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[AmpeROSE] How to measure the current – Part 2

Hello everyone!!

This week’s goal was to go through existing articles and bibliography, and discuss low current measurement. After doing a lot of reading, we now have a clearer vision of the methods used for current sensing and the main challenges in low current measurement.

Mmomo already discussed, in a separate post the two main methods used in current sensing (shunt & feedback – Spoiler: other methods exist: we are currently studying a third method in depth. We hope to share it later on this blog , so stay tuned!!).

In this post, I am going to present you with the main challenges that face low current ammeter designers. Accuracy is, evidently, crucial, when it comes to measuring low currents. That’s why I firmly believe that studying potential error sources in advance is extremely important in order to avoid getting to an impasse. Note that many articles found online study this issue in depth. Our main references are application notes from Keithley and National Instruments.

So here we go …

Measurement error sources

Leakage currents

Leakage currents are usually generated by stray resistance paths between the measurement circuit and nearby voltage sources. These currents can degrade the accuracy of current measurements considerably. This kind of errors is generally remedied by using good insulators (such as Teflon or polyethylene) and avoiding materials such as phenolic and nylon.

Zero Drift

Zero drift is the change of indicated zero offset when no input signal is applied. This offset must be corrected by “zeroing”, which means pulling the mass value back to zero(surprise!). Most electrometers include a means to correct zero drift. AmpeRose will make no exception of this rule!

Generated Currents

Any current generated in the measurement device will add up to the measured current and will introduce measurement errors. These currents include triboelectric effect currents and piezoelectric effect currents.

AC Interference

In order to solve this problem, we will use electrostatic shielding, and a battery.

To sum up, choosing the right measurement method is crucial but one must not ignore all the errors and negative effects that appear when measuring low currents. Some of these errors can be reduced by using appropriate shielding and proper cabling. Others must be resolved with more “tailored” methods. In all cases, we will have to take these errors into consideration in our design.

Next Week

Now that we have a better view on the subject, we will choose the appropriate current measurement method as well as the circuit to implement it. Stay tuned!

We would love to hear any suggestions or tips you might have concerning the current measurement step 🙂

[AmpeROSE] How to measure the current?

After having read some documentation about measurement here and there, we noticed that there are two main methods to do it. The ammeters using the first method are known as the shunt ammeters and the second as feedback ammeters. The measure of a current is done by converting a current to a voltage that we can directly read using an ADC (Analog to Digital Converter). Even though this is in common in the two method, they do it differently. We are going to see how it is done for each and the differences.

  1. Shunt ammeters

Source : « Linear Technology » (figure 1 from the appplication note of Linear Technology about current sensing)

In this method, a resistor is put in the circuit and we measure the voltage over this resistor. Using the Ohm’s law we can recover easily the current that flowed in it. This resistor is called “shunt”. Usually, the voltage is amplified with an operational amplifier before being measured.

We need to amplify the voltage over the shunt because we are trying to have the smallest voltage in order to keep the circuit under test as if it was not. This voltage is a drawback for this method and even has a name which is the “burden voltage”.

Trying to have a small burden voltage leads to minimizing the value of the resistance too but this is not so easy to do. Indeed, the more we reduce its value, the more it becomes sensitive to the thermal noise.

The small voltage also require to use an operational amplifier but even this has a drawback: the measurement needs some time to be stable. This happens because of the association of the resistor with the input capacitance of the operational amplifier. The time needed for the stabilization depends on the multiplication of the value of resistor by the value of the input capacitance. The bigger this product is, the longer this time will be.

Nevertheless, this method has the advantage of being used almost anywhere in the circuit.

Setting up this method consists in finding the good resistors that will not produce a too big burden voltage to allow the measured circuit to operate normally but big enough to be able to measure it.

  1. Feedback ammeters

Source : « National Instruments » (figure 4 of the article of National Instruments about the minimization of measurement errors of small currents)

This method works a bit differently but keeps the main idea of converting the current to a voltage with a resistor as you can see on the figure above. We measure the voltage called Vout on the figure which is proportional to the current Iin which is the measured current and Rf, the resistor used for the conversion.

This method has the advantage of allowing to choose a bigger resistor in order to have a bigger voltage. This is possible because this voltage has no influence on the tested circuit. This avoid us other problems such has the thermal noise.

It also reduce the problem of burden voltage, indeed, it depends only on the amplifier used. The burden voltage is generally in the range of the µV whereas the burden voltage of the shunt ammeters can reach one volt.

Since we use an operational amplifier, we also have a problem with the stabilization time of the measure but here the resistor is in the feedback loop so the effective resistor is smaller. The measurement can therefore be faster but the slew rate of the amplifier has to be adapted too.

Our following step is to build the best solution for our AmpeROSE based on these methods.