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Texas Instruments Comparing EMI Performance of LPV802 with Other Devices in a Gas Sensor App Application notes
Application Report
SNOA937 – October 2016
Comparing EMI Performance of LPV802 with Other
Devices in a Gas Sensor Application
Paul Grohe
ABSTRACT
Passing the IEC61000-4-3 Radiated EMI test is a requirement for preparing a product for many industrial,
medical and consumer markets. This application note presents the results of a simple comparison of the
LPV802 nanopower operational amplifier and two other competing devices in a Carbon Monoxide gas
sensor circuit while subjected to IEC61000-4-3 EMC test conditions.
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Contents
Introduction ................................................................................................................... 3
IEC61000-4-3 Test Conditions ............................................................................................. 3
Test Conditions Selected to Test EMI Performance of EUT ........................................................... 5
Test Setup .................................................................................................................... 5
Test Results ................................................................................................................. 10
Appendix..................................................................................................................... 18
References .................................................................................................................. 20
Acknowledgments .......................................................................................................... 20
List of Figures
1
Basic Schematic ............................................................................................................. 6
2
Test Board with Sensor ..................................................................................................... 7
3
Schematic with EMI Capacitors Added ................................................................................... 8
4
Test Setup In Chamber ..................................................................................................... 9
5
Test Results of 1 V/m With EMI Capacitors and Sensor Mounted .................................................. 10
6
Test Results of 3 V/m With EMI Capacitors and Sensor Mounted .................................................. 10
7
Test Results of 10 V/m With EMI Capacitors and Sensor Mounted ................................................. 11
8
Test Results of 30 V/m With EMI Capacitors and Sensor Mounted ................................................. 11
9
Test Results of 3 V/m With EMI Capacitors and No Sensor Mounted .............................................. 12
10
Test Results of 10 V/m With EMI Capacitors and No Sensor Mounted
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22
............................................
Test Results of 30 V/m With EMI Capacitors and No Sensor Mounted ............................................
Test Results of 1 V/m With No EMI Capacitors and Sensor Mounted ..............................................
Test Results of 3 V/m With No EMI Capacitors and Sensor Mounted ..............................................
Test Results of 10 V/m With No EMI Capacitors and Sensor Mounted ............................................
Test Results of 30 V/m With No EMI Capacitors and Sensor Mounted ............................................
Test Results of 10 V/m With No EMI Capacitors and No Sensor Mounted ........................................
Test Results of 30 V/m With No EMI Capacitors and No Sensor Mounted ........................................
Full Schematic ..............................................................................................................
Top 3D Board View (Flipped) .............................................................................................
Bottom 3D Board View ....................................................................................................
Top Component View ......................................................................................................
Bottom Component View (Flipped) ......................................................................................
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1
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23
24
...................................................................................................................
Bottom Layer ................................................................................................................
Top Layer
19
19
List of Tables
2
1
Defined Power Levels ....................................................................................................... 4
2
Selected "Spot" Frequencies ............................................................................................... 5
Comparing EMI Performance of LPV802 with Other Devices in a Gas Sensor
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Introduction
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1
Introduction
All monolithic operational amplifiers will react, to some degree, to the application of high intensity, high
frequency radio energy, also known as Electromagnetic Interference (EMI). Operational amplifiers that
have released in the last few years have EMI filtering added as part of the design to help minimize the
effects of EMI.
LPV80x devices do not however include the full input EMI filter as seen on many recently released
operational amplifiers. This was done intentionally as adding this EMI filter greatly increases the input
capacitance, which can cause peaking in sub microampere circuitry with large feedback resistor values
and source impedances. Instead, internal (proprietary) precautions were employed in the layout and
internal design of the LPV8xx to make it as EMI hardened as possible.
To verify the effectiveness of this approach, the LPV802 was tested and compared against two popular
and comparable competitor devices that do not have EMI protection. Under all the conditions, the circuit
that used LPV802 demonstrated better EMI immunity than circuits that used competitive devices.
The most common standard EMI test is the IEC61000-4-3 EMC Radiated Test, which defines the test
conditions and test setup.
