AN-1011
APPLICATION NOTE
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EMC Protection of the AD7150
by Holger Grothe and Mary McCarthy
EMC SOURCE
The AD7150 is a capacitance-to-digital converter (CDC)
designed for proximity applications. The device measures the
capacitance between two electrodes and compares its measurement 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 interference, 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).
RADIATIVE COUPLING
EMC SINK
DEVICE 1
DEVICE 2
CONDUCTIVE COUPLING
08144-001
INTRODUCTION
Figure 1. EMC Coupling Path
When a system is being designed to operate in a harsh environment, 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.
Rev. 0 | Page 1 of 8
AN-1011
Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1 AD7150 EMC Performance Without External Filters .............4 What is EMC? ................................................................................... 1 Choosing the External Filter ........................................................4 Capacitance-to-Digital Converter Architecture....................... 3 Conclusion .....................................................................................7 EMC Testing .................................................................................. 4 Rev. 0 | Page 2 of 8
Application Note
AN-1011
CAPACITANCE-TO-DIGITAL CONVERTER
ARCHITECTURE
To understand how EMC affects the operation of the AD7150,
an understanding of the architecture of the capacitance-todigital converter (CDC) is useful. A capacitance-to-digital
converter measures capacitance by using switching capacitor
technology to build up a charge balancing circuit (see Figure 2).
A brick wall filter achieves this response. However, since a CDC
device measures charge going from the excitation pin to the
capacitive input pin, the external filter must use passive
components only.
In practice, a passive filter has slower roll off. A tradeoff must
be made between passing the 32 kHz without attenuation and
attenuating multiples of 32 kHz (see Figure 4).
0
FEEDBACK LOOP
CREF
–10
CINT
–20
FILTER GAIN (dB)
Q=V×C
OFF-CHIP
DIGITAL
FILTER
COMPARATOR
EXCITATION
VOLTAGE
–30
–40
–50
–60
Figure 2. CDC Architecture
The sensing capacitor, CSENSOR, and the internal reference capacitor, CREF, are switched at a fixed sampling rate, and their
charge is pumped into the integrator. A comparator checks the
integrator output and controls the phase of the input switches
to close the feedback loop, which balances the charges flowing
through the CSENSOR and reference paths.
–70
A stream of zeros and ones, which can be seen on the comparator output, varies with the charge needed for the loop balance.
The charge is proportional to voltage and capacitance. Because
the voltages EXC and VREF have fixed values, the density of
zeros and ones represents the ratio between the input capacitance, CSENSOR, and the reference capacitance, CREF. The on-chip
digital filter then extracts the information carried by the timedomain pattern of zeros and ones to form the digital result.
Since the filtering is digital, the response around dc is repeated
around the sampling frequency and multiples of the sampling
frequency. Therefore, there is no rejection provided by the onchip digital filter around the sampling frequency and its
multiples.
–10
–80
0
8
16
24
32
40
48
56
64
INPUT SIGNAL FREQUENCY (kHz)
08144-003
0100110
08144-002
CSENSOR
INTEGRATOR
Figure 3. AD7150 Filter Response
0
FILTER GAIN (dB)
–20
–30
–40
–50
–60
–70
–80
27
32
37
INPUT SIGNAL FREQUENCY (kHz)
08144-004
VREF (–)
Figure 4. Frequency Response in 32 kHz Region
AD7150
0
IDEAL
–10
–20
FILTER GAIN (dB)
The AD7150 uses a second-order modulator and third-order
sinc filter. The excitation frequency, which is the capacitive
input sampling frequency, is equal to 32 kHz. Therefore, the onchip filter response is repeated around 32 kHz and multiples of
32 kHz (see Figure 3). In a noisy environment, some additional
filtering on the front-end is required to provide rejection at
multiples of 32 kHz. The 32 kHz signal must be present to
excite and measure the capacitance. Thus, an ideal external
filter should allow the 32 kHz signal to pass through unattenuated and then filter all frequencies around 64 kHz and higher.
