8.2.4 Detailed Design Procedure

The target distance, resolution and diameter are used as inputs to WEBENCH to design the sensor coil, The resulting coil design is a 2 layer coil, with an area of 2.5 cm², diameter of 17.7 mm, and 39 turns. The values for R_P, L and C are: R_P = 6.6 kΩ, L = 43.9 µH, C = 100 pF.

Using the L and C to determine ƒ_SENSOR = 1/2π√(LC) = 1/2π√(43.9*10^-6 * 100*10^-12) = 2.4 MHz

With a system reference clock of 40 MHz applied to the CLKIN pin allows flexibility for setting the internal clock frequencies. The sensor coil is connected to channel 0 (IN0A and IN0B pins).

After powering on the LDC, it will be in Sleep Mode. Program the registers as follows (this example sets registers for channel 0 only; channel 1 registers can use equivalent configuration):

1. Set the dividers for Channel 0.
   1. Because the sensor frequency is less than 8.75 MHz, the sensor divider can be set to 1, which means setting field FIN_DIVIDER0 to 0x1. By default, ƒ_IN0 = ƒ_SENSOR = 2.4 MHz.
   2. The design constraint for ƒ_REF0 is > 4 × ƒ_SENSOR. The 40 MHz reference frequency satisfies this constraint, so the reference divider can be set to 1. This is done by setting the FREF_DIVIDER0 field to 0x01.
   3. The combined value for Chan. 0 divider register (0x14) is 0x1002.
2. Program the settling time for Channel 0. The calculated Q of the coil is 10 (see Multi-Channel and Single Channel Operation).
   1. SETTLECOUNT0 ≥ Q × f_REF0 / (16 × f_SENSOR0) → 5.2, rounded up to 6. To provide margin to account for system tolerances, a higher value of 10 is chosen.
   2. Register 0x10 should be programmed to a minimum of 10.
   3. The settle time is: (10 x 16)/20,000,000 = 8 µs
   4. The value for SETTLECOUNT0 register (0x10) is 0x000A.
3. The channel switching delay is ~1 μs for f_REF = 20 MHz (see Multi-Channel and Single Channel Operation)
4. Set the conversion time by the programming the reference count for Channel 0. The budget for the conversion time is : T_SAMPLE – settling time – channel switching delay = 1000 – 8 – 1 = 991 µs
   1. To determine the conversion time register value, use the following equation and solve for RCOUNT0: Conversion Time (t_C0)= (RCOUNT0ˣ16)/f_REF0.
   2. This results in RCOUNT0 having a value of 1238 decimal (rounded down)
   3. Set the RCOUNT0 register (0x08) to 0x04D6.
5. Use the default values for the ERROR_CONFIG register (address 0x19). By default, no interrupts are enabled
6. Sensor drive current: to set the IDRIVE0 field value, read the value from Figure 55 using R_P = 6.6 kΩ. In this case IDRIVE0 value should be set to 18 (decimal). The INIT_DRIVE0 current field should be set to 0x00. The combined value for the DRIVE_CURRENT0 register (addr 0x1E) is 0x9000.
7. Program the MUX_CONFIG register
   1. Set the AUTOSCAN_EN to b1 bit to enable sequential mode
   2. Set RR_SEQUENCE to b00 to enable data conversion on two channels (channel 0, channel 1)
   3. Set DEGLITCH to b100 to set the input deglitch filter bandwidth to 3.3MHz, the lowest setting that exceeds the oscillation tank frequency.
   4. The combined value for the MUX_CONFIG register (address 0x1B) is 0x820C
8. Finally, program the CONFIG register as follows:
   1. Set the ACTIVE_CHAN field to b00 to select channel 0.
   2. Set SLEEP_MODE_EN field to b0 to enable conversion.
   3. Set RP_OVERRIDE_EN to b1 to disable auto-calibration.
   4. Set SENSOR_ACTIVATE_SEL = b0, for full current drive during sensor activation
   5. Set the AUTO_AMP_DIS field to b1 to disable auto-amplitude correction
   6. Set the REF_CLK_SRC field to b1 to use the external clock source.
   7. Set the other fields to their default values.
   8. The combined value for the CONFIG register (address 0x1A) is 0x1601.

We then read the conversion results for channel 0 and channel 1 every 1.00 ms from register addresses 0x00 to 0x03.