Creating Negative Patterns using Analog discovery 2

[Arpit Rao]

Greetings,

I am trying to generate 4 synchronized custom patterns that toggle between -0.9V and 0V. I am using two Analog Discovery 2 devices and accessing the Wavegen with 4 channels and connecting T2 of the two devices together. I am setting the offset voltage to -0.9V and amplitude to 0.9V. The ground of the Analog Discovery 2 I am connecting to a -0.9V supply which is the logic 0 for my DUT. My DUT is on a PCB and has a custom IC and can’t take too much current. Is there something wrong with my test setup? Also when I connected the outputs of the Wavegen to an external oscilloscope I see a lot of ringing in the waveform, is that expected?

 

Any feedback or suggestions are welcome.

 

 

Thanks,

Arpit

[JColvin]

Hi @Arpit Rao,

I am confused on your power supply setup. When connecting multiple devices together, you need to have them share a common ground so that they all have a consistent reference point to which they can compare their logic voltages to.

Where is the -0.9 V supply coming from? Is it from an external supply or are you using the Analog Discovery 2's power supplies?

What do mean by the -0.9V supply is the logic 0 for your DUT? Does your DUT use a logic standard where the logic low is a negative voltage or some sort of differential input (which to my knowledge still has a reference GND voltage that it compares to)? Or is this something entirely different that I'm not understanding?

Regardless, if using an external power supply, you should not attach a -0.9 V output from it to the ground of the Analog Discovery 2 because the ground of the Analog Discovery 2 is shared with the USB ground connected to the host computer and you would not want to have deliberately mismatched grounds.

As for the ringing on the wavegen output, are you using a BNC probe to view the signal or just a straight cable? What frequency are you sending the pulses at and do you have any sort of load on the output?

Thanks,
JColvin

[Arpit Rao]

Hi @JColvin

thanks for the reply, The DUT I am trying to test has a logic 0 at -0.9V and a logic 1 of 0V. The two AD2 devices I have I am connecting their Grounds together. Do I need to connect that common  AD2 ground to another supply or I can leave it floating to the external world?
 

I am giving the -0.9V to the DUT externally by using two positive external dc supplies. I connect the negative terminal of supply one to the positive terminal of supply 2 (0V) and taking the negative terminal of supply 2 to create -0.9V and giving it to the DUT.

 

For the ringing, I am connecting the Wavegen outputs to an external oscilloscope through jumper to SMA, SMA to BNC.

 

 

Thanks,

Arpit

[miketranch]

Just a reminder that USB connects ground from the Discovery2 to ground of PC. 

Depending on your PC, that could mean the Discovery2 ground has a path to AC power/safety ground. 

[Arpit Rao]

@miketranch

Suppose I connect the Ground of my pair of AD2s to the supply of my DUT which is at 0V, will there be any current flowing into my DUT from the AD2? 

also what happens if the ground of the AD2 is floating? Will the Wavegen outputs toggle between -0.9V and 0V? 

Edited January 14 by Arpit Rao

[attila]

Hi @miketranch

You should not leave any together used device floating since the communication between these would not work, unless you use dedicated isolator.
Voltage is potential difference ! You need to have a common or known relative level.

You could use different reference voltage for the DUT, like the GND of this to be connected to +0.9V relative to the AD2 GND. A 0.9V floating supply negative to be at AD-GND and positive at DUT-GND or use the AD V+ supply. This way the AD's 0V will be -0.9V for the DUT and for AD's +0.9V be 0V for the DUT.
Note, the AD-GND is connected to the computer GND via USB cable and this, depending on the computer supply, this may be connected to mains earth.
A wrong connection, like earth referenced supply output connected to AD GND will be a short circuit that could damaged the devices !

Usually, the safest option is to use the same GND for all devices and generate negative pulses.