The Equipment Under Test (EUT) is subjected to a calibrated RF field over a 80MHz to 6GHz frequency
range as defined by IEC61000-4-3 EMC standard.
To compare the EMI tolerance of the three devices, all three devices were exposed at the same time in
identical circuits with their output monitored for deviations.
Additionally, to test the effectiveness of a commonly used EMI filtering technique, two sets of boards were
tested. One set of boards had added external input EMI capacitors added, and one set did not have the
EMI capacitors present.
This application note describes the test conditions and compares the results of the three devices under
test. It is not a full IEC61000-4-3 certification.
2
IEC61000-4-3 Test Conditions
The IEC61000-4-3 standard defines a "radiated" RF electromagnetic field test. In short, a directional
broadband antenna is placed at a distance of 3 meters away and focused on the EUT. RF power is
applied at the specified power and frequency levels.
The IEC test conditions describe the physical setup, conditions, calibration, frequency range and required
power levels.
The next section briefly summarizes the IEC61000-4-3 standard conditions and required setup.
2.1
Physical Setup
The test is generally performed in a screened RF anechoic chamber to absorb reflections and prevent RF
interference to the surrounding outside environment (applied powers can be up to 1 kW).
The EUT is placed on a non-conductive 0.8m high platform (table) within a 1.5m x 1.5m square uniform
field area (UFA).
A wideband directional antenna (log-periodic or similar) is placed 3 meters from the platform and focused
on the uniform field area.
The EUT is set-up to operate normally within the uniform field area while being monitored for failures. Any
required external peripherals or accessories should also be included. If there are any peripheral,
monitoring or power cables, 3 meters of the cable(s) must exposed to the field (1 meter minimum if the
cables are <3m). The cables are usually passed through a RF filter before passing into the control point.
Field monitors are placed near the EUT to monitor the actual field levels.
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IEC61000-4-3 Test Conditions
2.2
2.2.1
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Test Conditions
Frequency
The standard specifies a frequency sweep of 80MHz to 6GHz, with frequency steps no larger than 1% of
the previous step. The range between 1.4 GHz and 6 GHz may be reduced to cover particular band
segments where interference is expected (DCS, DECT, RFID, WiFi, etc).
Time between steps should be no less than 0.5 second, but not longer than the time to verify proper EUT
operation.
2.2.2
Power Levels
The standard specifies four power levels, measured in Volts per Meter (V/m):
Table 1. Defined Power Levels
Theoretical Transmit Power Required
(0 dBi radiator at 3 meters)
Power Level
Field Strength
1
1 V/m
300 mW
2
3 V/m
2.7 W
3
10 V/m
30 W
4
30 V/m
270 W
x
100 V/m *
3000 W
* 100V/m is not part of the IEC standard, but some safety critical designs may be tested at this level.
As a point of reference, assuming 3m distance and no transmitting antenna gain (0dBi) and no path or
cable losses, Table 1 shows the theoretical transmitter power required. Of course, these are theoretical
calculations and will vary depending on actual antenna forward gain, cable losses, path loss and
reflections. Actual field values should be verified with a isotropic field monitor.
During the tests, a 1KHz, 80% sinusoidal AM modulation is applied to the transmitted signal. The IEC
specification gives limits on amplifier distortion and harmonic content allowances.
2.2.3
Polarity
Tests must be done with both vertical and horizontal polarity (but not at the same time!). This means the
antenna plane must be switched and the test sweep sequence repeated.
2.2.4
Position
Tests must be done on all four sides of the EUT. This requires the EUT to be physically turned 90° and
the entire test sequence repeated again for all remaining sides.
2.2.5
Calibration
The setup must be calibrated at 16 points within the uniform field area with a CW signal at 1.8X the test
power levels (to make up for the lack of AM modulation). A "calibration run" is usually done before the
acceptance tests and the actual required amplifier power level recorded for each point and recalled to
speed up the test measurement time.
2.2.6
Conditions Summary
As can be seen, with the numerous frequency points at four power levels, at two polarities and four sides,
the length of the test can be substantial, lasting several hours to days.
