How do you choose whether to use the 50 ohm input or the 1 megaohm 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 megaohm 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.  

Fundamentals of 100Base-T1 Ethernet

100Base-T1 Topology

The term Automotive Ethernet can be used to refer to any Ethernet-based network for in-vehicle electrical systems. It encompasses 100Base-T1, as well as several other variants/speeds of Ethernet (e.g., 10Base-T1, 1000Base-T1). Here, we’ll describe 100 Mb/s Automotive Ethernet as defined by the IEEE in its 802.3bp specification, which is nearly identical to BroadCom’s variant, BroadR-Reach.

What Is SENT SPC?

SENT SPC interrogation mechanism showing 
MTP preceding standard SENT frame.

Single Edge Nibble Transmission protocol, more commonly known as SENT (SAE J2716 JAN201604), has long been used by the Automotive industry to report low-speed sensor data to the Engine Control Unit (ECU). SENT was developed because the environment in a car was too noisy to transmit high resolution (10- or 12-bit) sensor data vertically on a 5 V bus. Instead, sensor signals are transmitted as a series of pulses, with data measured by falling-edge to falling-edge times. Information lies within the width of the pulses. Later specifications of SENT introduced Fast and Slow Channels to designate different streams of information carried within the same messages.

Probe Safety Demystified: Dynamic Range and Voltage Swing

One of the most basic things to know when using any probe is “what is the maximum voltage the device can safely measure?” The answer isn’t as straightforward as you might imagine, it requires understanding several key probe specifications as well as the nature of your signal.

Single-ended Range

Figure 1. Single-ended range is
measured voltage input to ground.

Everyone is pretty familiar with single-ended range: that's the maximum safe voltage input to ground, shown in Figure 1. Ground is directly tied to oscilloscope ground, which is tied to building ground. Therefore, when measuring voltage within this range using a single-ended probe, the ground connection cannot be a floating voltage, or you could damage the probe, the DUT, the oscilloscope...maybe yourself, as well. Single-ended voltage must be a grounded voltage on your board or something that could be tied to ground.

Probe Safety Demystified: CAT Ratings

Figure 1. CAT ratings required to
safely test different electrical sources.
(Sourced from “Measurement Categories,”
Wikipedia, Oct. 28, 2019.)

Any voltage probe will have several published specifications that are meant to indicate under what circumstances that probe is safe to use. They answer questions such as “What is the maximum voltage I can safely run through this probe?” and “Can I safely hold this probe while using it, or does it need to be mounted somewhere far away?” In this post, we’ll explain what the CAT rating is telling you.

CAT ratings are standardized ratings used to categorize the suitability of a voltage measurement device based on the source impedance of what it is used to measure. They are issued by the International Electrotechnical Commission (IEC).

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.

What Every Oscilloscope User Needs to Know About Transmission Lines

Eric Bogatin, Signal Integrity Evangelist, Teledyne LeCroy

Figure 1. Measured voltage at the oscilloscope from a
fast edge, low impedance DUT, with the oscilloscope at
1 megaohms (left) and 50 ohms (right).

It is easy to take a measurement with an oscilloscope and see a voltage waveform on the screen. It is sometimes hard to take a measurement without artifacts and interpret all the details of the measurement. 

Whenever you measure a signal with a rise time shorter than about 20 ns, assuming a 1 m long coax cable, transmission line effects should pop to the top of your list of potential artifacts to consider and avoid.