Current value changes with current range

Hello!

I am using the JS110 tool and I have to say it is really cool! This is my first analyzer tool (worked with oscilloscope and small resistor in series up to now to measure the current). The think is that when I change the range, then the power consumption seems to change in both oscilloscope and multimeter as well. I know that I am missing something here, so could you please help me find out what? Below the 4 measurements are done on the exact same board, but with different measurement range (see the red arrows). For range >= 180mA I get negative values for current!

Thanks in advance

Hi @alexxx ! Welcome to the forum, and thanks for selecting Joulescope for your measurement needs!

If I understand correctly, you are attempting to measure a target with something like 10 µA sleep current with some spikes to about 1 mA. I normally recommend using the default auto Current Range and allowing your Joulescope to autorange as it’s designed to do. However, if you select a fixed range, you have to be more aware of saturation and precision.

Here are the Joulescope accuracy specifications from page 12 of the Joulescope User’s Guide:

So, when you manually select the 2A range, the expected value is 0.010 mA * 0.003 ± 1.5 mA. In this range, the offset error is significantly bigger than your signal.

You may be able to select the 1.8 mA range if you want a fixed range. However, if your target ever draws more than 1.8 mA, then the measurement will saturate, so your energy and charge will be lower than the actual. If you manually select the 180 µA range, the measurement will definitely saturate.

With autoranging, your Joulescope will select the best current range to give you the highest accuracy. Then, as the current demands increase, your Joulescope changes the current range within 1.2 µs so that your target device keeps working.

Does this make sense and answer your question?

Hi @mliberty! Thank you for the detailed explanation. Yes it absolutely makes sense! But the auto range still confuses me a bit. I took a look in the user guide and in page 16 it is noted that “Current Range allows you to select Joulescope’s active shunt resistor.” So when the device wakes up, we see that the current range changes. So far so good.

On the other hand if you see the image below, there is a value changing (+5 when it sleeps and +3 when it wakes up). What do those values mean? How can I evaluate those values to make sense for the measurement?

Thanks.

The current ranges are numbered in order like so:
0: 10 A
1: 2 A
2: 180 mA
3: 18 mA
4: 1.8 mA
5: 180 µA
6: 18 µA

Based upon the image above, your Joulescope starts by measuring sleep current in the 180 µA range (5). When the target device wakes up, the Joulescope jumps to the 18 mA range (3) before dropping back to the 1.8 mA (4) range and finally returning to the 180 µA range (5).

Does this make sense for your measurement and test setup?

Aaah I see! Great thanks. Two more questions (and hopefully last).

  1. I draw very small current (~2uA). But I observe that although the average value is correct (~2uA) there are big negative spikes of -40uA! This happens only in autorange. How can we explain this?

  2. When I change the current range, I get different peak values. Also when I zoom in I get different peak values.
    a. When I have it in 180mA the peak value starts from 1.2mA and ends up to 4.5mA as I zoom in from 1 to 2.

b. When I have it in autorange the peak value starts from 2.5 mA and ends up to 2A (!) as I zoom in from 1 to 3.

So what is the actual peak value? I am confused.

Thanks for the support @mliberty!

I see that you turned off min/max. If you are concerned about min/max values, you should keep that on. The yellow line is the mean (average) value.

Your Joulescope collects samples 2 million times per second. When you are zoomed out in the UI, your screen cannot display that many individual points. Instead, each x-axis pixel on your screen may represent many, many samples. Each pixel on the yellow line is the mean of all samples for that pixel. The min line is the minimum value computed over each sample represented by that pixel, and the max line is the maximum value computed over each sample represented by that pixel. So, as you zoom in and out, the yellow line will change shape because each pixel represents different samples.

From another perspective, computing mean is a low-pass filter. As you zoom in, you increase the low-pass cut-off frequency. As you zoom out, you are decreasing the low-pass cut-off frequency. So, you are viewing the same signal at different bandwidths, which can change it’s shape.

When you are fully zoomed in, you see the actual samples, not the mean, so the shape will no longer change.


So, what’s up with the “spiky” events near current range switches? You have a circuit with a power supply, wires, Joulescope, and your target board. Joulescope is effectively a variable resistor. Your target board is a capacitor (bypass capacitance) in parallel with a variable resistor. Your wires are resistive and inductive, but you can often neglect these effects.

The current flowing through Joulescope creates a slight voltage drop, called burden voltage. According to Ohm’s law, voltage = current * resistance. Your Joulescope does a great job keeping this voltage very small, typically less than 20 mV. However, it’s not zero. When resistance changes, so does that voltage drop. However, the capacitance on the target resists that change. When the shunt resistor changes, the target capacitance needs to charge up or discharge to match the new burden voltage, and you can get large positive and negative spikes. This is real current! If you measure the RC time constant, you already know R (Joulescope shunt resistance), so you can compute your target capacitance. Since this is real current, it does not affect the accuracy of your measurement, but it is not entirely intuitive either.

For more detail, see this post.

Thank you very much @mliberty!
This is a detailed explanation and throws light on all my questions. I will continue using the tool extensively within the next few days and if something extra is required, I will let you know.

Have a nice day.

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