Is It OK to Use an External 50 Ohm Terminator with an Oscilloscope?

Recently, a reader posed the question in the Comment field on Dr. Eric Bogatin's blog post, How to Choose Between the Oscilloscope's 50 Ohm Input and 1 MOhm Input:  "Is there any difference between using an external 50 Ohm terminator instead of the internal 50 Ohm termination on the oscilloscope--for example, using a RG58/RG174 cable?"

Eric answered:

"In principle, you can use the oscilloscope input set for 1 MOhm termination, then add an external 50 Ohm termination resistor on a BNC Tee connector, for example. This has the advantage that you can actually use any resistor for a load, or terminate signals with an RMS voltage larger than 5 V.

However, there are two problems with using this approach for high-speed signals with rise times shorter than 1 nsec, which require an oscilloscope with bandwidth larger than 1 GHz.

How to Choose Between the Oscilloscope's 50 Ohm Input and 1 MOhm Input

Dr. Eric Bogatin

Figure 1. The same signal from a fast-buffer driver
measured with a 1-meter, 50 ohm cable with
1 megaohm input to the scope, and
same cable with 50 ohm input to the scope.

When every oscilloscope has both a 50 Ohm input and a 1 MOhm input, how do you choose which one to use? Here are my recommendations for when each input should be used. 

For additional information on this topic, check out my webinar on What Every Oscilloscope User Needs to Know About Transmission Lines.  

Build Your Own Low-Cost Power Rail Probe

Figure 1: The source series termination method is
a low-cost alternative for probing low-impedance,
fast-switching sources.

In an earlier post, we discussed the limitations of  Using 50-Ohm Coax from DUT to Oscilloscope  with low-voltage, high-bandwidth signals, like power rails. In this post, we’ll explain how to build your own, low-cost power rail probe to serve the purpose.

The source series termination method is a good alternative for probing a low-impedance, fast-switching source, comprising a 50-ohm resistor in series between the DUT and the coaxial-cable connection. The coaxial cable is then connected to the oscilloscope’s analog input set for 1 megaohm termination. An equivalent circuit model and a simple implementation appears in Figure 1.