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[AmpeROSE] Current generation for the initial calibration

Hello,

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.