sensor

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Output Data Rate

Response time

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LDC1041

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SNOSCY1 – MARCH 2014

8 Applications and Implementation

8.1 Application Information

8.1.1 Calculation of Rp_MIN and Rp_MAX

Different sensing applications may have a different range of the resonance impedance Rp to measure. The

LDC1041 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.

8.1.1.1 Setting Rp_MAX

Rp_MAX sets the upper limit of the LDC1041 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 4.

Note that setting Rp_MAX to a value not listed in Table 4 can result in indeterminate behavior.

8.1.1.2 Setting Rp_MIN

Rp_MIN sets the lower limit of the LDC1041 resonant impedance input range.

•

Configure the sensor such that the eddy current losses are maximized. As an example, for a proximity

sensing application, set the distance between the sensor and the metal target to the minimum sensing

distance.

•

Measure the resonant impedance Rp using an impedance analyzer.

•

Divide the Rp value by 2 and then select the next lower Rp value from Table 6.

Note that setting Rp_MIN to a value not listed on Table 6 can result in indeterminate behavior. In addition,

Rp_MIN powers on with a default value of 0x14 which must be set to a value from Table 6 prior to powering on

the LDC.

8.1.2 Output Data Rate

Configuration register(Address:0x04).

(6)

8.1.3 Choosing Filter Capacitor (CFA and CFB Terminals)

The Filter capacitor is critical to the operation of the LDC1041. The capacitor should be low leakage, temperature

stable, and it must not generate any piezoelectric noise (the dielectrics of many capacitors exhibit piezoelectric

characteristics and any such noise is coupled directly through Rp into the converter). The optimal capacitance

values range from 20pF to 100nF. The value of the capacitor is based on the time constant and resonating

frequency of the LC tank.

If a ceramic capacitor is used, then a C0G (or NP0) grade dielectric is recommended; the voltage rating should

be

≥10V. The traces connecting CFA and CFB to the capacitor should be as short as possible to minimize any

parasitics.

For optimal performance, the chosen filter capacitor, connected between terminals CFA and CFB, needs to be as

small as possible, but large enough such that the active filter does not saturate. The size of this capacitor

depends on the time constant of the sense coil, which is given by L/Rs, (L=inductance, Rs=series resistance of

the inductor at oscillation frequency). The larger this time constant, the larger filter capacitor is required. Hence,

this time constant reaches its maximum when there is no target present in front of the sensing coil.

Copyright © 2014, Texas Instruments Incorporated

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