Understand Characteristic Impedance in NI Cables

Issue Details

• I want to understand the characteristic impedance functionality.
• Why is my cable labeled 100Ω?
• The input impedance to the card is 1 MΩ or greater, so why would I need a cable rated for 100 Ω? Is this the correct cable for my application?

Solution

There is a signal integrity problem when working at very high frequencies or long distances, such a problem can be handled with a carefully crafted balanced transmission line and matching impedance in differential amplifiers.

The 100Ω rating on a cable is defined in transmission line theory as the characteristic impedance, it is not a simple cable resistance value. This property describes how a cable transmits signals, particularly at high frequencies. The characteristic impedance is the equivalent impedance of the cable infinite ladder network representation, thus its value is not dependent on the cable length. When a transmission line of finite length (lossless or lossy) is terminated at one end with an impedance equal to the characteristic impedance, it appears to the source like an infinitely long transmission line and produces no reflections (no signal distortion or loss).

A cable labeled as 100Ω has to be terminated with a matching impedance at the source. NI instruments considered this problem and implemented such solutions in acquisition devices and cables.