REV. E

ADuM1100

–11–

INPUT RISE TIME (10%–90%, ns)

4

1

2

0

36

8

10

3

1

5V INPUT SIGNAL

24

579

3.3V INPUT SIGNAL

Figure 5. Typical Propagation Delay Change due to

INPUT RISE TIME (10%–90%, ns)

0

1

–2

–4

36

8

10

–1

–3

24

579

3.3V INPUT SIGNAL

5V INPUT SIGNAL

Figure 6. Typical Propagation Delay Change due to

The impact of the slower input edge rates can also affect the

measured pulse width distortion as based on the input 50% level.

This impact may either increase or decrease the apparent pulse

that leads to the largest increase in pulse width distortion. The

change in this case is given by

∆∆

∆

PWD

LH

HL

ITH L H

ITH H L

rf

PWD

PWD

tV V

V

V

for t

t

t

=′

=

=

()

()

==

()

()

()

––

/.

–

–

,

––

08

1

where:

PWD

t

t

PWD

t

t

PLH

PHL

PLH

PHL

=

′= ′

′

–

–

This adjustment in pulse width distortion is plotted as a func-

tion of input rise/fall time in Figure 7.

INPUT RISE/FALL TIME (10%–90%, ns)

6

1

3

0

36

8

10

5

1

5V INPUT SIGNAL

24

579

3.3V INPUT SIGNAL

2

4

Figure 7. Typical Pulse Width Distortion Adjustment due

Method of Operation, DC Correctness, and

Magnetic Field Immunity

Referring to the functional block diagram, the two coils act as a

pulse transformer. Positive and negative logic transitions at the

isolator input cause narrow (2 ns) pulses to be sent via the trans-

former to the decoder. The decoder is bistable and therefore

either set or reset by the pulses indicating input logic transitions.

In the absence of logic transitions at the input for more than 2

µs,

a periodic update pulse of the appropriate polarity is sent to ensure

dc correctness at the output. If the decoder receives none of

these update 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 logic high state by the watchdog

timer circuit.

The limitation on the ADuM1100’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 analysis that follows defines the conditions under

which this may occur. The ADuM1100’s 3.3 V operating condi-

tion is examined because it represents the most susceptible mode

of operation.

The pulses at the transformer output are greater than 1.0 V in

amplitude. The decoder has sensing thresholds at about 0.5 V,

therefore establishing a 0.5 V margin in which induced voltages can

be tolerated. The induced voltage induced across the receiving

coil is given by

Vd

dt

rn

N

n

=

()

=

–/

;, ,.... ,

βπ

Σ

2

12

where:

β = magnetic flux density (Gauss).

N = number of turns in receiving coil.