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Texas Instruments EMI Considerations for Inductive Sensing Application notes
Application Report
SNOA962 – February 2017
EMI Considerations for Inductive Sensing
Rachel Liao, Luke LaPointe
ABSTRACT
This application note explains various EMI reduction techniques to help improve EMI performance for TI's
Inductance-to-Digital Converters (LDC). Each section details a general technique with references to other
useful online documents. A list of relevant EMI reduction techniques is provided for specific devices within
the LDC family of products.
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2
3
4
5
Contents
Introduction ...................................................................................................................
Design Techniques .........................................................................................................
Troubleshooting ..............................................................................................................
Conclusion ....................................................................................................................
References ...................................................................................................................
2
2
5
5
5
List of Figures
1
Layout of Effective Coil Shielding.......................................................................................... 3
2
Examples of Effective Coil Shielding Structures ......................................................................... 3
3
Common-Mode Choke and Capacitor Filtering Network on the LDC EVMs ......................................... 4
List of Tables
1
Suitable EMI Reduction Techniques ...................................................................................... 2
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1
Introduction
1
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Introduction
When an electronic system or device resides in a harsh and noisy environment, electromagnetic
interference (EMI) can occur, disrupting system level functions or causing a product to fail electromagnetic
compatibility (EMC) testing. EMI is essentially any unwanted radiated or conducted electrical signals that
negatively affect a system or device’s performance. Due to the increasing number of radiating wireless
devices, it is vital to ensure EMC and adhere to its standards. Each application may have different
compliance standards, for example, safety critical system automotive applications have more stringent
standards than personal electronics.
TI’s inductive sensing products are based on a narrowband resonant sensing architecture which provides
inherent immunity to broadband noise and targeted frequency ranges that fall outside of the resonant
frequency range. These devices also include internal EMI and deglitch filters to prevent high frequency
signals from disrupting the circuitry after the sensors which increases robustness against EMI
susceptibility. As part of normal operation, the LDC devices drive an inductive sensor that intentionally
radiates a magnetic field to sense nearby conductive objects. Therefore, it is important to be aware of the
radiated output spectrum which could disrupt nearby circuits that are sensitive to EMI. For more
discussion on the basics of inductive sensing, please refer to the TI Application Note LDC Device
Selection Guide.
There are a number of factors which affect the strength of the EMI radiated emissions from the LDC. At
high frequencies, the PCB traces, wires to remote coils, and the coil itself can serve as antennas that
generate far-field RF radiation or receive RF interference from the ambient environment. At low
frequencies, long wires to the supply are typically sources of radiation. Different mitigation techniques for
each LDC device are outlined below in Table 1 and a more detailed discussion can be found within each
section below.
Table 1. Suitable EMI Reduction Techniques
Device
Shielding
Passive Filters
Supply/Return Routing
LDC1101
X
X
X
LDC1312
LDC1314
X
X
X
LDC1612
LDC1614
X
X
X
LDC0851
X
LDC2112
LDC2114
X
2
Design Techniques
2.1
Shielding
X
X
X
Shielding represents an approach to prevent unwanted signals from leaving or entering critical or sensitive
areas of the system. Any time there is a moving charge, there will be a generated magnetic field (B-field)
and orthogonal electric field (E-field). If unmanaged, these fundamental fields can be the source of
potential EMI problems, each with their own coupling mechanisms to victim circuits. For this reason, the
orientation of the attacker and/or victim has a significant effect on the magnitude of the interference. The
magnetic field is created by current flow in closed loops and sensitive to large di/dt changes that can
couple to other loops. The electric field is created by large voltage transients on high impedance lines that
can radiate or couple to other high impedance lines which act like antennas to the incoming signal. For
inductive sensing products, a magnetic field is emitted by the inductive sensor to sense nearby conductive
materials. Therefore, the goal of shielding for the LDC products is to minimize the electric field coupling by
reducing the number of high-impedance nodes while still allowing the magnetic field to sense the desired
conductive targets.
2
EMI Considerations for Inductive Sensing
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Design Techniques
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2.1.1
Trace Shielding
For optimal EMI performance, signal traces should be routed on the middle layers of the PCB with a
ground shield (or COM for the LDC211x) above and below. This effectively creates a low impedance
shield above and below the traces which helps protect against incoming and outgoing electric fields. If the
application requires the sensors to be connected remotely by an external cable, then the cables should be
short, shielded, and in a twisted pair whenever possible.
