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LDC1614-CAPACITIVE INFLUENCES

Antonio Faggio
Expert 7990 points
Other Parts Discussed in Thread: LDC1614, TIDA-00508

Hi All,

I just joined the rotation program as an analog FAE and I would like to have an opinion from you about a request I received by a customer for the inductive sensing of an LDC1614.

The application is an encoder for motors. The customer wants to detect the rotation of a metallic disc.

The customer is facing some problems regarding the sensitivity of the LDC 1614 to capacitive influences, like moving a hand over the coils, which results in relatively large changes of the measurement result (about 0,5% of the measurement range).
Do you have some suggestions about how to compensate those influences, e.g. using differential coils? Or changing the coil shape or number of turns? Or adopting a particular method to improve the SRF?
 
The customer is going to use very small coils (2mm outer diameter) so with a very small inductance (approx. 0,41 µH according to WEBENCH tool), What about the resulting measurement range?

Using the LDC1614 EVM with the coils from 1-Degree-Reference-Design, the resulting range of the measurement result is 15,018,850 (decimal) which is why the noise has so small influence (percentaged).
Do you have some suggestions about how to estimate the resulting measurement range?

Many thanks in advance,

Regards,

Antonio Faggio

  • 0
    TI__Genius 13650 points
    Hi Antonio,
    using the 1-degree reference design is a great start; it shows how to use two coil pairs and compensate for z-axis variation and temperature.
    Are you using the algorithm from the reference design? If not, I would recommend trying this algorithm to find out how much the impact is.
    Secondly, I recommend trying to increase the sensor capacitor. If a larger capacitor is used, then the impact of any external capacitive contribution is decreased compared to the total sensor capacitance.
    If this is still insufficient, then you may try to put as much grounded metal as possible around the rotational encoder to decrease the capacitive change that the encoder target sees.
  • 0 in reply to Ben Kasemsadeh
    TI__Expert 7990 points
    Thank you very much Ben,

    I am at orientation now and cannot access to any reference design so far. I will do when I back to office.
    Why grounded metal around the rotational encoder would decrease the capacitive change that the encoder sees?
    Can differential trapezoidal coils be a good solution for this kind of applications? If so, why this shape could be a better option rather than circular spiral for this kind of application?

    Many thanks,

    Antonio Faggio
  • 0 in reply to Antonio Faggio
    TI__Genius 13650 points
    Hi Antonio,
    the ground would be four identical and symmetrical sections of copper flood on the same PCB layer as the coils. The copper floods should be located between the sensor coils. Adding these GND fills increases capacitive coupling between the GND and the target, and makes the target less sensitive to external capacitive coupling.

    For the LDC1614, the coil design as in the TIDA-00508 reference design guide works well. What is the reason for moving to trapezoidal coils? Do you have a drawing of what this would look like?
  • 0 in reply to Ben Kasemsadeh
    TI__Expert 7990 points

    Thank you Ben,

    I don't have a drawing for that, but I read on LDC sensor design written by Chris Oberhauser that trapezoidale coils are more suitable for small measurement rotational encoder since trapezoidal inductors allow for a larger area measurement compared to circular ones.

  • 0 in reply to Antonio Faggio
    TI__Genius 13650 points
    Hi Antonio,
    yes, coils that are designed as per Chris' apps note work also. The same methods for reducing capacitive coupling between the target and foreign objects apply.