Application Note
AN-1109
60
about 0.17 V p-p noise on the VDD1 pin generated on a 2-layer
board. The bottom section shows a PCB with ground and power
planes separated by a 0.1 mm core spacing with a substantial
improvement in noise to only 0.03 V p-p. This illustrates that
if tightly spaced ground and power planes are used, the power
supply noise can be dramatically reduced.
STANDARD BOARD 5V V
DD
GUARD BOARD 5V V
DD
STANDARD BOARD 3.3V V
DD
50
40
30
20
GUARD BOARD 3.3V V
DD
X
5.10
2 LAYER: NO GND AND PWR PLANES
5.08
5.06
5.04
5.02
X
5.00
4.98
4.96
4.94
4.92
4.90
10
0
2
4
6
8
10
12
14
16
18
20
0
50
100
150
200
250
300
350
400
5.10
5.08
5.06
5.04
5.02
STITCHING CAPACITANCE (pF)
4 LAYER: 4 MIL SPACING GND
TO PWR PLANE
Figure 15. Peak Emissions at Frequencies of 230 MHz to 1000 MHz at 1 Mbps
Rate for Stitching Capacitance and Guard Options
5.00
4.98
4.96
60
STANDARD BOARD 5V V
DD
4.94
4.92
4.90
GUARD BOARD 5V V
DD
STANDARD BOARD 3.3V V
DD
50
40
30
20
0
2
4
6
8
10
12
14
16
18
20
GUARD BOARD 3.3V V
DD
TIME (µs)
Figure 13. VDD Voltage Noise for Different PCB Layouts
3.3 V OPERATION
Many iCoupler products can operate with 3.3 V input and
output supplies. Operation at lower voltages reduces generated
noise as well as production of radiated emissions. Figure 14 to
Figure 17 show how emissions are reduced using a standard
4-layer evaluation board with the 4-channel ADuM1402 when
3.3 V supplies are used instead of 5 V supplies.
60
10
0
50
100
150
200
250
300
350
400
STITCHING CAPACITANCE (pF)
STANDARD BOARD 5V V
DD
Figure 16. Peak Emissions at Frequencies of 30 MHz to 230 MHz at 10 Mbps
Rate for Stitching Capacitance and Guard Options
GUARD BOARD 5V V
DD
STANDARD BOARD 3.3V V
50
40
30
20
DD
60
GUARD BOARD 3.3V V
DD
STANDARD BOARD 5V V
DD
GUARD BOARD 5V V
DD
STANDARD BOARD 3.3V V
DD
50
40
30
20
GUARD BOARD 3.3V V
DD
X
X
10
0
50
100
150
200
250
300
350
400
STITCHING CAPACITANCE (pF)
Figure 14. Peak Emissions at Frequencies of 30 MHz to 230 MHz at 1 Mbps
Rate for Stitching Capacitance and Guard Options
10
0
50
100
150
200
250
300
350
400
STITCHING CAPACITANCE (pF)
Figure 14 to Figure 17 also show the emissions for a variety of
4-layer evaluation boards that vary in amount of primary side
to secondary side stitching capacitance and guard options.
The data in these figures is used for Table 4 to Table 7 in the
Example Board section to show how to apply layout techniques
to reduce emissions to meet CISPR 22 Class B emissions
standards.
Figure 17. Peak Emissions at Frequencies of 230 MHz to 1000 MHz at
10 Mbps Rate for Stitching Capacitance and Guard Options
Rev. 0 | Page 9 of 20
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