2.1.2
Coil Shielding
For large sensors or sensors that are directly exposed to large voltage transients such as ESD, it can also
be helpful to shield the sensor coil from picking up strong E-field emissions nearby. A solid ground plane
should not be used directly over the sensor because this would block the B-field, preventing the LDC from
sensing intended targets. Instead, the ground shield should have cut-outs to prevent eddy currents from
forming on the surface. Figure 1 and Figure 2 show some examples. A coil shield with orthogonal lines to
the sensor traces is recommended as it minimizes the parasitic capacitance to the sensor coils
underneath.
Figure 1. Layout of Effective Coil Shielding
Figure 2. Examples of Effective Coil Shielding Structures
2.1.3
Ground Plane
In addition to shielding the sensors and the routing, adding a solid un-cut ground plane beneath all the
components can be one of the biggest factors for improving emissions up to 40 dB (1). See Section 2.3 of
the TI Application Note High-Speed Layout Guidelines for more discussion.
(1)
R. F. German, H. W. Ott and C. R. Paul, "Effect of an image plane on printed circuit board radiation," IEEE International Symposium on
Electromagnetic Compatibility, Washington, DC, 1990, pp. 284-291. doi: 10.1109/ISEMC.1990.252775
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Design Techniques
2.2
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Passive Filters
2.2.1
Common-Mode Chokes
There are two significant types of signals that need to be considered when looking at EMI effects. First,
there are differential-mode signals. These are signals that appear on the two lines of a closed loop, equal
in magnitude and opposite in direction. Differential-mode signals can be conceptualized as the desired
signal and it's return path. Long traces or poor supply bypassing often causes differential-mode signals to
radiate. Secondly, there are common-mode signals which appear on the two lines of a closed loop but are
unequal in magnitude resulting in a net current flow in one direction. Common-mode radiation is one of
typical sources of EMI issues, especially with signals that have a large di/dt or dv/dt component. Sources
of common-mode signals include return paths that have a shared or common impedance with noisy
signals or unintentional coupling to a reference ground or metal chassis through stray capacitance. Good
layout can resolve many of these issues, however, shunting the noise at the source or adding a commonmode filter at the source can also provide a significant reduction in EMI if additional suppression is
needed. Many commercially available common mode filters exist and can be quite useful in debugging or
fixing issues which are identified late in the product development process. A good practice is to integrate
the common-mode filter on the PCB itself through the use of a common-mode choke or simple L and C
filtering as seen in Figure 3 below. A common-mode choke produces a high-impedance node which
prevents any common-mode signals from coupling into the system. In order to effectively use a commonmode choke, the chokes are placed as close to the device under test as possible. This way, any commonmode signals traveling through the device will have a short path, minimizing the possibility of any radiation
before being filtered out by the chokes.
2.2.2
Shunt Capacitors
For even further EMI filtering, shunt capacitors can be placed near the device in conjunction with the
common-mode chokes. Footprints for these are already implemented on the LDC131x/161x EVMs as
seen in Figure 3. These capacitors should be much smaller than the sensor capacitor itself to prevent a
loss in sensitivity. The latest LDC devices such as the LDC2114 and LDC2112 have a dedicated COM
plane which can incorporate much larger shunt capacitors near the device to more effectively mitigate
EMI. Please also refer to Section 3.3.4 EMI Emissions Testing of the TI Application Note Inductive
Sensing Touch-On-Metal Buttons Design Guide for more discussion on the use of passive filtering for
emissions.
In case of EMI issues with long wires, replace these EMI filter
component placeholders:
Populate in case
of EMI
susceptibility
issues.
Replace with
Common-mode
chokes (e.g.
SRF3216-222Y) in
case of EMI emissions
or susceptibility
Populate in case
of EMI emissions
issues.
