Answer to View Challenge:
Before you see the answer, these are NOT the reason for the difference:
-The difference is power supply quality.
-One uses onboard Arduino regulator, the other does not.
Final hint: If the probe ground is connected for either circuit, then the waveform is a clean square wave like we saw at the start of the video.
As most people noticed, the core difference is that Circuit A was powered by USB while Circuit B is powered by the barrel jack. The barrel jack was a little bit of a red herring. If you did not notice earlier in the video, the barrel is powered by the bench power supply. (However, if it was a wall-wart, the result would be similar.)
The USB supply came from an instrument (or PC) connected to the wall with an Earth ground tab. (The oscilloscope, like most scopes, is also connected to Earth ground.) The bench power supply’s output channels are isolated (or floating) from Earth ground!
So, Circuit A is finding a ground path through the earth ground connection of the USB supply and the scope. While Circuit B is likely to find a path through the Neutral/Ground connection back at my electrical box.
- WorkBench Wednesdays #47: What is Oscilloscope Bandwidth?
- WorkBench Wednesdays #42: Compare Ideal vs. Real Filter with an LCR Meter
- Tektronix: Application note for Fundamentals of Floating Measurements and Isolated Input Oscilloscopes
Bill of Materials
|Product Name||Manufacturer||Quantity||Buy Kit|
|RTM3000 - 4 Channel Oscilloscope (RTM3K-COM4)||Rohde & Schwarz||1||Buy Now|
|TPS2012B - 2 Channel Isolated 100 MHz Oscilloscope||Tektronix||1||Buy Now|
|[Bundle] Development Kit, Analog Discovery 2 Pro Bundle, 30MHz Oscilloscope, 12MHz Waveform Generator||Digilent||1||Buy Now|
|RTH1054, 500 MHz, 4 Analog, 8 Digital, 5 Gsps, complete||Rohde & Schwarz||1||Buy Now|