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Test Conditions Selected to Test EMI Performance of EUT
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Test Conditions Selected to Test EMI Performance of EUT
Because the intention was not trying to fully certify this design to IEC standards, but only to compare the
behavior of unique devices on identical boards under the IEC conditions, it was decided to use a subset of
frequencies using the IEC test setup conditions.
To reduce the large number of 1% frequency steps, 14 individual "spot" frequencies were chosen so that
they were located in strategic parts of the RF spectrum (where troublesome interference would be
expected).
Table 2. Selected "Spot" Frequencies
Step
Frequency
Services Affected
1
80 MHz
FM Radio, Mobile Radio, TV
2
100 MHz
FM Radio, Aeronautical
3
140 MHz
Mobile Radio
4
200 MHz
Mobile Radio, TV, DAB
5
300 MHz
TV, Aeronautical
6
450 MHz
Mobile Radio, TV, GSM
7
600 MHz
TV
8
700 MHz
TV, Mobile Radio, GSM, 4G
9
750 MHz
4G, Mobile Radio
10
800 MHz
Mobile Radio, GSM
11
850 MHz
1G/2G/3G GSM, Mobile Radio
12
900 MHz
Mobile Radio, ISM, GSM
13
950 MHz
GSM, Mobile Radio, Radar
14
1 GHz
GSM, Radar
The EMI hardening of the LPV8xx is designed to be effective at frequencies above 300MHz. This makes
all the devices susceptible to frequencies below 300MHz and rely on external filtering for this range. For
this reason, frequencies below 300MHz tend to be the most influenced by external layout and are of the
main interest of this investigation. Frequencies above 1GHz were not measured as the responses are
characterized in the EMIRR performance tests and preliminary tests showed little influence (and also
required a second expensive test setup).
Since this is a small board, and are comparing like devices, TI chose to test only one side and one polarity
(horizontal).
4
Test Setup
4.1
Test Circuit
The test circuit is a conventional transimpedance amplifier with reference voltage buffer, commonly used
with gas sensors. The buffer is required to supply the varying currents required by the sensor that would
otherwise load down a simple resistor divider network and cause errors.
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Test Setup
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GND
GAS_SENSOR1
City 2CF+
52nA/ppm
J1
1
2
3
4
CF1
Vs
Vs
0.1 µF
VTIA
R3
Power
CE
1
CE
WE
10k
RL1
RE
Actual RL value
recommended by
sensor manufacturer
Vs
Actual RF
value
determined
by sensor
8
3
Vs
143k
C6
10µF
C5
0.1 µF
RF1
TIA_IN
10.0
GND
Vs
2 WE
2
6
Vref
5
B
V+
V-
R4
7
3
A
V+
V-
10.0k
C1
10µF
R2
806k
4
4
100mV @ 3V Supply @ 131nA
N/C
8
R1
22M
TIA_OUT
1
RO1
VTIA
49.9k
U1A
LPV802DGKR
C12
1µF
U1B
LPV802DGKR
GND
GND
GND
GND
GND
Figure 1. Basic Schematic
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Test Setup
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The circuit was built on a standard 62mil, 2-layer FR4 board with ground planes on both sides. A 4-pin
connector was used to allow quick board changes. The sensor pins are socketed to allow easy removal of
the sensor (and soldering of the sensor pins is not recommended by the manufacturer).
Figure 2. Test Board with Sensor
The sensor used is a City Technology ECO-Sure 4 Series two terminal Carbon Monoxide Sensor (City
PN# 2112B3000A).
The zero current reference level was approximately 120mV while running off two AA batteries. This allows
the measurement of bipolar (±) currents from the sensor and allows observation of sensor health which
may not be possible with a ground referenced design. The feedback time constant was kept small to allow
observation of small transients and fast settling time to minimize time between measurement steps.
The board layout was purposely not "optimized' for RF rejection (traces have long surface traces and no
ferrite beads), to allow some introduction of RF signals to ensure some interaction.
4.2
EMI Filter Capacitors
A common EMI-hardening technique is to add small (15pf to 33pF) capacitors across the inputs in a
"delta" formation (from each input to RF GND, and one between the inputs). Provisions were made for
these capacitors to test the effectiveness of this technique. Figure 3 shows these 15pF capacitors added
to the schematic as C2-C4, C9-C11, C7, C8 and C13.