ACTUAL
–30
–40
–50
–60
–70
–80
10
100
INPUT SIGNAL FREQUENCY (kHz)
Figure 5. Anti-Aliasing Filter
Rev. 0 | Page 3 of 8
1000
08144-005
VREF (+)
AN-1011
Application Note
EMC TESTING
DPI
SYSTEM
For EMC testing, the DPI setup, as shown in Figure 6, is used.
This diagram is taken from the IEC 62132—Part 4 document.
The DPI setup consists of an RF signal generator, an RF
amplifier, a directional coupler (which is connected through
probes to an RF power meter, which measures the forward
power to the DUT). Measuring the reflected power is optional,
because the forward power level must remain constant during
the DPI sweep.
The DPI test was repeated over a lower frequency range of
1 MHz to 3 MHz using a smaller step size of 200 Hz. This test
was included because the AD7150 was expected to be sensitive
to tones around 32 kHz and its multiples, and the external EMC
filter was expected to be less efficient in this range.
For all the EMC testing, the AD7150 was configured with an
input range of 2 pF and the sensitivity was set to 10 decimal.
50Ω
COAX
RF
AMPLIFIER
TEST
PCB
DIRECTIONAL
COUPLER
PFOR
PREFL
RF
GENERATOR
BUS
RF
POWER
METERS
DECOUPLING
NETWORK
DC
BLOCK
RF
INJECTION
PORT
OPTIONAL:
CONTROL PC
Figure 6. DPI Test Setup
EXC
0.1µF
08144-007
VDD
6.8nF
10µF
Figure 7. AD7150 to DPI System Connections
AD7150 EMC PERFORMANCE WITHOUT
EXTERNAL FILTERS
The AD7150 was EMC tested without the external filters to
determine the EMC performance of the device. Because the
CIN pins were the most sensitive, they were used for the DPI
testing. As shown in Figure 8, the power level that causes false
triggers is much lower than the target of 50 mW.
Note that the AD7150 remained functional when EMC tested
using a target power level of 50 mW. Although the part false
triggered when tested to this power level, it never locked up.
0.06
TARGET
0.05
0.04
0.03
0.02
0.01
0
1
10
100
FREQUENCY (MHz)
1000
Figure 8. DPI Testing from 1 MHz to 100 MHz in 1 MHz Steps and from
100 MHz to 1000 MHz in 10 MHz Steps on CIN Pins Without External Filters
CHOOSING THE EXTERNAL FILTER
After evaluating several passive filter structures, a second-order
filter was selected for the CIN pins because it uses a reasonably
small amount of components and it provides good performance
in terms of frequency response and roll off using nonprecision
components.
DUT
DUT
MONITOR
FILTER
47pF
A brick wall filter gives the optimum filter response—the
32 kHz signal is unattenuated, though all frequencies around
64 kHz and higher are rejected. Because active components
could not be used with the AD7150, different types of passive
filters were evaluated on the CIN and EXC pins.
=
08144-006
DC
SUPPLY
CIN
CSENSOR
1pF
08144-008
The frequency was increased from 1 MHz to 100 MHz in
1 MHz steps and from 100 MHz to 1000 MHz in 10 MHz steps.
Analog Devices, Inc., used a target power level of 50 mW. If the
AD7150 did not false trigger when 50 mW was injected during
the frequency sweep, this was considered a pass. If a false
trigger occurred when 50 mW was injected, this was considered
a fail. If the device failed to pass the 50 mW target level at any
frequency, the maximum RF power level at which the device
did not false trigger was determined.
FILTER
47pF
DPI POWER LEVEL (W)
All pins of the AD7150 are EMC tested. The EXC, CIN, and
VDD pins were found to be the most sensitive. Therefore,
this application note focuses on these pins. A continuous RF
frequency was applied individually to the CIN, EXC, and VDD
pins using ac-coupling (see Figure 7) as per IEC 62132—Part 4.
