PPK2 -> strange Source-mode current readings after sudden change in range

[WhiteFang]

Hi,

EDITED: This appears to NOT be related to any open circuit, the SAME behavior comes even with abrupt changes to the current scale, without any interruptions. Additional pictures at bottom of post to show this as well.  So then that seems like something goes wrong with the range selection logic internally in the PPK2 then?

I am using a PPK2 and doing some simple testing with an LED and two resistors in parallel. I have one resistor running via a momentary switch, and I have Vout from PPK2 feeding the circuit via a second momentary switch.
I have it set to source mode, 3000mV.

If I press and hold the Vout button, then turn on the output and start sampling, I see values as expected around 1.4mA, if I press the second switch, the current increase to a bout 2.3mA. If I momentarily let go of the Vout button the readings go haywire and show 0-20mA "all over the place" (see snippet below).  Once in this mode, the only way to reliably recover is to turn source off and then on again, and make sure the Vout is connected before turning it on. 

I suspect this has something to do with the PPK2 entering a bad state when the circuit is "broken" but I am very surprised that it is unable to recover once the circuit is completed again?!

It would imagine there would be a number of ways the DSP on the PPK could detect this type of condition and avoid getting "stuck" in some weirdo mode?

Has anyone else seen this issue?

Of course one could argue that this is a corner case, and one would be correct.  This is not how the device would be expected to be used in most cases, but imagine you want to measure some relay operated load for example, then this type of behavior could happen?

I did a PPK2 power-cycle and repeated the experiment so I could include a "clean" sampling metric output from the shell:

New snippets showing the same behavior without any interruption in the circuit.  In the below I temporarily connect a 100Ohm in parallel to suddenly increase the current draw of the circuit. Upon removing the 100Ohm I would expect to fall back to the initial low 1.x mA, instead it goes into "freak mode".

[Sigurd Hellesvik]

Hi,

Thank you for the thourogh testing and report on this!

Here is what I plan to do:

When I get back to the office 6th of January I will take out my PPK2 and test this (Might be later in that week depending on the my workload).

Then if I can reporduce this, I will create an internal bug report, and we'll have our PPK people look into this.

How does this sound to you?

Regards,
Sigurd Hellesvik

[WhiteFang]

Hi Sigurd,
That sounds very good, and a Happy New Year with best wishes for 2025 to us all! 

As for me, I will test on a different PPK2 unit, probably tomorrow, to check that it behaves the same.

I have looked some more at it, but the below is pure speculation and mostly a summary of my opening post:

* "It" happens when abruptly changing current draw by more than one "range".

* after "It" happens, the range bits can often be seen fluctuating between 0 and the previous range (for me, typically 3 or 4).

* Once "It" happens, results are bogus until output is disabled or device is reset. 

I did my testing with a simple red LED running at nominal 1.4mA and using a temporary 100Ohm resistor that I would temporarily put in parallel with R_LED to intermittently boost current to 10-12mA range and forcing an expected range change. "It" starts when removing the temporary resistor and the current is supposed to drop.  

(I did not take additional screenshots.)

EDIT: 

It was exactly the same behavior on a second PPK2 unit, so this should be easily reproduced across "any" device, most likely.

[WhiteFang]

Hi Sigurd,

I hope Oslo survived the (multiple) snow/weather related incidents the past few weeks!

Do you have any updates you can share with me? Even if only that someone is investigating?

[WhiteFang]

Hi Sigurd, have you had a chance to look at this? can you reproduce the issue we see?

[Sigurd Hellesvik]

Hi Kjell,

Sorry for missing your last query. I must have forgot to scroll beneath the other replies.

Anyhow: Nothing new from the devs. I will ping them.

For my part: Here is how I try to reproduce this:

And here is me pressing the button repeatedly:

In other words, I cannot reproduce the issue with this setup. Can you check if the setup looks correct, or if I missed anything?

[WhiteFang]

Hi Sigurd,

It is a bit hard to see, but it looks like in your setup the button turns the diode on or off.  I observed the issue when the current went from "low" to "high" then back to "low".  I would suggest changing the setup so that when the button is not pressed, the diode current is flowing through 1k resistance, and that the switch when pressed/on connects the 100Ohm in parallel with the 1k so the current increases significantly.

In my setup the "low current" was 2 "ranges" lower than the "high current" - not sure if the issue is also related to that, but ideally you would adapt Vout and/or R values so that the jump is (at least) 2 ranges.

[Sigurd Hellesvik]

Alright, so i connected the LED on the 1k resistor instead.
And the button on the 100R:

The LED does not light now, as it is an 1k resistor, so too low voltage over the LED.
However, from the below graph, we can see that it consumes a bit of current when I plug in the LED:

No, this shows only ~1.5uA with 1k + LED only.

Then when I press the button to connet the 100R resistor, I get the following:

This jumps from ~1.5uA to ~35mA, which should be more than two "ranges" , right?

Here is also one test wiht 100,000 samples per second. I also put the supply voltage at the same that you got:

EDIT:
And for good measure, here is how the circuit looks: