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AN-1011
APPLICATION NOTE
One Technology Way • P. O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Te l: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
EMC Protection of the AD7150
by Holger Grothe and Mary McCarthy
Rev. 0 | Page 1 of 8
INTRODUCTION
The AD7150 is a capacitance-to-digital converter (CDC)
designed for proximity applications. The device measures the
capacitance between two electrodes and compares its measure-
ment result with a threshold. If the input capacitance is altered,
by the presence of a hand, for example, an output flag is set
to signify that a threshold has been exceeded, thus indicating
proximity.
Electromagnetic interference affects the conversion results
since it distorts the electric field around the capacitive sensor
and, therefore, alters the capacitance. To protect the AD7150
and the capacitive sensor from this electromagnetic interfe-
rence, some external filtering is used. However, including filters
is challenging because the filters degrade the accuracy of the
capacitance-to-digital conversions. This application note
discusses the EMC performance that can be achieved with an
external filter on the AD7150 pins as well as the affect of the
filter on the accuracy of the AD7150.
WHAT IS EMC?
Electromagnetic compatibility (EMC) refers to the ability to
operate in, without overly contributing to, an environment
of electromagnetic radiation. When this goal is met, all
electronic equipment operates correctly in the presence of
other equipment. In a system, there are several EMC coupling
paths: radiative, conductive, inductive, and capacitive (see
Figure 1).
DEVICE 1
EMC SOURCE
DEVICE 2
EMC SINK
RADIATIVE COUPLING
CONDUCTIVE COUPLING
08144-001
Figure 1. EMC Coupling Path
When a system is being designed to operate in a harsh environ-
ment, the system must be designed with EMC in mind and
EMC testing must be performed. There are different levels of
EMC testing: testing at system level, testing subsystems of the
overall system, and testing at the IC level. Test methods are
defined for each level of EMC testing.
The EMC performance required from a subsystem or IC device
depends on the function of the device as well as its location
in the system. For example, a device must have high EMC
performance in automotive applications if it is connected to
the car battery or chassis. If a device is confined within a
printed circuit board, then the EMC level required from the
device is less.
The AD7150 is an integrated circuit. Therefore, EMC testing
was performed using direct power injection (DPI) in accordance
with the international standard IEC62132—Part 4. The AD7150
is used for proximity sensing, for example, keyless entry. It is
confined within a PCB and it has a local connection to the
sensor. Therefore, the level of electromagnetic interference is
expected to be low.
