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

2.5.1 Measurement of SRF

The SRF of an inductor can be easily measured with an impedance analyzer. With an impedance analyzer, the SRF is measured by simply connecting the inductor and plotting the magnitude of the impedance and phase versus frequency. The frequency at which θ=0° corresponds to the SRF. In the example shown in Figure 2-15, the SRF occurs at 4.6 MHz. Note that the SRF is a function of the target interaction — when the target is interaction is stronger, the SRF almost always increases.

GUID-57AA22F5-ECB8-4FD1-878E-25CF815CF14C-low.pngFigure 2-15 Inductance T Versus Frequency

A Vector Network Analyzer (VNA) is also able to perform this measurement. As always with VNAs, it is important to properly calibrate the VNA. The calibration plane must be at the connections to the inductor, otherwise the offset introduces a transmission line into the measurement that results in a measurement error. Once calibrated, simply measure S11 over an appropriate frequency span. The SRF is the frequency where the S11 graph crosses the real axis close to the open, as indicated in Figure 2-16.

GUID-1DDDA1A9-D1BE-4594-A482-CA0F1635BF17-low.pngFigure 2-16 Measurement of SRF with a VNA