I was looking at the differential output driver of a RS422 transceiver with my scope, I had one output connect to the scope probe and the other connected to the scope probe ground. With this set up I measured +5 and -5 10 volts differentially however the + had about 3 volt of noise riding on the square wave I was measuring. Is the normal? What should I see?

Please help,

Kevin

 

Grounding an output driver is a pretty egregious error. The scope is however doing what you are telling it to do, which is to display the scope input with respect to what it thinks is "GND", which in fact is trying to beat up and down instead of being a reference.

GND should be connected to GND the world around. If you want to look at a differential output then use two channels for the inputs and set the scope up to display CH1 + CH2-INVERTED. Your scope may have separate buttons to invert channel 2 and to display the sum of two channels.

It is a good thing the RS422 drivers are so robust. You did manage to avoid toasting it -- right?

 

Thanks, your right don’t know what I was thinking, no damage though my scope is a Fluke scopemeter ground is floating.

KK