Datasheet
44
LDC1612
,
LDC1614
SNOSCY9A –DECEMBER 2014–REVISED MARCH 2018
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8.1.5.2 Automatic IDRIVE Setting with RP_OVERRIDE_EN
The LDC1612/LDC1614 can automatically determine the appropriate sensor current drive when entering Active
Mode. For the majority of applications, it is recommended to program a fixed current drive for consistent
measurement performance. The automatic sensor amplitude setting is useful for initial system prototyping if the
sensor amplitude is unknown. When this function is enabled, the LDC attempts to find the IDRIVEx setting which
results in a sensor amplitude between 1.2 V
P
and 1.8V
P
. For systems which have a large variation in target
interaction, the LDC1612/LDC1614 may select a current drive setting which has poorer repeatability over the
range of target interactions. In addition, measurement repeatability will be poorer with different sensor current
drives. To enable the automatic sensor amplitude, set RP_OVERRIDE to b0.
The following sequence uses auto-calibration to configure sensor drive current for a sensor with an unknown R
P
:
1. Set target at the maximum planned operating distance from the sensor.
2. Place the device into SLEEP mode by setting CONFIG.SLEEP_MODE_EN to b0.
3. Program the desired values of SETTLECOUNT and RCOUNT values for the channel.
4. Enable auto-calibration by setting RP_OVERRIDE_EN to b0.
5. Take the device out of SLEEP mode by setting CONFIG.SLEEP_MODE_EN to b1.
6. Allow the device to perform at least one measurement, with the target stable (fixed) at the maximum
operating range.
7. Read the channel current drive value from the appropriate DRIVE_CURRENTx register (addresses 0x1e,
0x1f, 0x20, or 0x21), in the INIT_DRIVEx field (bits 10:6). Save this value.
8. During startup for normal operating mode, write the value saved from the INIT_DRIVEx bit field into the
IDRIVEx bit field (bits 15:11).
9. During normal operating mode, the RP_OVERRIDE_EN should be set to b1 for a fixed current drive.
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 This may cause the sensor frequency to shift so that the output values no longer represent a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. If the current drive is set at a lower value, the SNR performance of the system will decrease,
and at near zero target range, oscillations may completely stop, and the output sample values will be all zeroes.
If there are significant differences in the sensor construction for different channels, then this process should be
repeated for each channel.
8.1.5.3 Determining Sensor IDRIVE for an Unknown Sensor R
P
Using an Oscilloscope
If the sensor R
P
is not known, probing the sensor amplitude with an oscilloscope can be used set IDRIVEx.
An iterative process of adjusting the drive current setting while monitoring the signal amplitude on INAx or INBx
to ground is sufficient. Simply move the sensor target to the farthest planned operating distance from the sensor,
and measure the channel amplitude after the amplitude has stabilized. If the sensor amplitude is less than 1.5
V
P
, increase the channel IDRIVE setting. If the sensor amplitude settles to greater than 1.75 V
P
, decrease the
channel IDRIVE setting. If there are significant differences in the sensor construction for different channels, then
this process should be repeated for each channel.
8.1.5.4 Sensor Auto-Calibration Mode
The LDC includes a sensor current Auto-calibration mode which can be dynamically set the sensor drive current.
The auto-amplitude correction attempts to maintain the sensor oscillation amplitude between 1.2V and 1.8V by
adjusting the sensor drive current between conversions.
This functionality is enabled by setting AUTO_AMP_DIS to b0, and applies to all active channels. The
INIT_IDRIVEx register field will be updated with the current drive value as the sensor current drive setting
changes. The value of the INIT_IDRIVEx register field matches the setting of the IDRIVEx register field. For
example, an INIT_IDRIVEx field with b10001 corresponds to a current drive of 195 µA.
When auto-amplitude correction is active, the output data may experience offsets in the channel output code due
to adjustments in drive current. Due to these offsets, Auto-amplitude correction is generally not recommended for
use in high precision applications.