The test method recommends using ac-coupling capacitors
of 6.8 nF. This capacitance value was used on the VDD pin.
However, lower value capacitors (47 pF) were used on the EXC
and CIN pins because the value suggested in IEC 62132—Part 4
exceeds the maximum allowed capacitance to ground that can
be connected to the AD7150.
AD7150
For the EXC pins, a first-order filter was sufficient to achieve
the desired EMC performance. Finally, the VDD pin used
standard decoupling capacitors (a 0.1 μF ceramic capacitor in
parallel with a 10 μF tantalum capacitor to GND). The desired
EMC performance was met with these decoupling capacitors.
Rev. 0 | Page 4 of 8
Application Note
AN-1011
AD7150 Performance with External Filters
The values of the components were chosen so that the best
tradeoff between EMC performance and the accuracy of the
AD7150 in the context of proximity detection was achieved.
Even though the accuracy of the AD7150 was degraded, the
part still functioned in a proximity application.
39kΩ
82kΩ
CIN
68pF
22pF
CSENSOR
1pF
AD7150
10kΩ
08144-009
EXC
47pF
FILTER CHARACTERISTICS
NO FILTER: y = 1.0452x – 0.0006
ADI FILTER: y = 0.5697x + 0.724
4
MEASURED CAP VALUE
The values of the components used in the second-order filter
connected to the CIN pins and the first-order filter connected
to the EXC pins are shown in Figure 9. A 1 pF ceramic capacitor
was used in place of the capacitive sensor. The second-order
filter has a cutoff frequency of 72.76 kHz, the phase shift is −48°
at 32 kHz, and the attenuation at 32 kHz is −1.62 dB.
The external filters connected to the CIN and EXC pins
influence the accuracy of the AD7150 conversions. Figure 12
shows how the input-to-output transfer function is altered.
With the external filters, the offset error is 0.724 pF, while the
gain error is −0.859 pF when a 2 pF input capacitance is used
(this equates to −42.9%). The power supply rejection is reduced
to 40 fF/V.
3
2
1
0
0
0
0
–60
–120
PHASE (Degrees)
LEVEL (dB)
–90
3
4
Figure 12. Input-to-Output Transfer Function of the AD7150 with and
without the External Filter
–20
–40
2
SET CAP VALUE
–30
–60
1
08144-012
Figure 9. Passive Filters on Front End of AD7150
AD7150 EMC Performance with External Filters
DPI on the CIN Pins
0.06
–100
–180
100k
1
10
100
1k
10k
FREQUENCY (kHz)
Figure 10. Frequency Response of Second-Order Passive Filter
1.1
DPI POWER LEVEL (W)
–150
08144-010
0.05
–80
1.0
0.04
0.03
0.02
0.01
0
1
0.7
10
100
FREQUENCY (MHz)
0.6
1000
Figure 13. CIN: Sweep from 1 MHz to 1000 MHz
0.5
0.4
0.3
0.2
0.1
0
0
2
4
6
8
10
12
14
16
18
TIME (µs)
Figure 11. Step Response of Second-Order Passive Filter
20
08144-011
VOLTAGE (V)
0.8
08144-013
0.9
When the RF frequency was swept from 1 MHz to 100 MHz
in 1 MHz steps and from 100 MHz to 1000 MHz in 10 MHz
steps, no false triggers occurred on the AD7150 output as
shown in Figure 13. When the DPI testing was repeated over
a range of 1 MHz to 3 MHz in steps of 200 Hz (Figure 14), the
external passive filter provided full immunity to frequencies
above 1.9 MHz. At low frequencies, the external filter was less
efficient—there is still some sensitivity in narrow bands
around multiples of 32 kHz.
