Discovery BNC Scope distortion

[andy128]

Let me apologize in advance, but I'm new to the Discovery2 and not an analog person.

My question has to do with the Discovery2 BNC adapter board.  In particular, I am talking a look at I2C signals (~400Khz) using the analog inputs.  When connected to the AD2 directly, I get the expected signal going from rail to rail.  When connected through the BNC adapter and scope probe (jumpers set to DC coupling) I get a distorted signal.  Not sure if I can trust the BNC adapter.  Any suggestions on what's going on and if it can be corrected?

Thanks in advance,  +andy

[jpeyron]

Hi @andy128,

The Analog Discovery BNC Adapter board is a pass through circuit so I would check the gain on the probes (i.e 1x, 10x).  @attila could you confirm.

cheers,

Jon

[andy128]

Yes, that's why I'm confused.  I looked that the board schematic and noticed that it's pretty much a pass through short of added a little capacitance because of the board traces and such.  I ohmed out the ground and found 0.4 ohm from the probe itself. The input impedance is ~ 9 Mohm.  The probes I have are the ones from "Analog Discovery 2 Pro Bundle".  I also tried some Tex P5050 with the same result.  The gain on the channel is set to 10x which is correct for both probes.  I had a hardware engineer at my company also looked at it and he can't explain it.  Thus this post asking for help.  I'm confused.

[jpeyron]

Hi @andy128,

I have tagged the engineer that will be able to give you the best response to this issue.( @attila).

thank you,

Jon

[attila]

Hi @andy128

The scope probe at 10x needs to be tuned.

Connect Scope probe to W1 (JP4 0), generate a 1kHz square wave and adjust the trimmer on the probe with a screwdriver until you see a perfect square wave.
The trimmer is near the BNC connector on the other side of the label.
Gray is undercompensated, orange is good:

For fine tuning you could use the Network Analyzer.
Orange is undercompensated, green is good.

 

[andy128]

That did it!  Very cool instrument  Thanks so much.

 

+andy