Analog Discovery 2 Grounding Help for Newbie

[Jeromed]

I'm thoroughly confused by the note in the Analog Discovery 2 Reference manual, Section "2. Scope" about having a common ground reference. I understand (at least I think I do) about the dangers of floating ground but I'm not sure how to fix it.

My setup at present is a breadboard with power supplied by a linear DC power supply usually set to 9v (I got tired of replacing 9 volt batteries).  The power supply has - terminal, + terminal, and Earth Gnd terminal. Presently I'm powering my board with the + and - terminals, treating the - terminal as ground.

The Analog Discovery 2 (AD2) is being powered by a usb port on a laptop.  Since the power to the laptop is being supplied by an AC/DC brick, I have no idea what the ground is on the AD2.

Is it simply a matter of taking a ground wire on the 2x15 fly wire and plugging it to the - terminal via the breadboard?  Should I take the - terminal on the power supply and short it to the earth ground?  Or should I be looking at the solution mentioned in the note above:

Universal Serial Bus (USB) Peripheral Isolator Circuit
http://www.analog.com/en/design-center/reference-designs/hardware-reference-design/circuits-from-the-lab/cn0160.html#rd-overview

Thanks in advance for your help.
 

[Alex]

Perhaps, you can refer to this post 

 

[Jeromed]

That post basically says I need a common ground but it does NOT say how to do it.

On ‎5‎/‎30‎/‎2017 at 8:25 AM, attila said:

The Scope inputs of Analog Discovery are differential but not isolated. There needs to be a ground connection between the device and circuit to meet the common mode voltage limitations.

So do I simply connect a ground wire (not one of the four scope wires) from the 2x15 flywire to the ground on my breadboard?

Do I still need to run my breadboard ground to earth ground through my linear power supply?

On ‎5‎/‎30‎/‎2017 at 8:25 AM, attila said:

To use it in single ended mode, connect the negative input to the ground, like it is on the BNC adapter board.

Again, do I simply connect the BNC ground to breadboard ground?

On ‎5‎/‎30‎/‎2017 at 8:25 AM, attila said:

For more information see: https://reference.digilentinc.com/reference/instrumentation/analog-discovery-2/reference-manual

Which basically states this that started the whole question in the first place:

On ‎5‎/‎30‎/‎2017 at 8:25 AM, attila said:

2. Scope

Important Note: Unlike traditional inexpensive scopes, the Analog Discovery 2 inputs are fully differential. However, a GND connection to the circuit under test is needed to provide a stable common mode voltage. The Analog Discovery 2 GND reference is connected to the USB GND. Depending on the PC powering scheme, and other PC connections (Ethernet, audio, etc. – which might also be grounded) the Analog Discovery 2 GND reference might be connected to the whole GND system and ultimately to the power network protection (earth ground). The circuit under test might also be connected to earth or possibly floating. For safety reasons, it is the user’s responsibility to understand the powering and grounding scheme and make sure that there is a common GND reference between the Analog Discovery 2 and the circuit under test, and that the common mode and differential voltages do not exceed the limits shown in equation 11 . Furthermore, for distortion-free measurements, the common mode and differential voltages need to fit into the linear range shown in Figs. 12 and 13. For those applications which scope GND cannot be the USB ground, a USB isolation solution, such as what is described in ADI’s CN-0160 can be used; however, this will limit things to USB full speed (12 Mbps), and will impact the update rate (screen refresh rates, not sample rates) of the Analog Discovery 2.

I realize all of this would be a lot easier if I simply used the power supply from AD2 (as I assume everything would now have a common ground) for my breadboard but the experiments I'm currently going through all require 9 volts thus the linear power supply as I got tired of changing batteries.

Any help for a poor befuddle newbie would be appreciated.

[Alex]

Sorry, I may confuse you since that BNC adapter (optional accessory) was also mentioned in that post. You don't need BNC adapter in this case. I will say as long as you use the ground wire (black color) from the 2x15 flywire to ground the whole circuit on the breadboard, that should be fine. There is a similar tutorial using 9V battery in the passive RC circuit https://reference.digilentinc.com/learn/instrumentation/tutorials/ad2-oscilloscope/start

[Jeromed]

Well that certainly helped.  When I plugged 1+  1-  into a circuit I was testing, it changed what was happening, like it was introducing voltage.  When I then added a black wire to ground (-) it starting acting normally again.

And I do have the BNC Adapter as well.  Do I just put ground on the probe to ground on the breadboard?  Or do I need to also add a black wire from 2x15 flywire to the breadboard ground.

Thanks for your help.

[Alex]

I think so. You can check the schematics https://reference.digilentinc.com/_media/analog_discovery_bnc_adapter_board:discoverybnc_sch.pdf