, using the recommended cables and connection practices described in the respective Getting Started Guides. Under these conditions, no performance difference was observed between the connector variants.