# What does 50ohm input/output resistance mean on a oscilloscope?

Joined Jul 11, 2014
24
Im a little confused as to what the purpose of the the 50ohm resistance means on the oscilloscope BNC connections and what do i connect to my op amp circuit to match the resistance?

#### MrChips

Joined Oct 2, 2009
29,825
Oscilloscopes usually have an input impedance of 1MΩ or 10MΩ with a probe.

#### studiot

Joined Nov 9, 2007
4,998
The word is impedance, not resistance and it refers to the impedance seen by the signal as it travels along its path.

To understand the answer to this you have to have some idea what happens when a signal passes along a cable or in fact along any path, and also the difference between resistance, reactance and impedance.

This theory is called transmission line theory.

Common values are 50Ω for instruments including scopes and computer work, 75Ω, 150Ω or 300Ω for Radio work (RF equipment).

The connectors (BNC, TNC, Coax or whatever) and cables are all designed to present a continuous path at the desired impedance to the signal.

Any irregularities in this path will result in different frequency components of the signal being transmitted differently, destroying the result at the receiving end or causing reflections.

Since you are coupling an op amp this may be academic since it the whole system really only affects high frequency signals and has little effect on power and audio frequencies.

#### crutschow

Joined Mar 14, 2008
33,342
Typically oscilloscopes have 1 megohm input resistance in parallel with perhaps 30-50pF of capacitance but some are selectable for 50Ω resistance also. The 1 MΩ input is commonly used with a 10:1 probe or for direct connection with low frequency signals. The 50Ω is normally only used for high frequency signals where you want to match a 50Ω characteristic impedance coax cable carrying the signal.

A typical op amp can't drive a 50Ω load. For that you want to use the 1MΩ input, probably with a 10:1 probe to minimize loading of the circuit.