adum1210 Analog Devices, Inc., adum1210 Datasheet - Page 14

• Manufacturer Part Number: adum1210
• Description: Dual-channel Digital Isolator
• Manufacturer: Analog Devices, Inc.

ADuM1210
APPLICATIONS INFORMATION
PC BOARD LAYOUT
The ADuM1210 digital isolator requires 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 can differ from the propagation
delay to a logic high output.
Pulse width distortion is the maximum difference between the
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 ADuM1210 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 ~1 μs at the input, 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 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 12) by the
watchdog timer circuit.
The ADuM1210 is extremely immune to external magnetic
fields. The limitation on the ADuM1210 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 can occur. The 3 V operating
condition of the ADuM1210 is examined because it represents
the most susceptible mode of operation.
INPUT (V
OUTPUT (V
Ix
)
Ox
)
Figure 9. Propagation Delay Parameters
t
PLH
t
PHL
50%
50%
Rev. C | Page 14 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).
r
N is the number of turns in the receiving coil.
Given the geometry of the receiving coil in the ADuM1210 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 10.
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 occurred 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
ADuM1210 transformers. Figure 11 expresses these allowable
current magnitudes as a function of frequency for selected
distances. As seen in Figure 11, the ADuM1210 is 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, a 0.5 kA current would have to be
placed 5 mm away from the ADuM1210 to affect the
component’s operation.
n
is the radius of the nth turn in the receiving coil (cm).
V = (−dβ / dt) ∑ π r
0.001
0.01
Figure 10. Maximum Allowable External Magnetic Flux Density
100
0.1
10
1
1k
10k
MAGNETIC FIELD FREQUENCY (Hz)
n
2
; n = 1, 2, … , N
100k
1M
10M
100M