SNOA930C March   2015  – May 2021 LDC0851 , LDC1001 , LDC1001-Q1 , LDC1041 , LDC1051 , LDC1101 , LDC1312 , LDC1312-Q1 , LDC1314 , LDC1314-Q1 , LDC1612 , LDC1612-Q1 , LDC1614 , LDC1614-Q1 , LDC2112 , LDC2114 , LDC3114 , LDC3114-Q1

 

  1.   Trademarks
  2. 1The Sensor
    1. 1.1 Sensor Frequency
    2. 1.2 RS and RP
      1. 1.2.1 AC Resistance
      2. 1.2.2 Skin Effect
  3. 2Inductor Characteristics
    1. 2.1 Inductor Shape
      1. 2.1.1 Example Uses of Different Inductor Shapes
    2. 2.2 Number of Turns
    3. 2.3 Multiple Layers
      1. 2.3.1 Mutual Inductance of Coils in Series
      2. 2.3.2 Multi-Layer Parallel Inductor
      3. 2.3.3 Temperature Compensation
    4. 2.4 Inductor Size
    5. 2.5 Self-Resonance Frequency
      1. 2.5.1 Measurement of SRF
      2. 2.5.2 Techniques to Improve SRF for Wire-wound Inductors
  4. 3Capacitor Characteristics
    1. 3.1 Capacitor RS, Q, and SRF
    2. 3.2 Effect of Parasitic Capacitance
      1. 3.2.1 Recommended Capacitor Values
    3. 3.3 Capacitor Placement
  5. 4Physical Coil Design
    1. 4.1 Example Design Procedure Using WEBENCH
      1. 4.1.1 General Design Sequence
    2. 4.2 PCB Layout Recommendations
      1. 4.2.1 Minimize Conductors Near Sensor
      2. 4.2.2 Sensor Vias and Other Techniques for PCBs
  6. 5Summary
  7. 6References
  8. 7Revision History

3 Capacitor Characteristics

The capacitor used in the sensor is a critical, yet often overlooked component. In LDC applications, we recommend using C0G-grade ceramic capacitors (they are also referred to as NP0 capacitors). These capacitors use the highest quality ceramic dielectric, and as a result they are very stable and avoid many of the non-idealities capacitors using other dielectrics:

  • They are not polarized.
  • They exhibit minimal aging shifts.
  • Excellent temperature stability at only ±30 ppm/°C.
  • Very low ESR (equivalent series resistance) – typically less than a few mili-Ω.
  • No piezoelectric effects – some dielectrics convert physical stresses into electrical shifts; which is undesirable.
  • Can operate at very high frequencies.
  • Exhibit almost no dC/dV effects, in which the voltage across the capacitance affects the capacitance.
  • Generate minimal distortion.
  • Available in voltage ratings suitable for LDC operation.

However, compared to other dielectrics, they do not have a large amount of capacitance per unit volume, so it becomes difficult to find C0G capacitors of more than 0.47 µF. For capacitance values of 47 pF to 3300 pF typically used for LDC sensors, C0G capacitors are available in small footprints and are reasonably priced.