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Texas Instruments Measuring Rp of an L-C Sensor for Inductive Sensing Application notes
Application Report
SNOA936 – October 2015
Measuring Rp of an L-C Sensor for Inductive Sensing
Don Liu
ABSTRACT
Three methods are introduced for the measurement of the RP value of an L-C tank circuit using common
lab equipment.
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Contents
Introduction ..................................................................................................................
RP Measurement Method 1 – Using a Network Analyzer ..............................................................
RP Measurement Method 2 – Using an Impedance Analyzer .........................................................
Measurement Method 3 – Using a Signal Generator and an Oscilloscope .........................................
Conclusion ....................................................................................................................
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Introduction
When designing an application using TI’s LDC series of inductive sensors, it is necessary to know the L-C
sensor’s equivalent parallel resistance RP at the sensor’s resonant frequency. The RP value changes as
the target is moved; the minimum RP occurs when the metal target is closest to the sensor. The maximum
RP occurs when the target is at the farthest distance. Accordingly, both values should be measured to
ensure that the RP value is within the proper range as defined in the datasheets.
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RP Measurement Method 1 – Using a Network Analyzer
A vector network analyzer can measure the complex impedance of the inductor over a range of frequency.
The XL (reactance) and RS (series loss resistance) values are displayed at a selected frequency. Then use
the formula shown in the figure to calculate the RP at desired frequency.
Figure 1. Using a Network Analyzer to Find RP.
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RP Measurement Method 2 – Using an Impedance Analyzer
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RP Measurement Method 2 – Using an Impedance Analyzer
An impedance analyzer can measure the impedance of the inductor over a range of frequency. Some
impedance analyzer has built-in RP measurement function. If not, the Rp value can be calculated using the
LS (Inductance) and RS (series loss resistance). Use the formula shown in the figure to calculate the RP at
desired frequency.
Figure 2. Using an Impedance Analyzer to Find RP.
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Measurement Method 3 – Using a Signal Generator and an Oscilloscope
Use a sine-wave signal generator and oscilloscope, as illustrated in Figure 3, below. Note that the LC
sensor capacitor is required for this method, therefore the RP value is measured at the resonant frequency
only. Adjust the frequency so that the VPP1 reaches maximum value (resonance occurs). Adjust the R
such that VPP2=2VPP1. Repeat the above steps if needed to get better accuracy. Then use an ohm
meter to measure R. The value of R is the RP of the LC sensor.
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Measuring Rp of an L-C Sensor for Inductive Sensing
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Conclusion
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Figure 3. Using a Signal Generator and Oscilloscope to Find RP.
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Conclusion
To properly configure an LDC for optimum system operation, it is necessary to know the RP range for the
system. The methods described in this application note can be used to measure the expected RP range,
and combined with the guidance found in application note Configuring Inductive-to-Digital-Converters for
Parallel Resistance (RP) Variation in L-C Tank Sensors, enable the proper configuration of the relevant
LDC device.
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