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I will be experimenting with the Keysight company's N2792A differential probe.

Looking at the datasheet of the probe, the input impedance is 1Meg, but it is said to be used with the 50ohm impedance mode of the scope.

As far as I know, the 50 ohm mode of the scope is used for impedance matching, but the input impedance of the probe is not 50 ohm, so I am curious about the reason for using this mode.

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    \$\begingroup\$ The probe also has output impedance, and even in passive probes the two rarely match! E.g. when using a typical 10x probe, the DC input impedance is 10MΩ, while the output impedance is 1MΩ. But in a 1x probe, the input and output DC impedances match. High voltage passive probes may well have DC input impedances in GΩ range with output impedance of 1MΩ or less. \$\endgroup\$ – Kuba hasn't forgotten Monica Apr 21 at 18:42
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The probe itself is an active device which has input and output, each one with its characteristics (given in the manual):

enter image description here

Impedance matching, like with a standard signal generator with 50 Ohm output, minimizes signal degradation caused by reflection.

Edit: also check the answer provided by JRE on why 50 Ohm is commonly chosen.

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The N2792A is an active probe. It has a built in amplifier and a battery. Since it has an amplifier, you aren't restricted to the high impedances you need with passive probes.

  • A 50 ohm connection picks up less stray noise and interference from the environment than a high impedance connection - the 50 ohm connection is more immune to external influences so you get a cleaner measurement.
  • 50 ohms is a commonly used impedance for radio frequency equipment. The amplifier in the probe is probably designed using common circuits and components for RF circuits, which means the design is somewhat simpler if you use the same output impedance as those circuits are normally used with.
  • The scope has a 50 ohm input. Proper termination of a properly matched cable means you don't have to worry about signal reflection or losses between the probe and the scope.
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  • \$\begingroup\$ thanks, JRE. You means, N2782A differential probe output impedance is 50ohms. so I have to match scope 50ohm mode for suppressing reflection. Did I understand well? \$\endgroup\$ – journey Apr 22 at 1:15

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