Rev. 0 | Page 5 of 8
AN-1011
Application Note
0.06
DPI on the VDD Pin
DPI testing was also performed on the VDD pin. With the
standard decoupling capacitors connected to VDD, there were
no false triggers on the AD7150 output when the RF frequency
was swept from 1 MHz to 100 MHz in 1 MHz steps and from
100 MHz to 1000 MHz in 10 MHz steps (see Figure 17). When
the frequency was swept from 1 MHz to 3 MHz in 200 Hz steps,
again, no false triggers occurred (see Figure 18). Therefore, the
decoupling capacitors provide a high level of EMC performance.
0.04
0.03
0.02
0.06
0.01
2.5
3.0
FREQUENCY (MHz)
0.05
Figure 14. CIN: Fine Sweep from 1 MHz to 3 MHz in 200 Hz Steps
DPI on the EXC Pins
0.02
0
1
0.06
0.05
0.05
0.03
0.02
1
10
100
1000
FREQUENCY (MHz)
Figure 15. EXC: Sweep from 1 MHz to 1000 MHz
0.04
0.03
0.02
0.05
0.04
0.03
0.02
2.0
2.5
3.0
FREQUENCY (MHz)
08144-016
0.01
1.5
0
1.0
1.5
2.0
2.5
3.0
FREQUENCY (MHz)
Figure 18. VDD: Sweep from 1 MHz to 3 MHz in 200 Hz Steps
0.06
0
1.0
1000
0.01
08144-015
0
0.1
100
Figure 17. VDD: DPI Testing in 1 MHz to 1000 MHz Range
0.06
0.04
10
FREQUENCY (MHz)
0.01
DPI POWER LEVEL (W)
0.03
0.01
DPI POWER LEVEL (W)
DPI POWER LEVEL (W)
When the RF frequency applied to EXC was swept from 1 MHz
to 100 MHz in 1 MHz steps and from 100 MHz to 1000 MHz in
10 MHz steps, no false triggers occurred on the AD7150 output
as shown in Figure 15. When the DPI testing was repeated over
a narrower range of 1 MHz to 3 MHz in 200 Hz steps, again no
false triggers occurred (see Figure 16). Thus, the EXC pin
provides a high level of EMC performance when the first
order filter is connected to the pin.
0.04
08144-017
2.0
DPI POWER LEVEL (W)
1.5
08144-014
0
1.0
Figure 16. EXC: Sweep from 1 MHz to 3 MHz in 200 Hz Steps
Rev. 0 | Page 6 of 8
08144-018
DPI POWER LEVEL (W)
0.05
Application Note
AN-1011
CONCLUSION
functional. A power level of 50 mW does not cause the AD7150
to lock up.
When EMC testing the AD7150 using a target power level of
50 mW, the device does not lock up. The part continues to
convert during the EMC event and it returns to the expected
accuracy after the EMC disturbance is removed.
The AD7150 continues to meet the requirements for proximity
detection applications when the external EMC filters are used.
The filters cause some degradation in the accuracy of the
AD7150, however the system is still sufficiently accurate for
proximity detection systems.
In this application note, external passive filters are recommended to improve the EMC performance of the AD7150.
When external filters are included on the CIN and EXC pins
and standard decoupling is used on the VDD pin, the device
passes the EMC test for frequencies above 1.9 MHz. For
frequencies less than 1.9 MHz, the device shows some
sensitivity in the regions around multiples of 32 kHz.
Without external filters, the AD7150 does not pass the EMC
test described in IEC 62132—Part 4 when a target power level
of 50 mW is used. However, the AD7150 always remains
The external EMC filters discussed in this application note
optimize the EMC performance of the AD7150. If a less
stringent filter is used, the AD7150 accuracy will degrade by a
smaller amount. There is a tradeoff between EMC performance
and AD7150 accuracy.
Rev. 0 | Page 7 of 8
AN-1011
Application Note
NOTES
©2009 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN08144-0-6/09(0)
Rev. 0 | Page 8 of 8