7.3.2 Measuring R_P With LDC1001

The LDC1001 supports a wide range of LC combinations, with oscillation frequencies ranging from 5 kHz to 5 MHz and R_P ranging from 798 Ω to 3.93 MΩ. This range of R_P can be viewed as the maximum input range of an ADC. As shown in Figure 9, the range of R_P is typically much smaller than the maximum input range supported by the LDC1001. To get better resolution in the desired sensing range, the LDC1001 offers a programmable input range through the R_P_MIN and R_P_MAX registers. Refer to Calculation of RP_MIN and RP_MAX for information on setting these registers.

When the resonance impedance R_P of the sensor drops below the programed R_P_MIN, the R_P output of the LDC will clip at its full scale output. This situation could, for example, happen when a target comes too close to the coil.

Figure 10. Transfer Characteristics of LDC1001 With R_P_MIN = 16.160 kΩ and R_P_MAX = 48.481 kΩ

Use Equation 2 to calculate the resonance impedance from the digital output code:

Equation 2.

where

• R_P = Measured sensor parallel resistance in kΩ.
• R_P_MIN is the resistance (in kΩ) selected in register 0x02
• R_P_MAX is the resistance (in kΩ) selected in register 0x01
• Y = Proximity Data÷2¹⁵
• Proximity data is the LDC R_P output = (Contents of Register 0x22) × 2⁸ + (Contents of register 0x21).

Example: If Proximity data (address 0x22:0x21) is 5000, R_P_MIN is 2.394 kΩ, and R_P_MAX is 38.785 kΩ ,the resonance impedance is given by:

Y = 5000/2¹⁵ = 0.1526

R_P = (38785 × 2394) ÷ (2394 × (1 – 0.1526) + 38785 × 0.1526) = (92851290 ÷ (2028.675 + 5918.591))

R_P = 11.683 kΩ