1
\$\begingroup\$

I am planning to measure an RS485 signal with an oscilloscope. Since I want to see the signal that is "seen" by the receiver side, I was planning to plug the reference terminal of probe to Sig(-), and the measurement terminal to Sig(+).

However, I saw in many places that a differential probe should be used for measuring differential signals, and I couldn't understand the reason.

Measuring differential signal with single ended probe

What is the reason? Why does a differential probe exist after all, since all measurements in electronics are inherently differential?

\$\endgroup\$
  • 2
    \$\begingroup\$ Because the signal's ground and the scope's ground are probably connected to the same ground (i.e. earth). Since differential signals' (e.g. RS485, CAN) plus and minus signals are referenced to the same ground, it's always a good practice to use either a differential probe or two probes/channels with mathematical subtraction. If you use a single probe you probably short one signal to ground. \$\endgroup\$ – Rohat Kılıç Oct 30 '19 at 6:21
  • \$\begingroup\$ @RohatKılıç In my application, the transmitter circuit's reference ground(also the signal's ground) and Earth ground are isolated, so it is unlikely to be a problem. Are there any other possible problems? \$\endgroup\$ – C K Oct 30 '19 at 6:36
  • 2
    \$\begingroup\$ differential probes will have better common-mode rejection, giving your scope a more honest waveform of the "difference". \$\endgroup\$ – analogsystemsrf Oct 30 '19 at 6:59
1
\$\begingroup\$

I am planning to measure an RS485 signal with an oscilloscope.

And...

In my application, the transmitter circuit's reference ground(also the signal's ground) and...

Here's the contradiction. There are two basic signals in RS485 - they are differential and balanced and they need to be so. If they are not then it's not RS485. Then you say that the transmitter's reference ground is also the signal ground and that cannot be so for RS485.

If you are trying to measure a proper RS485 signal, you should not connect one of the transmit lines to the oscilloscope ground reference because that will load imbalance the signal (at best) or at worst short that signal out.

Use two single ended probes for each of the transmitted signals and subtract the signals using the oscilloscope features.

|improve this answer|||||
\$\endgroup\$
  • 2
    \$\begingroup\$ worth pointing out more explicitly, rather than hinting, that almost all two channel scopes have a display Y1-Y2 feature, which is the 'poor man's' differential probe. \$\endgroup\$ – Neil_UK Oct 30 '19 at 8:19
  • \$\begingroup\$ Hello, I am not very experienced with such signalling. Can you elaborate a little on a "balanced" signal and load imbalance? Does it mean that the different impedances on receiving side (probe reference and probe measurement tip) will cause problems? \$\endgroup\$ – C K Oct 30 '19 at 9:22
  • \$\begingroup\$ @CK exactly - it affects the edges of the data being transmitted and can easily cause data corruptions. In telecom applications it is called earth impedance balance and is quite a hot subject from simple telephones to high speed data links like ISDN and internet. \$\endgroup\$ – Andy aka Oct 30 '19 at 9:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.