Datasheet
0
20
40
60
80
100
120
140
±1.00
±0.75
±0.50
±0.25
0.00
0.25
0.50
0.75
1.00
Count
Deviation (µm)
C001
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.0 0.2 0.4 0.6 0.8 1.0 1.2
RMS Noise (µm)
Distance (mm)
C001
LDC1000
www.ti.com
SNOSCX2 –SEPTEMBER 2013
The resonance impedance can be calculated from the digital output code as follows:
R
P
= (R
P
MAX × R
P
MIN ) / ( R
P
MIN×(1-Y) + R
P
MAX × Y ) , in Ω.
Where:
• Y=Proximity Data/2
15
• Proximity data is the LDC output, register address 0x21 and 0x22. (1)
Example: If Proximity data (address 0x22:0x21) is 5000, Rp_MIN is 2.394 kΩ, and Rp_MAX is 38.785 kΩ ,the
resonance impedance is given by:
Y=5000/2
15
= 0.1526
RP=(38785*2394)/(2394×(1-0.1526) + 38785×0.1526) =(92851290)/(2028.675 + 5918.591)
RP =11.683 kΩ
Figure 13 and Figure 14below show the change in RMS noise versus distance and a histogram of noise, with the
target at 0.8mm distance from the sensor coil. Data was collected with a 14mm PCB coil(23 turns,4 mil trace
width, 4mil spacing between trace,1oz Cu thickness,FR4) with a sensing range of 0.125mm to 1.125mm. At a
distance of 0.8mm, the RMS noise is approximately 250nm.
Figure 13. Typical RMS Noise Vs. Distance with Figure 14. Histogram of Output Codes at 0.8mm
PCB Coil Distance
Note that while the LDC1000 has high resolution, its absolute accuracy depends on offset and gain correction
which can be achieved by two-point calibration.
Calculation of Rp Min and Rp Max
Different sensing applications may have a different ranges of the resonance impedance Rp to measure. The
LDC1000 measurement range of Rp is controlled by setting 2 registers – Rp_MIN and Rp_MAX. For a given
application, Rp must never be outside the range set by these register values, otherwise the measured value will
be clipped. For optimal sensor resolution, the range of Rp_MIN to Rp_MAX should not be unnecessarily large.
The following procedure is recommended to determine the Rp_MIN and Rp_MAX register values.
Rp_MAX
Rp_MAX sets the upper limit of the LDC1000 resonant impedance input range.
• Configure the sensor such that the eddy current losses are minimized. As an example, for a proximity sensing
application, set the distance between the sensor and the target to the maximum sensing distance.
• Measure the resonant impedance Rp using an impedance analyzer.
• Multiply Rp by 2 and use the next higher value from Table 6.
Copyright © 2013, Texas Instruments Incorporated 11
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