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LDC1614-Q1: Coil design - inner unwound area

Part Number: LDC1614-Q1
Other Parts Discussed in Thread: LDC1614

Hi team,

I have a question about sensor coil design for LDC1614.

In page 24, 4.2.2 of LDC Sensor Design application note (SNOA930A), there is a description as below.

- Keep the inner 30% of the sensor area unwound for most applications, but for inductive metal button
replacement place as many turns as possible.

Q1. Could you advise what is the purpose of keeping 30% of unwound area in center of the coil?

Q2. How critical is this 30%?

Q3. which types of application doesn't require this unwound area?

regards,

  • Hello,

    The sensing distance and resolution of a coil is determined by the outer diameter of the coil. For most applications, the inner turns do not affect the resolution nor the sensing distance. The inner turns only add series resistance to the coil which reduces the Q factor and requires more drive current to keep the sensor oscillating. 

    It is not required to keep the inner 30% of the coil unwound, but it is recommended.

    Metal button applications do need the inner turns for two reasons. First, some buttons are extremely small, and the coils require as many turns as possible to increase the inductance enough for the LDC devices to effectively drive them. Second, the deflection of the metal target is primarily in the center of the coil, so the inner turns will increase the resolution. Inner turns are recommended for any application where the target movement will primarily affect the center of the coil and not the outer turns. Note that the strongest inductive response will occur when the target size is at least as large as the coil size, so this is not recommended unless absolutely necessary.

    Best Regards,

  • Perfect. Thank you.