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Test Setup
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GND
GND
GND
GAS_SENSOR1
City 2CF+
52nA/ppm
J1
CF1
C9
15pF
C2
15pF
Vs
VTIA
R3
Power
CE
1
CE
WE
10k
RL1
RE
Actual RL value
recommended by
sensor manufacturer
Vs
3
Vs
143k
B
V+
V-
R4
7
3
A
V+
V-
10.0k
4
5
4
C1
10µF
R2
806k
C4
15pF
U1B
LPV802DGKR
C7
15pF
C8
15pF
RO1
VTIA
49.9k
U1A
LPV802DGKR
C12
1µF
C13
15pF
GND
GND
GND
TIA_OUT
1
C11
15pF
GND
GND
Actual RF
value
determined
by sensor
2
6
Vref
100mV @ 3V Supply @ 131nA
C10
15pF
N/C
8
C3
15pF
R1
22M
C6
10µF
C5
0.1 µF
RF1
TIA_IN
10.0
GND
Vs
2 WE
Vs
0.1 µF
8
1
2
3
4
GND
GND
GND
Figure 3. Schematic with EMI Capacitors Added
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Test Setup
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Eight boards were produced to make two sets of four boards. Four boards would be tested at the same
time. One entire board was a "spare" LPV802 in a grounded reference configuration and is not used in
this report.
One set of boards (#1 to #4) contained the external 15pF EMI capacitors, and one set (#5 to #8) did not
have the EMI capacitors populated.
The same sensors were used for each set of boards and were swapped-out between runs.
4.3
Chamber Setup
The boards were distributed evenly within the calibrated test area, as shown in Figure 4.
Figure 4. Test Setup In Chamber
Each of the boards was connected to a central battery box (2 x AA cells) through one meter of four
conductor shielded cable with EMI chokes on both ends. The battery box was connected to the control
room via 15 meters of UTP CAT-5 cable, with appropriate EMI chokes, to deliver the output voltages to
the logging system.
The two white boxes with the cones are the field sensors for monitoring the field during the test.
4.4
Test Sequence
The tests performed in the following sequence:
1. Boards 1-4 with sensors installed
2. Boards 1-4 with sensors removed
3. Boards 5-8 with sensors installed
4. Boards 5-8, with sensors removed
The sensors and/or the capacitors were removed to provide a data point on how much the they
contributed to the overall performance.
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Test Results
5
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Test Results
The following section show the results of the tests that were conducted.
It should be noted that the noise of the sensor causes the output voltage to change randomly up to
±10mV. So some random noise in the traces is expected from the sensor.
5.1
Test Results With EMI Capacitors and Sensor Mounted
The following tests are with the 15pF EMI capacitors fitted and gas sensor installed. This represents what
is considered good engineering practice.
1
LPV802
Comped B
Output Voltage (V)
Comped M
0.1
0.01
0.001
10
100
1000
Frequency (MHz)
C001
Figure 5. Test Results of 1 V/m With EMI Capacitors and Sensor Mounted
Figure 5 shows very little disturbance across the field. All devices are undisturbed.
1
LPV802
Comped B
Output Voltage (V)
Comped M
0.1
0.01
0.001
10
100
1000
Frequency (MHz)
C002
Figure 6. Test Results of 3 V/m With EMI Capacitors and Sensor Mounted
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Figure 6 starts to show some variations, with competitor "M" starting out properly at 80MHz, but failing at
100-200MHz. Competitor "B" and the LPV802 are fairly undisturbed.
1
LPV802
Comped B
Output Voltage (V)
Comped M
0.1
0.01
0.001
10
100
1000
Frequency (MHz)
C003
Figure 7. Test Results of 10 V/m With EMI Capacitors and Sensor Mounted
Figure 7 shows very little disturbance across the field. All devices are undisturbed. It is unknown why
Competitor "M" failed the 3V/m but passed the 10V/m run.
1
LPV802
Comped B
Output Voltage (V)
Comped M
0.1
0.01
0.001
10
100
1000
Frequency (MHz)
C004
Figure 8. Test Results of 30 V/m With EMI Capacitors and Sensor Mounted
Now some major changes can be observed. Figure 8 shows both competitors started to fail at 140MHz,
while the LPV802 held-on down to 100MHz. Remember that the lower frequencies are more troublesome.
