Datasheet

ADuM1200/ADuM1201 Data Sheet
Rev. K | Page 24 of 28
APPLICATIONS INFORMATION
PCB LAYOUT
The ADuM1200/ADuM1201 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 must 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 must not exceed 20 mm.
See the AN-1109 Application Note for board layout guidelines.
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.
INPUT (
V
Ix
)
OUTPUT (V
Ox
)
t
PLH
t
PHL
50%
50%
04642-012
Figure 12. Propagation Delay Parameters
Pulse width distortion is the maximum difference between
these two propagation delay values and is an indication of
how accurately the timing of the input signal is preserved.
Channel-to-channel matching refers to the maximum amount
that the propagation delay differs between channels within a
single ADuM1200/ADuM1201 component.
Propagation delay skew refers to the maximum amount that
the propagation delay differs between multiple ADuM1200/
ADuM1201 components operating under the same conditions.
DC CORRECTNESS AND MAGNETIC FIELD IMMUNITY
Positive and negative logic transitions at the isolator input send
narrow (~1 ns) pulses 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 17 and Table 18) by the watchdog timer
circuit.
The ADuM1200/ADuM1201 are extremely immune to external
magnetic fields. The limitation on the magnetic field immunity
of the ADuM1200/ADuM1201 is set by the condition in which
induced voltage in the receiving coil of the transformer is suf-
ficiently 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 ADuM1200/ADuM1201 is
examined because it represents the most susceptible mode of
operation.
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
V = (−dβ/dt)Σ∏r
n
2
; n = 1, 2, … , N
where:
β is the magnetic flux density (gauss).
N is the number of turns in the receiving coil.
r
n
is the radius of the nth turn in the receiving coil (cm).
Given the geometry of the receiving coil in the ADuM1200/
ADuM1201 and an imposed requirement that the induced
voltage be 50% at most of the 0.5 V margin at the decoder, a
maximum allowable magnetic field is calculated, as shown in
Figure 13.
MAGNETIC FIELD FREQUENCY (Hz)
100
MAXIMUM ALLOWABLE MAGNETIC FLUX
DENSITY (kgauss)
0.001
1M
10
0.01
1k 10k 10M
0.1
1
100M100k
04642-013
Figure 13. Maximum Allowable External Magnetic Flux Density
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 occurs during a transmitted pulse (and
has the worst-case polarity), it reduces 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
ADuM1200/ADuM1201 transformers. Figure 14 expresses these
allowable current magnitudes as a function of frequency for
selected distances. As seen, the ADuM1200/ADuM1201 are
extremely immune and can be affected only by extremely large
currents operating very close to the component at a high frequency.
For the 1 MHz example, place a 0.5 kA current 5 mm away from
the ADuM1200/ADuM1201 to affect the operation of the
component.