Analog Discovery 2, broken scope?

[EdWall]

Hi! I hope this ends up in the right section!

I had a weird thing happen to my Analog Discovery 2 just now. I was monitoring some uart communication with both scopes when my 2 scope started acting strange. It doesn't "hold" the voltage, initially it samples the right voltage but then it decreases exponentially, it kind of looks like a capacitor discharging or something like that (see pictures). If I instead connect 2+ to ground and use 2- to probe the signal it works fine although it's inverted... I cant imagine what would have whacked it since I'm just looking at some low voltage signals but it definitely seems broken to me...

If anyone has an idea of what might have happened or how to fix it that would be greatly appreciated, I've been really happy with the product so far and I really want that scope back in working order!!  

 

The first pic is the broken scope, and the second is the same measuring the same square wave scope but "reversed" like i mentioned.

 

Thanks!

[attila]

Hi @EdWall

In case of the first screenshot if you have used BNC adapter this was probably set to AC coupling.
The scope inputs wires are differential. The scope channel reads the difference between 1+ and 1- or 2+ and 2-. This for instance to measure the voltage drop (current) on a shunt resistor.
Using it in single ended mode the negative inputs should be connected to ground. Connecting the positive to ground and negative to signal you will have voltage reading with "opposite sign".
And always make sure to have common ground connection between the devices and circuits!

[EdWall]

Thanks for your answer @attila I kind of forgot that I posted here so sorry for the late reply! 

I'm actually not using a BNC adapter, I'm just using the jumpers that are included in the base unit so DC coupling is the only option. I'm quite sure that all the setting are correct and that I'm using it correctly since it worked well before. I think something must have happened with one of the components in channel 2, it kind of looks like there is an inductive connection between 2+ and ground that grounds the probe when the voltage is static. I would guess that there could have been some current spikes from the device I was probing that could have caused this. Any idea if there is a fix? Is it possible to fix myself if I identify the faulty part?

 

[attila]

Hi @EdWall

Due to high voltages usually the ADG612 gets damaged, resulting in positive or negative top of scale readings (like +/-25V, +/-2.5V)

First you could verify the resistors and the signal propagation on in the input path.

[EdWall]

Hello again!

This is going really slow for me!:)

 

Anyhow, I just checked the resistors and circuits you mentioned, everything seems good excep R40, I'm having some trouble getting a reading from it, mostly the the multimeter shows that there is no connection (infinite resistance). One time i dig get a reading of 1250K tho but cant seem to reproduce that reading. Could it be this resistor that has burned out? Do i need to replace it? Whats the recommended proces, dont do a lot of smd soldering..

 

Thanks!

[attila]

Hi @EdWall

First, you could try to resolder the resistor ends with a bit of solder wire. This may fix solder or end cap separation. If it does not solve try to replace it.

To confirm the problem is with R40. I've unsoldered this and with 1kHz 1V square I'm capturing the same.

[EdWall]

Hi again!

 

So I got the replacement resistor on there, only problem is that vin_sc2_p now somehow has a short circuit to ground.. im not sure if it was there before or if I messed something up with the soldering... I cant see any visual connection either.. might be time to give this up..

 

Thanks for the help tho!

[attila]

Hi @EdWall

Here the GND is highlighted with red and VIN_SC2_P with with contour.
The ground plane around C55 and R40 pads has low clearance. This has solder mask but could have been scratched during rework...

Edited October 12, 2021 by attila