Datasheet
2
SENSO
inf
R
1
L(d) L M )
C
d
¦
(
)(2
S
0
2
4
6
8
10
12
14
16
18
0 1 2 3 4 5 6 7 8
R
P
(kΩ)
Distance (mm)
36
LDC1612
,
LDC1614
SNOSCY9A –DECEMBER 2014–REVISED MARCH 2018
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Product Folder Links: LDC1612 LDC1614
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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 51 shows an example of R
P
variation based on 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, on FR4 material).
This curve is a typical response where the target distance scales based on the sensor size and the sensor R
P
scales based on the free-space of the inductor.
Figure 51. Example R
P
vs. Distance with a 14 mm PCB Coil and 2 mm Thick Stainless Steel Target
It is important to configure the sensor current drive so that the sensor will still oscillate at the minimum R
P
value
(which typically occurs with maximum target interaction). As an example, if the closest target distance in a
system with the response shown in Figure 51 is 1mm, then the sensor current drive needs to support a R
P
value
is 5 kΩ. Both the minimum and maximum R
P
conditions should have oscillation amplitudes that are within the
device operating range. See section Sensor Current Drive Control for details on setting the current drive.
The inductance that is measured by the LDC is:
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
• ƒ
SENSOR
= sensor oscillation frequency for a distance d between the sensor coil and target
• C = C
SENSOR
+ C
PARASITIC
(3)
Figure 52 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
material). The frequency and inductance graphs will scale based on the sensor free-space characteristics, and
the target distance scales based on the sensor diameter.