Categories

[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.