adum1200 Analog Devices, Inc., adum1200 Datasheet - Page 15

• Manufacturer Part Number: adum1200
• Description: Dual-channel Digital Isolators
• Manufacturer: Analog Devices, Inc.

APPLICATION INFORMATION
PC BOARD LAYOUT
The ADuM120x digital isolators require no external interface
circuitry for the logic interfaces. Power supply bypassing is
strongly recommended at the input and output supply pins. 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.
PROPAGATION DELAY-RELATED PARAMETERS
Propagation delay is a parameter that describes the time it takes
a logic signal to propagate through a component. The
propagation delay to a logic low output may differ from the
propagation delay to a logic high.
INPUT (V
OUTPUT (V
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.
Channel-to-channel matching refers to the maximum amount
that the propagation delay differs between channels within a
single ADuM120x component.
Propagation delay skew refers to the maximum amount that the
propagation delay differs between multiple ADuM120x
components operating under the same conditions.
DC CORRECTNESS AND MAGNETIC FIELD IMMUNITY
Positive and negative logic transitions at the isolator input cause
narrow (~1 ns) pulses to be sent to the decoder via the
transformer. The decoder is bistable and is therefore either set
or reset by the pulses, indicating input logic transitions. In the
absence of logic transitions of more than 2 µs at the input, a
periodic set of refresh pulses indicative of the correct input state
are sent to ensure dc correctness at the output. If the decoder
receives no internal pulses for more than about 5 µs, the input
side is assumed to be unpowered or nonfunctional, in which
case the isolator output is forced to a default state (see Table 8)
by the watchdog timer circuit.
The ADuM120x are extremely immune to external magnetic
fields. The limitation on the ADuM120x’s magnetic field
immunity is set by the condition in which induced voltage in
the transformer’s receiving coil is sufficiently large to either
falsely set or reset the decoder. The following analysis defines
the conditions under which this may occur. The 3 V operating
condition of the ADuM120x is examined because it represents
the most susceptible mode of operation.
IX
)
OX
)
Figure 12. Propagation Delay Parameters
t
PLH
t
PHL
50%
50%
Rev. B | Page 15 of 20
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
where:
β is the magnetic flux density (gauss).
N is the number of turns in the receiving coil.
r
Given the geometry of the receiving coil in the ADuM120x 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 13.
For example, at a magnetic field frequency of 1 MHz, the
maximum allowable magnetic field of 0.2 kgauss induces a
voltage of 0.25 V at the receiving coil. This is about 50% of the
sensing threshold and does not cause a faulty output transition.
Similarly, if such an event were to occur during a transmitted
pulse (and had the worst-case polarity), it would reduce the
received pulse from > 1.0 V to 0.75 V—still well above the 0.5 V
sensing threshold of the decoder.
The preceding magnetic flux density values correspond to
specific current magnitudes at given distances away from the
ADuM120x transformers. Figure 14 expresses these allowable
current magnitudes as a function of frequency for selected
distances. As seen, the ADuM120x are extremely immune and
can be affected only by extremely large currents operated at
high frequency and very close to the component. For the 1 MHz
example, one would have to place a 0.5 kA current 5 mm away
from the ADuM120x to affect the component’s operation.
n
is the radius of the nth turn in the receiving coil (cm).
0.001
V
0.01
100
Figure 13. Maximum Allowable External Magnetic Flux Density
0.1
10
1
1k
(

β d
/
dt
10k
)
¦
MAGNETIC FIELD FREQUENCY (Hz)
3
r
n
2
;
100k
n
ADuM1200/ADuM1201
1
2 ,
,...
N
1M
10M
100M