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DRV421EVM: Core design in very small current measurements (1uA to 5mA)

Part Number: DRV421EVM
Other Parts Discussed in Thread: DRV421, DRV425


I'm trying to test the idea of measuring a very small current signal through a toroid of some sort.  I do not want to use a shunt.  The current would be on the order of 5 to 50uA.  I came across the DRV421, and it seems like my best option so far.  Assuming appropriate amplfiication, do you think the DRV421 would be able to measure on this scale?

If so, could you help with the magnetic core/compensation winding design?  I tried to order cores and cut them myself, but I can't seem to get enough inductance on the compensation winding after I cut the core.  How can I go about getting the core/winding to test my design?



  • Jake,

    This is not easy by any means.  You will notice you will have challenges on a few fronts. I have cover some before here

    The first is getting the inductance required.  This is simply a physics problem that will be with the dimensions of the DRV425 and how close you can get to the air gap, how small you can make the air gap and the number of windings.  You can use simulations to determine this fast by using the scripts on this post.

    DRV421 Offset.  Offset = 8µT, Core Gain 500µT/A;  Offset error = 8µT/(500µT/A)= 16mA (error due to offset)

    Magnetization of core-  The core can get magnetize and create an offset.  This is in the same magnitude of the offset.  The deguass will also have issues as you increase the Rshunt and you can no longer saturate the core to degauss it.  It would be best to create your own deguass circuit.

    Outside fields-  You will need to be able to shield external fields as this is small field you will attempt to measure.

    If you are doing differential measurements you need to account for one current source closer to the air gap relative to another current source.  This is because one can couple to the air gap differently than the other creating another mis-match in coupling causing an error if you are attempting to measure differential current.

  • I understand the 16mA offset would be hard to overcome.  Does this represent itself in a DC form?  I'm hoping to measure signals from 10 Hz to 150 Hz.  If I use a HPF to remove any DC offset or error, then amplify the signal from there, do you think that could help with the error?

    The differential measurement is correct, and I see you're right.  Distance from wires to air gap will be important to control.

    I will use the simulations you've provided.  Thank you!


  • Hello Jake,

    Yes the offset will be in a DC form but this take away from any head room and decrease your range of your measurement.  Also if you are doing AC is the 10Hz to slow to just use a current transformer.  A current transformer will keep all the flux on the core and you will have less influence of external fields.

    A HPF could help but the 10Hz is so close to DC.  This will require some large value components.

    You will need to do some adjustments for the simulations to get a representation of multiple conductors and the distance from the air gap.  You need to add another conductor and current in the opposite direction.  If you have them match ideally the field at the air gap will be zero.  This is not the case and will improve based on permeability of the core and the distance/orientation each conductor is from the air gap.  Please understand that as you increase the common current on the conductor you will have a proportional increase on the field seen at the gap. 

  • Thanks Javier,

    Yes, you're right.  The 10 Hz makes it difficult to use a current transformer, but this is still an option.  I'm trying to look at all possibilities.  It makes sense about the headroom though.  

    Do you know someone I could contact about making the core?  Or maybe an off the shelf part?  I've found it's an odd gap design, and many manufacturers don't have the ability to provide this.  I would like to test out the idea, but getting a core is the most difficult part of the process.



  • Hello Jake,

    Here is where thing can also get complicated.  I have not found many core development myself that will do small prototyping and require a large investment.  We have previously worked with Wurth, Sumida, Pulse Electronics and GCI as well as others but these are on the front of my mind.  Most of the core companies deal with power and large volume and not much with special cores.  Even though I named some companies the prototype and full scale volume needs to make business sense for them.   I have add other customers find a supplier but they did not share the core manufacture.

  • Thanks Javier.  Really appreciate your help.  I'll try those suppliers and see what they have to offer.