The registers listed in Table 38 are used to control the sensor drive current so that the sensor signal amplitude is within the optimum range of 1.2 VP to 1.8 VP (sensor amplitudes outside this optimum range can be reported in the status register - refer to Device Status Registers ). The device can still convert with sensor amplitudes lower than 0.6 VP, however the conversion noise will increase with lower sensor amplitudes. Below 0.6 VP the sensor oscillations may not be stable or may completely stop and the LDC will stop converting. If the current drive results in the oscillation amplitude greater than 1.8 V, the internal ESD clamping circuit will become active. This may cause the sensor frequency to shift so that the output values no longer represent a valid system state. Figure 53 shows the block diagram of the sensor driver. Each channel has an independent setting for the IDRIVE current used to set the sensor oscillation amplitude.
|CHANNEL(1)||REGISTER||FIELD [ BIT(S) ]||VALUE|
|All||CONFIG, addr 0x1A||SENSOR_ACTIVATE_SEL ||Sets current drive for sensor activation. Recommended value is b0 (Full Current mode).|
|RP_OVERRIDE_EN ||Set to b1 for normal operation (RP Override enabled)|
|AUTO_AMP_DIS ||Disables Automatic amplitude correction. Set to b1 for normal operation (disabled)|
|0||CONFIG, addr 0x1A||HIGH_CURRENT_DRV ||b0 = normal current drive (1.5 mA)
b1 = Increased current drive (> 1.5 mA) for Ch 0 in single channel mode only. Cannot be used in multi-channel mode.
|0||DRIVE_CURRENT0, addr 0x1E||IDRIVE0 [15:11]||Drive current used during the settling and conversion time for Ch. 0 (auto-amplitude correction must be disabled and RP over ride=1 )|
|INIT_IDRIVE0 [10:6]||Initial drive current stored during auto-calibration. Not used for normal operation.|
|1||DRIVE_CURRENT1, addr 0x1F||IDRIVE1 [15:11]||Drive current used during the settling and conversion time for Ch. 1 (auto-amplitude correction must be disabled and RP over ride=1 )|
|INIT_IDRIVE1 [10:6]||Initial drive current stored during auto-calibration. Not used for normal operation.|
|2||DRIVE_CURRENT2, addr 0x20||IDRIVE2 [15:11]||Drive current used during the settling and conversion time for Ch. 2 (auto-amplitude correction must be disabled and RP over ride=1 )|
|INIT_IDRIVE2 [10:6]||Initial drive current stored during auto-calibration. Not used for normal operation.|
|3||DRIVE_CURRENT3, addr 0x21||IDRIVE3 [15:11]||Drive current used during the settling and conversion time for Ch. 3 (auto-amplitude correction must be disabled and RP over ride=1 )|
|INIT_IDRIVE3 [10:6]||Initial drive current stored during auto-calibration. Not used for normal operation.|
If the RP value of the sensor attached to Channel x is known, Table 39 can be used to select the 5-bit value to be programmed into the IDRIVEx field for the channel. If the measured RP (at maximum spacing between the sensor and the target) falls between two of the table values, use the current drive value associated with the lower RP from the table. All channels that use an identical sensor/target configuration can use the same IDRIVEx value. The appropriate sensor drive current can be calculated with:
|IDRIVEx Register Field Value||Nominal Sensor Current (µA)||Minimum Sensor RP (kΩ)||Maximum Sensor RP (kΩ)|
Sensors with RP greater than 90 kΩ can be driven by placing a 100 kΩ resistor in parallel with the sensor inductor to reduce the effective RP.
Sensors which have a wide range of RP may require more than one current drive setting across the range of operation - the current would need to be dynamically set based on the target position. Note that some high-resolution applications will experience an output code offset when the current drive is changed. Another approach for systems which have a wide range of RP is to place a discrete resistor in parallel with the inductor to limit the range of RP variation in the system. This will also reduce the sensor Q, and so may not be feasible for some implementations.