Beginner Q : Scope connections for the Discovery 2

[Dig2]

Hi,

I recently purchased the Discovery 2 and the BNC adapter board (Pro kit). Could someone please clarify where the positive and negative connectors for the scope should be attached when monitoring an AC signal (+/- 12 V). Should the negative connector be attached to the negative rail? And when using the BNC adapter board should the negative connector be connected to ground (because the inputs are not differential)?

I wanted to confirm the connections as I had a mishap and toasted my pcb's power supply whilst trying to use the waveform generator and the oscilloscope. Just trying to debug why that happened. I had the oscilloscope's negative connector connected to ground.

 

[HansV]

 

Hi, i am not a digilent expert but experienced the same problem as you have so i thought i give you the topics that helped me. 

This helped me, I use the USB-isolator which you can see at the end of the video.

 

 

 

 

[NAEastland]

Hey Dig2,

So to your questions, the negative (alligator clip) of the BNC probe should not be connected arbitrarily, since it could lead to either erroneous measurements at best or damage to your devices at worst. A way I try to think about it is to ask what voltage do you want to measure? For instance, if you are measuring an AC output signal that you expect to swing between +/- 12V, you would not want to connect your reference point (alligator clip) to a voltage other 0V. If you connected your reference to -3V hypothetically, once the output signal reached its max you would actually get a reading of 12V - (-3V) = 15V, and the minimum output would read -12V - (-3V) = -9V. This is of course just a hypothetical example of an erroneous measurement.

The worst case would be that you end up creating a short between the ground of your scope and the device you are measuring and you end up frying something. The sources that Hans has linked in his reply should help you with an explanation of what may have happened with your particular set up. In the video, Dave explains the couple cases where connecting the negative (alligator clip) end of the probe to wherever on does not make a difference (these being when the device you are measuring is isolated from the ground of your scope). He also explains how doing so will cause problems if both devices are connected to the ground of the mains voltage. As a general rule you should always connect the negative of the BNC probe to a ground/reference point on the device you are measuring.

As you said though, you had the probe's negative clip connected to ground, so you should not have had a problem. Without having seen your set up I really can't say what happened (obviously something was shorted, but where), but yes, as the BNC probes are not differential the negative for the probe you are using should be connected to ground of the device you are measuring. Sorry this got a bit wordy, but I hope this and the info shared by Hans was more helpful than confusing.

Regards,

Nate

[Dig2]

Thanks for the answers HansV and Nate. Part of the confusion I had was when I was using the 'standard' connectors (orange/striped orange and purple/striped purple). I wasn't sure if the  negative connector was placed at the most negative point on the board or at ground.

From the videos and feedback it seems that this connector should be placed at ground to avoid a short.

My set up  has a laptop , powered by an AC/DC adapter with a ground reference, connected to the Discovery 2. The Discovery 2 is powered by an adapter with a floating reference, and a PCB board is connected to an AC/DC adapter with a ground reference. 

So as long as the negative reference (striped orange / striped purple)  of the oscilloscope  is attached to ground on the PCB board everything is ok. 

Thanks guys for improving my understanding