ADuM2210/ADuM2211
Data Sheet
APPLICATIONS INFORMATION
PCB LAYOUT
DC CORRECTNESS AND MAGNETIC FIELD
IMMUNITY
The ADuM221x digital isolator requires no external interface
circuitry for the logic interfaces. Power supply bypassing is
strongly recommended at the input and output supply pins (see
Figure 12). Bypass capacitors are most conveniently connected
between Pin 1 and Pin 3 for VDD1 and between Pin 14 and
Pin 16 for VDD2. The capacitor value should be between 0.01 μF
and 0.1 μF. The total lead length between both ends of the
capacitor and the input power supply pin should not exceed
20 mm. Bypassing between Pin 3 and Pin 7 and between Pin 9
and Pin 14 should be considered unless the ground pair on each
package side is connected close to the package.
Positive and negative logic transitions at the isolator input cause
narrow (~1 ns) pulses to be sent via the transformer to the
decoder. The decoder is bistable and is, therefore, either set or
reset by the pulses, indicating input logic transitions. In the
absence of logic transitions at the input for more than ~1 μs, a
periodic set of refresh pulses indicative of the correct input state
is sent to ensure dc correctness at the output. If the decoder
receives no internal pulses for more than approximately 5 μs,
the input side is assumed to be without power or nonfunctional;
in which case, the isolator output is forced to a default state (see
Table 11 and Table 12) by the watchdog timer circuit.
GND
GND
1
2
NC
NC
V
The limitation on the ADuM221x magnetic field immunity is
set by the condition in which induced voltage in the transformer
receiving coil is large enough to either falsely set or reset the
decoder. The following analysis defines the conditions under
which this can occur. The 3 V operating condition of the
ADuM221x is examined because it represents the most suscept-
ible mode of operation.
V
DD2
DD1
V
V
/V
V
/V
OA IA
OB
IA OA
V
IB
NC
NC
GND
1
NC
NC
GND
2
Figure 12. Recommended Printed Circuit Board Layout
The pulses at the transformer output have an amplitude greater
than 1.0 V. The decoder has a sensing threshold at about 0.5 V,
therefore establishing a 0.5 V margin in which induced voltages
can be tolerated. The voltage induced across the receiving coil is
given by
In applications involving high common-mode transients, care
should be taken to ensure that board coupling across the
isolation barrier is minimized. Furthermore, the board layout
should be designed such that any coupling that does occur
equally affects all pins on a given component side. Failure to
ensure this could cause voltage differentials between pins
exceeding the device’s Absolute Maximum Ratings, thereby
leading to latch-up or permanent damage.
2
V = (−dβ/dt)Σπrn ; n = 1, 2,…, N
where:
β is the magnetic flux density (gauss).
N is the number of turns in the receiving coil.
rn is the radius of the nth turn in the receiving coil (cm).
PROPAGATION DELAY-RELATED PARAMETERS
Propagation delay is a parameter that describes the length of
time it takes for a logic signal to propagate through a compo-
nent. The propagation delay to a logic low output can differ
from the propagation delay to logic high.
Given the geometry of the receiving coil in the ADuM221x and
an imposed requirement that the induced voltage be at most
50% of the 0.5 V margin at the decoder, a maximum allowable
magnetic field is calculated as shown in Figure 14.
100
INPUT (V
)
50%
Ix
tPLH
tPHL
OUTPUT (V
)
50%
Ox
10
1
Figure 13. Propagation Delay Parameters
Pulse width distortion is the maximum difference between
these two propagation delay values and is an indication of how
accurately the input signal’s timing is preserved.
0.1
Channel-to-channel matching refers to the maximum amount
the propagation delay differs among channels within a single
ADuM221x component.
0.01
0.001
Propagation delay skew refers to the maximum amount the
propagation delay differs among multiple ADuM221x
components operated under the same conditions.
1k
10k
100k
1M
10M
100M
MAGNETIC FIELD FREQUENCY (Hz)
Figure 14. Maximum Allowable External Magnetic Flux Density
Rev. A | Page 16 of 2
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