[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 🙂

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