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[AmpeROSE] Unveiling the dark side of feedback ammeters

Hello everyone,

In the bibliographic review momo and I made last week about low current measurements, two main designs were mainly considered : Shunt Ammeters & Feedback Ammeters. We also mentioned a third design that we were studying.

In this post, we will start by discussing that third design, then we will present its disadvantages and the disadvantages of feedback ammeters in general and finally we will conclude that shunt ammeters are the way to go.

So here we go …

General Impedance Converters

Let us consider the following circuit (Widely known as General Impedance Converter):

We will label nodes 1-5 from top to bottom. We have the same voltage at nodes 1, 3 & 5 (Inputs of Op-amps). The input current is the current traversing the impedance Z1 while the output current is the current traversing the impedance Z4 (and then the load impedance Z5). In addition the current traversing Z2 is the same current traversing Z3.

Combining all these information we get the following results:

Therefore by choosing Z1 = Z4 and Z2 = Z3, we have a circuit capable of measuring the current while maintaining a very small burden voltage. Cool !!

But wait ?! The simple feedback ammeter does the same thing ! What is then the advantage of such design ?

When we started studying that design, we thought that feedback ammeters can only be used in low side configuration (How deluded we were !). But in fact, both designs can be used in both low side and high side configurations.

However, the main advantage of GIC based ammeter is actually that it can be used to measure current flowing in both directions while simple feedback ammeters can only measure current flowing from the source to the DUT. But since we are measuring IoT devices, bidirectional measurement is not a necessary feature. Thus this design will only complicate our lives. It was interesting however considering and studying new ideas.

Back to Feedback Ammeters

For now, feedback ammeters look like the perfect match for our specifications. Very low burden voltage, used in both low side & high side configurations, used in all pico-ammeters. Awesome ! However, following a discussion with Alexis Polti, we found out that feedback ammeters are not that awesome.

In the low side configuration, the current that passes through the feedback resistor will re-enter the op-amp (it will sink the current to the ground). In the high side configuration, it is the op-amp who will source current to the DUT. In both cases, current will pass through the op-amp and will be modified by the op-amp consequently. The problem comes from the fact that operational amplifiers act like low pass filters. Thus all current peaks (or even smaller variations) will go undetected. This type of ammeters cannot be used in applications where high dynamic range is needed (our application for example 🙂 ).

Shunt Ammeters … Again …

Finally, after taking all these facts into consideration, we found out that shunt ammeters are actually the most adapted design to AmpeROSE. The main challenge for now is to choose the right shunt resistors (and choosing the right intervals) in order to have an acceptable burden voltage.

Next week, we should have the final schematics of the measurement part. We are looking forward to sharing it with you !!

Until next week !!

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