[AmpeROSE] Current generation for the initial calibration


As you saw in a previous post, we are going to do some measurements on the AmpeROSE to do an initial calibration, to correct eventual imprecisions. To do that we need to be able to put a current that we know and control precisely. In order to achieve our goal, we decided to use a current mirror, the one we are going to use was built by Analog Devices and has the reference ADL5315. The following figure shows you how we will use it.

Since it is a current mirror there is some current being copied from somewhere to somewhere. Here it is the current flowing through INPT that is copied to IOUT. Then this current is injected in our measurement system : MIR+ and MIR- are connected to the shunts.

To generate a current on INPT, we used a resistor to force the current which depends on the voltage on the INPT pin. So, we also need to control the voltage on this pin. That is simple here because we have the same voltage on INPT and VSET, so we only need to force one voltage on the VSET pin to have the same on INPT. Then we know the voltage and the resistor so we can deduce the current. In our application, we want to have some precise current, so we chose the current and then we compute and apply the voltage  we need of VSET.  As you can see, we have 2 resistors, so depending the current we need to generate, we use one or both (which is almost equivalent as saying the other here).

To control the voltage on VSET, we are using a voltage divider. This voltage divider is built upon a digital potentiometer. So, we can use it to vary  the VSET voltage to generate multiple values of currents.

Here are the equations that allow us to know how to generate a current :

  • VSET= Rpot /(Rpot + 100K)
  • Iout=Iin= VSET/Rinpt

According to these equation these are the values of current we can generate : from 100nA to 500nA and from 500uA to 2.5mA.

2 comments to [AmpeROSE] Current generation for the initial calibration

  • jblescher


    A little question that is not related to this specific post but to your whole project (so anybody in the group can feel free to answer).

    If I understand correctly the goal of the project is to measure the power consumption of IOT devices within a given range of voltage / current / frequency change.
    For this you rely on:
    1) a hardware circuit and
    2) a piece of software that will address the bias introduced by your hardware circuit

    How will you confront your device to reality to see if it is accurate?
    Do you have a device able to feed accurate input to your system to be able to calibrate it?


  • MichelElHABR


    In order to confirm the accuracy of our device we will compare its results with a proven device.

    As for the accurate input, well this is the subject of this post as well as a previous one:
    We have, on AmpeRose itself a calibration circuit, that works by feeding very accurate current to the measurement stage, which can then be properly calibrated.

    When we settle on the final definitive schematic, we will post an estimation of the accuracy and errors of our device.

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