Measuring a signal line instead of power rail

Technical Discussion
1

Dear community,
I am currently investigating an issue in a device and I am looking into measuring the current flow and voltage levels in a signal line instead of power supply. This would be especially for monitoring purposes, since I am not quite sure what the root cause of the issue with the device is, since I only have incomplete information on how the device was handled before running into the issue.
So my question basically is the following:
Could I just connect the JS110 between the output of one IC (Line driver/signal buffer) and the input of the other IC (Schmitt Trigger as Buffer. The signal would obviously go from the output buffer into the IN+ and then from the OUT+ to the input of the next IC.
I could then either connect the OUT+ and OUT- together and leave IN- unconnected, but then I would not have any voltage reading on the signal anymore, which would be unpractical, because there might be a short somewhere to a higher voltage which potentially leads to the destruction of the device.
But I guess since the IN- and OUT- are anyway shorted together, I could also connect the Device Ground with the to have the ground reference for the voltage measurement in the JS110 correct?

I am sorry if this description is incomplete in some parts. I am happy to answer further questions of the setup for clarification.

I am also curious if anyone already performed such measurements and if its practical in any way.

Best regards,
Alex

1 Like
2

Hi @Conrise and welcome to the Joulescope forum!

Yes, your proposed setup of IC output to JS110’s IN+ and JS110’s OUT+ to IC input should work. You can optionally connected either IN- or OUT- to your target system ground to measure the signal voltage level, too.

For reference, here is the simplified block diagram of your JS110:

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One potential concern is autoranging. It is very easy for a signal to exceed the JS110’s autoranging bandwith, which can cause it to have autorange more than necessary significantly decreasing accuracy. The JS110 is unable to measure for up to 5 µs following a current range switch. The JS220 features our newer Enwavity™ technology which measures through the current range changes.

If you notice excessive current range changes, you can either manually select a fixed range or add some capacitance between the IC input and ground. Several µF is enough, but depending upon this signal, even this amount could prevent proper operation of your circuit.

Does this make sense?

I am happy to take a closer look at what you are actually seeing. Post a picture of the Waveform widget during typical operation, and please enable the current range signal r, too. If you would rather not post this publically, feel free to DM me!