R19
U1
ADDR
SCL
SDA
4
1
2
INTB
SD
5
6
7
ADDR
SCL
SDA
INTB
SD
VDD
PAD
GND
13
8
GND
IN1B
IN1A
12
11
IN1B
IN1A
IN0B
IN0A
10
9
IN0B
IN0A
CLKIN
C20
18pF
C23
18pF GND
C44
18pF
C7
330pF
1
2
J13
1
2
1729018
C17
330pF
TSW-102-07-G-S
COIL1
GND
0
J4
C39
18pF
C10
330pF
CLKIN
C38
18pF GND
LDC1612DNT
C34
R20
J5
C42
18pF
0
R23
C37
18pF
3
0
R33
C41
18pF
1
2
J6
1
2
1729018
C24
330pF
TSW-102-07-G-S
COIL0
GND
0
C6
Figure 3. Common-Mode Choke and Capacitor Filtering Network on the LDC EVMs
2.3
Supply/Return Routing
For optimal EMI performance, it is crucial to have a stable and clean power supply and low impedance
return routing. For more in-depth discussions on these subjects, please see the references below.
4
EMI Considerations for Inductive Sensing
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Troubleshooting
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2.3.1
Rise-Time and Fall-Time Considerations
Fast rise and fall times on critical signals can translate directly to EMI problems even if the failing
frequencies are outside the expected operating range. See Section 1.2 of the TI Application Note HighSpeed Layout Guidelines.
2.3.2
Appropriate Bypass Capacitor Selection
Bypass capacitors play a critical role in ensuring stable device operation. Proper selection of the values
and location, based on the supply characteristics and device requirements, can greatly improve system
performance. Incorrect selections may create unexpected resonances in impedance making the supply
lines more susceptible to EMI at those frequencies. See Section 2.4 of the TI Application Note High-Speed
Layout Guidelines.
2.3.3
Routing Considerations
Long traces can be thought of as antennas which can be sources of radiation and susceptibility depending
on its length and excitation signal. The previous generation of LDC devices expects the PCB sensor coil to
be placed close to the device, thus limiting the length of the traces. For guidance on the maximum
recommended length for trace routing, please refer to the Remote Coil Length tab of the LDC Calculator
Tool. The latest LDC devices (LDC211x) are designed to incorporate the long traces as part of the sensor
and can be represented as transmission lines in series with the PCB coil. This allows the sensor tank
capacitor to be placed near the device which provides extra EMI filtering towards any unwanted signals
that may have coupled onto the traces after the PCB sensor coil.
For high frequency signals, the lowest impedance return path is directly underneath the forward going
signal trace. Therefore, it is important to have a solid return path underneath the trace by minimizing cuts
and splits in the ground plane which otherwise forces the return current to flow around the cutout, forming
a loop. If unmanaged, additional loops may overlap potentially disrupting sensitive circuits. See Section
2.3 of the High-Speed Layout Guidelines application report.
3
Troubleshooting
During EMC testing, it can be important to debug problems in real time. If the PCB fails a radiated EMI
test, there are a couple of tips to determine its root cause. For example, if the failure occurs around 300
MHz or less, it is likely that the issue is coming from long external cabling since this frequency has an
effective wavelength of 1 m or more. These problems can be mitigated by adding ferrite beads to the
cabling and retesting. On the other hand, if the failure occurs at a frequency greater than 300 MHz, it is
likely that the issue is coming from the PCB itself either due to issues in routing, shielding, or bypassing. If
this is the case, then a board spin with the techniques outlined above may be required.
4
Conclusion
EMI is often a crucial but neglected topic for robust system performance. However, using the techniques
discussed above, good EMI performance can be achieved.
5
References
R. F. German, H. W. Ott and C. R. Paul, "Effect of an image plane on printed circuit board radiation," IEEE
International Symposium on Electromagnetic Compatibility, Washington, DC, 1990, pp. 284-291.
doi: 10.1109/ISEMC.1990.252775
A. Weiler and A. Pakosta, "High-Speed Layout Guidelines", Clock Drivers, Texas Instruments Application
Report. SCAA082, November 2006.
C. Oberhauser, "LDC Device Selection Guide", Texas Instruments Application Report. SNOA954A,
January 2017.
B. Kasemsadeh and L. LaPointe, "Inductive Sensing Touch-On-Metal Buttons Design Guide", Texas
Instruments Application Report. SNOA951, June 2016.
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