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Test Results
5.2
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Test Results With EMI Capacitors and No Sensor Mounted
The following tests have the EMI capacitors installed, but the sensor was removed. This test is to show
how much the sensor contributes to the overall performance - either as an "antenna", or the sensor itself
being affected. It also shows the low "noise floor" of the test circuit.
1 V/m test was skipped since it was known the effects would be minimal.
1
LPV802
Comped B
Output Voltage (V)
Comped M
0.1
0.01
0.001
10
100
1000
Frequency (MHz)
C005
Figure 9. Test Results of 3 V/m With EMI Capacitors and No Sensor Mounted
As was expected, no change detected. Note how "flat' the traces are, this shows how much of the random
variation is due to the sensor noise.
1
LPV802
Comped B
Output Voltage (V)
Comped M
0.1
0.01
0.001
10
100
1000
Frequency (MHz)
C006
Figure 10. Test Results of 10 V/m With EMI Capacitors and No Sensor Mounted
All the competitive devices showed very slight variation at 80MHz at 10V/m, as shown in Figure 10. The
LPV802 was undisturbed.
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1
LPV802
Comped B
Output Voltage (V)
Comped M
0.1
0.01
0.001
10
100
1000
Frequency (MHz)
C007
Figure 11. Test Results of 30 V/m With EMI Capacitors and No Sensor Mounted
Figure 11 shows the competitor devices with variation at 80MHz at 30V/m, while the LPV802 still
remained undisturbed.
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Test Results
5.3
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Test Results With No EMI Capacitors and Sensor Mounted
The following test have the 15pF EMI capacitors removed and sensor installed. This test represents a
design where EMI has not been taken into consideration, and shows the effectiveness of the EMI
capacitors.
1
LPV802
Comped B
Output Voltage (V)
Comped M
0.1
0.01
0.001
10
100
1000
Frequency (MHz)
C008
Figure 12. Test Results of 1 V/m With No EMI Capacitors and Sensor Mounted
Some variation is seen at 80MHz for all devices.
1
LPV802
Comped B
Output Voltage (V)
Comped M
0.1
0.01
0.001
10
100
1000
Frequency (MHz)
C009
Figure 13. Test Results of 3 V/m With No EMI Capacitors and Sensor Mounted
All devices failed at 80MHz, though the LPV802 recovers at 100MHz. All have recovered by 200MHz.
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1
LPV802
Comped B
Output Voltage (V)
Comped M
0.1
0.01
0.001
10
100
1000
Frequency (MHz)
C010
Figure 14. Test Results of 10 V/m With No EMI Capacitors and Sensor Mounted
All devices failed at 80MHz and 100MHz. All devices also show a error that decreases as the frequency
increases up to 700MHz.
1
LPV802
Comped B
Output Voltage (V)
Comped M
0.1
0.01
0.001
10
100
1000
Frequency (MHz)
C011
Figure 15. Test Results of 30 V/m With No EMI Capacitors and Sensor Mounted
All the devices fail at 80-100MHz. The LPV802 recovers at 140MHz and remains fairly flat to 1GHz.
Competitor devices recover by 200MHz.
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Test Results
5.4
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Test Results With No EMI Capacitors and No Sensor Mounted
This test is without the EMI capacitors and without the sensor. This test is meant to show the susceptibility
of the amplifier and the board itself (minus the antenna effect of the sensor).
Only the 10V/m and 30V/m tests were run to save time.
1
LPV802
Comped B
Output Voltage (V)
Comped M
0.1
0.01
0.001
10
100
1000
Frequency (MHz)
C012
Figure 16. Test Results of 10 V/m With No EMI Capacitors and No Sensor Mounted
1
LPV802
Comped B
Output Voltage (V)
Comped M
0.1
0.01
0.001
10
100
1000
Frequency (MHz)
C013
Figure 17. Test Results of 30 V/m With No EMI Capacitors and No Sensor Mounted
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5.5
Observations
During the tests, it was found that it was best to sweep the frequency from high to low until the devices
failed. When the devices failed, the sensors could take anywhere from seconds to tens of minutes to
recover from the overload. This delay is caused by the malfunctioning amplifier placing voltage across the
sensor, which causes a long delay as the sensor "recovers" back to zero current.
