LDC1612_15 Datasheet, PDF (38/58 Pages) Texas Instruments – LDC1612, LDC1614 Multi-Channel 28-Bit Inductance to Digital Converter (LDC) for Inductive Sensing

LDC1612, LDC1614
SNOSCY9 – DECEMBER 2014
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Application Information (continued)
Texas Instruments' WEBENCH design tool can be used for coil design, in which the parameter values for R_P, L
and C are calculated. See http://www.ti.com/webench.
R_P is a function of target distance, target material, and sensor characteristics. Figure 55 shows that R_P is directly
proportional to the distance between the sensor and the target. The graph represents a 14-mm diameter PCB
coil (23 turns, 4-mil trace width, 4-mil spacing between traces, 1-oz copper thickness, FR4).
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Distance (mm)
Figure 55. Example RP vs. Distance with a 14-mm PCB Coil and 2mm Thick Stainless Steel Target
It is important to configure the LDC current drive so that the sensor will still oscillate at the minimum RP value.
As an example, if the closest target distance in a system with the response shown in Figure 55 is 1mm, then the
LDC R_P value is 5 kΩ. The objective is to maintain a sufficient sensor oscillation voltage so that the sensor
frequency can be measured even at the minimum operating distance. See section Current Drive Control
Registers for details on setting the current drive.
The inductance that is measured by the LDC is
L(d)
L_inf  M(d)
1
(2S ¦_SENSOR )² C
where
• L(d) is the measured sensor inductance, for a distance d between the sensor coil and target
• L_inf is the inductance of the sensing coil without a conductive target (target at infinite distance)
• M(d) is the mutual inductance
• f_SENSOR = sensor oscillation frequency for a distance d between the sensor coil and target
• C = C_TANK + C_PAR
(11)
Figure 56 shows an example of variation in sensor frequency and inductance as a function of distance for a 14-
mm diameter PCB coil (23 turns, 4-mil trace width, 4-mil spacing between traces, 1-oz copper thickness, FR4).
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