# Texas Instruments | Measuring Rp of an L-C Sensor for Inductive Sensing | Application notes | 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.
1
2
3
4
5
1
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 ....................................................................................................................
1
1
2
2
3
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.
2
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.
SNOA936 – October 2015
Submit Documentation Feedback
Measuring Rp of an L-C Sensor for Inductive Sensing
1
RP Measurement Method 2 – Using an Impedance Analyzer
3
www.ti.com
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.
4
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.
2
Measuring Rp of an L-C Sensor for Inductive Sensing
SNOA936 – October 2015
Submit Documentation Feedback
Conclusion
www.ti.com
Figure 3. Using a Signal Generator and Oscilloscope to Find RP.
5
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.
SNOA936 – October 2015
Submit Documentation Feedback
Measuring Rp of an L-C Sensor for Inductive Sensing
3
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other
changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest
issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and
complete. All semiconductor products (also referred to herein as “components”) are sold subject to TI’s terms and conditions of sale
supplied at the time of order acknowledgment.
TI warrants performance of its components to the specifications applicable at the time of sale, in accordance with the warranty in TI’s terms
and conditions of sale of semiconductor products. Testing and other quality control techniques are used to the extent TI deems necessary
to support this warranty. Except where mandated by applicable law, testing of all parameters of each component is not necessarily
performed.
TI assumes no liability for applications assistance or the design of Buyers’ products. Buyers are responsible for their products and
applications using TI components. To minimize the risks associated with Buyers’ products and applications, Buyers should provide
TI does not warrant or represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, or
other intellectual property right relating to any combination, machine, or process in which TI components or services are used. Information
published by TI regarding third-party products or services does not constitute a license to use such products or services or a warranty or
endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of the
third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of significant portions of TI information in TI data books or data sheets is permissible only if reproduction is without alteration
and is accompanied by all associated warranties, conditions, limitations, and notices. TI is not responsible or liable for such altered
documentation. Information of third parties may be subject to additional restrictions.
Resale of TI components or services with statements different from or beyond the parameters stated by TI for that component or service
voids all express and any implied warranties for the associated TI component or service and is an unfair and deceptive business practice.
TI is not responsible or liable for any such statements.
Buyer acknowledges and agrees that it is solely responsible for compliance with all legal, regulatory and safety-related requirements
concerning its products, and any use of TI components in its applications, notwithstanding any applications-related information or support
that may be provided by TI. Buyer represents and agrees that it has all the necessary expertise to create and implement safeguards which
anticipate dangerous consequences of failures, monitor failures and their consequences, lessen the likelihood of failures that might cause
harm and take appropriate remedial actions. Buyer will fully indemnify TI and its representatives against any damages arising out of the use
of any TI components in safety-critical applications.
In some cases, TI components may be promoted specifically to facilitate safety-related applications. With such components, TI’s goal is to
help enable customers to design and create their own end-product solutions that meet applicable functional safety standards and
requirements. Nonetheless, such components are subject to these terms.
No TI components are authorized for use in FDA Class III (or similar life-critical medical equipment) unless authorized officers of the parties
have executed a special agreement specifically governing such use.
Only those TI components which TI has specifically designated as military grade or “enhanced plastic” are designed and intended for use in
military/aerospace applications or environments. Buyer acknowledges and agrees that any military or aerospace use of TI components
which have not been so designated is solely at the Buyer's risk, and that Buyer is solely responsible for compliance with all legal and
regulatory requirements in connection with such use.
TI has specifically designated certain components as meeting ISO/TS16949 requirements, mainly for automotive use. In any case of use of
non-designated products, TI will not be responsible for any failure to meet ISO/TS16949.
Products
Applications
Audio
www.ti.com/audio
Automotive and Transportation
www.ti.com/automotive
Amplifiers
amplifier.ti.com
Communications and Telecom
www.ti.com/communications
Data Converters
dataconverter.ti.com
Computers and Peripherals
www.ti.com/computers
DLP® Products
www.dlp.com
Consumer Electronics
www.ti.com/consumer-apps
DSP
dsp.ti.com
Energy and Lighting
www.ti.com/energy
Clocks and Timers
www.ti.com/clocks
Industrial
www.ti.com/industrial
Interface
interface.ti.com
Medical
www.ti.com/medical
Logic
logic.ti.com
Security
www.ti.com/security
Power Mgmt
power.ti.com
Space, Avionics and Defense
www.ti.com/space-avionics-defense
Microcontrollers
microcontroller.ti.com
Video and Imaging
www.ti.com/video
RFID
www.ti-rfid.com
OMAP Applications Processors
www.ti.com/omap
TI E2E Community
e2e.ti.com
Wireless Connectivity
www.ti.com/wirelessconnectivity
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265