If it is known that the circuit will fail at the low frequencies, starting at the high frequencies will maximize
the amount of data points collected. Maximizing test time is important when paying for chamber time by
the hour, and waiting several minutes for a sensor to recover can be expensive!
5.6
Summary
The LPV8xx showed a definite advantage over the non-EMI hardened devices, particularly in the 100200MHz range. All of the devices were mostly unaffected by the upper (>400 MHz) frequencies.
Frequencies below 200MHz are mostly reliant on the external filtering.
Adding the external EMI input capacitors also helped overall performance, and should be added as part of
normal design process.
EMI protection does not completely eliminate the effects of EMI, but it does help to reduce the effects.
Adding the external filtering further reduces the effects, and external filtering is recommended even with
the use of EMI protected devices.
The LPV8xx devices help the System Designer to reliably pass EMC tests helps eliminate the need for
expensive redesigns to comply with EMI standards. This in turn ensures the successful to market launch
of products.
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100mV @ 3V Supply @ 131nA
Power
J1
GND
GND
R2
806k
Vref
R1
22M
Vs
1
2
3
4
GND
VTIA
C1
10µF
Vs
GND
GND
C4
15pF
C3
15pF
C2
15pF
5
6
Vs
V+
V-
GND
B
R3
10k
8
18
4
7
U1B
LPV802DGKR
CE
1
CE
GAS_SENSOR1
City 2CF+
10.0k
R4
N/C
WE
RL1
10.0
TIA_IN
Actual RL value
recommended by
sensor manufacturer
2 WE
GND
GND
C11
15pF
C10
15pF
C9
15pF
2
3
V+
V-
GND
A
Vs
143k
RF1
1
Vs
TIA_OUT
C5
0.1 µF
U1A
LPV802DGKR
Actual RF
value
determined
by sensor
0.1 µF
CF1
RO1
49.9k
GND
C12
1µF
C6
10µF
GND
C7
15pF
C13
15pF
VTIA
C8
15pF
Full Schematic
RE
6.1
3
Appendix
8
6
4
GND
Appendix
www.ti.com
Figure 18. Full Schematic
Comparing EMI Performance of LPV802 with Other Devices in a Gas Sensor
Application
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Appendix
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6.2
Board Layout
Figure 19. Top 3D Board View (Flipped)
Figure 20. Bottom 3D Board View
Figure 21. Top Component View
Figure 22. Bottom Component View (Flipped)
Figure 23. Top Layer
Figure 24. Bottom Layer
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Comparing EMI Performance of LPV802 with Other Devices in a Gas Sensor
Application
Copyright © 2016, Texas Instruments Incorporated
19
References
7
References
•
•
•
•
8
www.ti.com
International Electrotechnical Commission, "Electromagnetic compatibility (EMC) - Part 4-3: Testing
and measurement techniques - Radiated, radio-frequency, electromagnetic field immunity test", Third
Edition, February 2006, http://www.iec.ch
City Technology, Ltd., "Ecosure Datasheet, AO4206 Issue 2 ECN I 3183 Issue 5", November 2013
Datasheet, Retrieved from http://www.citytech.com
Giangrandi, Iacopo. (n.d.). "Field generated by a transmitter at a given distance", Retrieved from
http://www.giangrandi.ch/electronics/anttool/tx-field.shtml
Advanced Test Equipment Rentals. (n.d.). "IEC 61000-4-3: Radiated, radio-frequency, electromagnetic
field immunity test", Retrieved from http://www.atecorp.com/compliance-standards/iec-standards/iec61000-4-3-electromagnetic-compatibility-emc.aspx
Acknowledgments
TI would like to thank Jay Gandhi and Kevin Bothmann of Electro Magnetic Test in Mountain View, CA for
their invaluable help and making special accommodations for our "unique" test requirements.
20
Comparing EMI Performance of LPV802 with Other Devices in a Gas Sensor
Application
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