Recomendations about distance between stacked coils needed

Hi TI team,

Are there any distinct recomendations concerning the distance between two coils in a stacked configuration like in LDC0851 applications?

I need criteria to choose the PCB thickness or just a gap between the two stacked coils.

Best regards,

    Igor Gorbounov

  • Hi Igor,

    More separation is better. The reason why is because when the LDC0851 compares the inductance between sense and reference an incoming metal that produces a large inductance difference between the two coils gives the most margin in the design. For a stacked coil arrangement, both of the coils sense incoming metal, therefore the difference in inductance is not as large as if one of the coils was placed far away from the metal. Having a close separation would have the effect of reducing sensing range for proximity applications or reducing margin for event counting/inductance comparison.

    By increasing the separation, it looks more like a side-by-side coil arrangement which would have the effect of increasing sensing range and adding more margin to the design.

    So as a general rule of thumb, more is better. We've used 36mil separation on the coil EVM, but most separation possible is ideal. 

    Regards,
    Luke LaPointe
    SSP Appilcations

  • Dear Igor,

    Please share your observation when you tried for the more distance  between the two coils.i.e. Sense coil & reference coil

    i am planning for keeping 2.4mm of the distance between mid1 & mid2 layers.

    Regards

    Seema Joshi

  • In reply to seema joshi:

    Hi Josh,
    We at last placed the reference coil perpendicular to the sense coil. This reference coil is on a PCB along with the other components, this PCB is inside of a metal body (copper tube) of our sensor. The sense coil is attached near the end of that tube and is wrapped (on its perimeter) with a ferrite film.

    Best wishes,
    Igor Gorbounov
  • In reply to Igor Gorbounov:

    Hi Igor,

    how much distance proximity sense you are targeting ? with coil of how much OD?
    In my case i have coil size constraint not more than 15mm OD
    i have checked my latest coil of 15mm OD with various capacitor as well as resistors at Radj. still can not achieve proximity more than 3.85mm.
    Principally with 15mm OD coil i can be able achieve the 5mm proximity.
  • In reply to seema joshi:

    Hi Joshi,
    I do not quite remember now all details because this sensor development was stopped but its diameter was about 30 mm and it reliably worked at about 6 mm.
  • In reply to Luke LaPointe:

    Dear Luke,

    Can you please comment on the good separation between coils.
    As informed right now with 1 mm i.e. 38 mil separation we are getting 3.85 proximity,shall we go for 2.4mm separation?
    Also let us know can we connect ADJ pin to GND to get the highest distance proximity with the coil?

    Regards
    Seema Joshi
  • In reply to seema joshi:

    Dear SIr,

    Please replay we are solely dependent on your suggestion ,Please suggest any other way to tuen 15mm OD to get the 5mm proximity.
    With the latest we tried keeping Id/OD > .3 it is .6 in latest.


    Regards
    Seema Joshi
  • In reply to seema joshi:

    Hello Seema,
    As this thread mentions, increasing the separation between the coils can effectively increase the sensitivity for stacked coils. Therefore if you are able to introduce a 2.4mm separation between the coils, then this should result in a longer sensing range.
    Regards,
    Luke
  • In reply to Luke LaPointe:

    Dear Luke,

    You are right! With the 1 mm to   2.4 mm lamination width change  between reference & sense coil effectively increased the proximity from 3.8 mm to 7 mm .i.e. 3.2 mm.

    As per requirement this coil & interface( LDC0851 chip board ) is needed to be inside the  metal encapsulation.When we  engage the coil &  interface in the encapsulation  & do the proximity check  the sensing distance reduces reached to 4.85 mm instead of 7 mm.

    We can not predict reason for the phenomenon.Please guide how to overcome the effect due to  metal encapsulation .

    Regards

    Seema Joshi