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AN-1055
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 AD7746
by Holger Grothe and Mary McCarthy
Rev. 0 | Page 1 of 20
INTRODUCTION
The AD7746 is a 24-bit capacitance-to-digital converter (CDC).
The device measures the capacitance of a floating capacitive
sensor directly, that is, the sensor connects directly to the
AD7746 input pins.
Electromagnetic interference affects the conversion results
since it distorts the electric field around the capacitive sensor
and, therefore, alters the capacitance measurement. To protect
the AD7746 from this electromagnetic interference, some exter-
nal filtering is used. However, including filters in the signal path
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 AD7746 pins as well as the affect of the
filter on the accuracy of the AD7746.
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
08743-001
CONDUCTIVE COUPLING
Figure 1. EMC Coupling Path
A system designed to operate in a harsh environment must
be designed with EMC in mind and EMC testing must be
performed during the design phase. There are different levels
of EMC testing: testing at the 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
a car battery or chassis. If a device is confined within a printed
circuit board (PCB), then the EMC level required from the
device is less.
The AD7746 is an integrated circuit. Therefore, EMC testing
was performed using direct power injection (DPI) in accordance
with the international standard IEC 62132—Part 4. The
AD7746 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.
