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Current measurement equipment: Capabilities, limitations, and best practices

In this topic, we will cover the capabilities, limitations, and best practices of different equipment: power analyzers/SMU, oscilloscopes, and multimeters, and when to use each.

Power analyzer/SMU

The most complete tool for current measurements and power profiling is a power analyzer or SMU (Source Measuring Unit). SMUs, in addition to measuring the current, also supplies the voltage to the device under test (DUT). Power analyzers can often act as an SMU, or it can act as an ampere meter, where the voltage is supplied externally, and the current is measured in series with the DUT. The terms SMU and power analyzer is often used interchangeably.

The most important aspects of power analyzers is that they have automatic range switching, so we get a very large dynamic range. They usually also have quite good sample rate, giving us a detailed plot timewise. But this of course varies from equipment to equipment.

Limitations with dynamic range switching

As mentioned above, when the current increases or decreases, the measurement shunt value will be changed. This change leads to a change on the output impedance of the voltage source. Or in other words, there will be a sudden voltage step or drop on VDD, when the the measurement range changes. Since we have decoupling capacitors on VDD, the sudden jump in voltage will lead to an inrush to the decopling capacitors, or the oposite when voltage drops. This can be seen as a spike or overshoot on the current plot whenever there is a large positive change in current consumption, or undershoot when the current drops.

Many power analyzers will compensate for this in software. But how well this is done, depends on the tool.

Here is a comparison between the PPK and professional lab equipment, Keysight power analyzer. The Keysight power analyzer is much better at compensating for this inrush current. It is important to note that the total charge/energy will (should) still be the same.

Oscilloscope

If we need to inspect the power profile with detail, and we do not have access to a good power analyzer, using a scope can be a solution.

This method involves manually adding a fixed measurement shunt in series with the DUT, and then measure the voltage drop with the scope. This also means that we can not have dynamic range switching in this setup, which results in a limited use case.

Measure the current profile of an nRF54L15 DK with an oscilloscope

The diagram above shows how to use an oscilloscope with the nRF54L15 DK for current measurement. The unpopulated R24 has a PCB footprint on the board’s back for soldering a resistor

Using a oscilloscope involves choosing a fixed shunt resistor that matches the dynamic area you want to measure, and then measuring the voltage drop over the shunt resistor with a probe on each side. You then use the math function on the scope to plot the voltage difference between the two probes, and convert to a current by diving by the shunt value.

Since the method involves using two probes, one on each side of the shunt, you will get issues with differential mode offsets, making it hard to measure the lowest idle currents. This method is therefore mostly used for getting the power profile of active current above 1mA. Also, the probes themselves will add a current draw. 10:1 probes have a 10M Ohm impedance, meaning that e.g. VDD=3.0V will give 0.6uA leakage combined on both probes.

Usually 10 Ohm resistor is a good choice for active currents.

Variations in resistor accuracy might give a linear offset to the result. This can be mitigated by calibration, meaning that you must be able to accurately measure the resistor value first, and then adjust for this in the scope’s math function.

Important

WARNING!

You can not use one probe and connect it to one side of the shunt, and then the probe’s ground point on the other side of the shunt to measure the voltage drop. It will short circuit VDD to GND, and could potentially damage the probe or the scope.

Configuring the oscilloscope

In order to get the best results on the scope, here is a step by step guide on how to configure the scope

Multimeter/amperemeter

While the oscilloscope is good for higher currents and detailed power profiling, an amperemeter is kind of the opposite, complementing each other. An amperemeter is often accurate enough for the lowest current, when fixed at the lowest measurement range, but it does not give you any information about the power profile. So you can use it for measuring idle current, verifying your setup, etc. And it will be much less error prone than using a complex instrument you have configured wrong. Can be used for sanity checking measurement setups.

You should keep in mind that if you measure a periodic signal, the period should be significantly shorter than the refresh interval of the display, otherwise it will miss the periodic signal and only show the idle current in between.

Note

Keeping the multimeter in series with the DUT, in the lowest current range (purpose is to measure idle current), The chip can fail to boot, because the multimeter will limit the current. So you fail to measure the idle current, but instead you measure the DUT in a reset loop. A solution is to set the multimeter to the highest range when turning on the power, then switching to the lowest range when measuring. If the device resets when switching to the lower range, because of a voltage glitch, you can keep it in the lowest range, but short the multimeter terminals when power is first applied, letting the device boot, before the short is removed, and current is measured.

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The following settings are most likely under the channel configuration on your scope Bandwidth filtering: The analog inputs on the scope usually have a bandwidth filter. Turning this on will reduce the noise level. We do not need a high bandwidth to measure DC current. Invert: Since we measure the voltage drop, it is convenient to invert the signal, since a negative voltage correspond to a positive current
Channel configuration Offset: Adjust the offset on each channel so that the idle current, or lowest current is at the bottom of the screen (if you did not invert the signal, it should be at the top of the screen). In this example we use VDD=3.0V, meaning that the offset should be close to -3.0V. V/div: Change the voltage per div on each channel so that the signal fills the whole screen, but does not clip the signal. Same V/div on both probes. AC coupling: Normally we would use DC coupling. However, on some scopes we can not have a large offset (3V in this case) and at the same time use a V/div small enough for detailed plots. An alternative is to use AC coupling instead of DC coupling on both channels and then set the offset to 0V.
Trigger It is convenient to set the edge trigger to the the first spike in the event, on the channel that is on the DUT side of the shunt resistor, since it has the largest voltage drop. In this case CH2. You will find this under the “Trigger” menu/button.
Oversampling Most scopes have digital post processing of the signal to filter out noise. This is called different things. “High resolution”, “sin x interpolation”, etc. You will find this under the “Acquire” menu/button.
Cursors We can set our measurements in the right column to be gated by the cursors, meaning that we only measure what is between the cursors. The default is to measure the entire screen. This way we can use cursors to inspect the different parts of the plot You will find this under the “Meas” menu/button.
Math 1 M1 = CH2 – CH1 Since we want to measure only the voltage drop over the shunt resistor, we need to use the math function on the scope to subtract the supply side voltage (CH1) from the DUT side voltage (CH2) You will find this under the “Math” menu/button
Math 2 *M2 = M1A + B** It is convenient to add another math function that takes the output of the first math function and scales it according to the shunt value, and also fixes any offset issues. This way we can read the current directly out from the scope measurements. (Measurements will still be displayed as Voltage, but we can just swap this out with Ampere) We set the gain, A, to 0.1 since our shunt resistor is 10Ohm And we also change the offset, B, so that the idle period in the plot gets close to 0. This is because that this method often adds quite a large offset, which we need to compensate for. We can not use this method to measure the idle current reliably, unless we use a much larger shunt resistor.
Measurements Now we are ready to add measurements to our plot. We want to add measurements for the Math 2 function (M2). You will find this under the “Meas” menu/button. In this plot we have added 4 measurements, avg, max, min and area. We can read the following information out from these measurements: Avg – average current = 2.01 mA Max – max current = 11.95 mA Min – min current = -0.27 mA Area – charge = 8.40 uC Since our measurements are gated by the cursors, we can move these around and capture other things like TX run current, etc

Designing Low-Power Bluetooth LE Products

Current measurement equipment: Capabilities, limitations, and best practices

Power analyzer/SMU

Limitations with dynamic range switching

Oscilloscope

Important

Configuring the oscilloscope

Multimeter/amperemeter

Note

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