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You can set the NI 5122/5124/5142/5152 analog input impedance to either 50 Ω or 1 MΩ. The 50 Ω path provides slightly better noise and distortion performance because the high impedance buffer for the 1 MΩ path is bypassed. The 1 MΩ path is required in applications that require minimal loading or that require using a standard 10:1 oscilloscope probe.
The 50 Ω inputs of the NI 5122/5124/5142 are protected by a thermal disconnect circuit. If an overvoltage event is large and sudden enough, however, the protection circuits might not have enough time to react before permanent damage occurs. It is therefore important that you observe the specified maximum signal input levels, especially when the inputs are set for 50 Ω.