Current Range auto / man

Hi,

actually i can see a strange behavior on the Joulescope if i switch the current range from auto to manually 180uA.
While I measure approx. 60 uA at a voltage of 5V in the automatic measuring range, the measured value only reaches 50uA when there is a jump to the measuring range of 180uA. Shouldn’t it be the same value in both measuring ranges? Have I missed something? is there a calibration that has to be done? I have added some screenshots here. If it’s needed i could add also the log files of my measurement.

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Hi @Dominik and welcome to the forum!

Until proven otherwise, I would trust the higher “auto” value, but you are definitely right to question, investigate, and confirm.

No. Your Joulescope does not need any additional calibration to operate correctly.

Assuming that your system is not drawing more than 180 µA, then those measurements should be the same. I suspect that your system may be saturating the 180 µA range which would explain why it is reading lower.

Here’s something easy to try. Set Min/Max to lines. If you see the fixed 180 µA range measurement maxing out around 190 µA, then it is saturating. You can also zoom in on those areas to confirm.

If your system is drawing more than 180 µA but less than 1.8 mA, you can try the 1.8 mA range, which still gives suitable accuracy to discern a 13 µA difference:

I am also happy to take a look. Here’s how you can share data:

  1. Confirm that you are capturing full-rate 2 Msps data. Select FilePreferencesDevicesettingsampling_frequency2 MHz.
  2. Set Current Range to auto.
  3. Capture data.
  4. Add dual markers
  5. Select a short region showing a “normal” segment of this activity that is causing the autoranging. Ideally, less than 0.5 seconds so you can post the file to the forum.
  6. Right click on the marker and select Export data.
  7. Repeat 2-6 for the 180 µA range.
  8. Post both files here.

Look forward to hearing what you find!

First Thank you very much for your fast response !

So the first thing i’ve done is set Min/Max to Lines. As you can see it shows a maximum current of 80mA what I think can’t be a correct value.

I had a look into the preferences; the sampling frequency is set up right so 2 MHz.

So in my post here are the measurements attached as you described here.


Let me know what you think about it.

Best regards

Dominik

Hi @Dominik,

Thank you for posting the two jls files. The 180 µA fixed range capture clearly shows that the target device draws more than 200 µA causing the measurement to saturate:

The saturation occurs at 200 µA, and the idle current is 21 µA. The saturation lasts for 688 µs and occurs approximately every 7 ms. It also has a very nice RC decay [see Wikipedia]. τ occurs when the value crosses

(1 - 0.632) * (200 - 21) + 21 = 86.8 µA

I measure τ = 602 µs

Now, we know that Joulescope uses a 111 Ω shunt resistor in the 180 µA range, so your target capacitance is:

τ = R * C
602 µs = 111 Ω  * C
C = 5 µF

If we assume that the pulse is infinitely short, we can project back the RC decay curve to estimate your actual maximum current:

1 / e ^ (-688 µs / 602 µs) * (200 - 21) µA + 21 µA = 582 µA

We also know that your device experienced a voltage drop of

V = 111 Ω * 582 µA = 65 mV

which is 1.3% (not too bad) given your 5V supply.

Let’s take a look at the autoranging capture. We see the same events occurring approximately every 7 ms:

Zooming in on these events reveal that they are really short.

The duration is so short that they exceed Joulescope’s autoranging bandwidth, which is another way of saying that you will not see a clear picture of the actual pulse with autoranging on. To get a clear picture of the pulse, you should set the smallest fixed range that does not saturate. You can find a similar discussion here. While in the fixed range, make sure that you never see a pulse exceeding that fixed range’s value. If you do, try the next range up.

As you move to less sensitive current ranges, your system’s measurement bandwidth also increases, which means that the RC decay will be much reduced, but your instantaneous current will also increase.

If you want, I am happy to take a look at this fixed range capture. You can consider adding capacitance to your target to get better measurements. I normally recommend at least 10 µF, but I am happy to recommend a value for your system based upon the fixed range capture.

Thank you for your very detailed explanation. I can understand the process much better now!
It’s great to get such quick and detailed feedback.

:slightly_smiling_face:

For the moment i will go on with autoranging on. I think that will be enough for my actual measuring purposes.

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Great! If you have further questions, don’t hesitate to post more here.