Datasheet

INISO
ISO
N2GND
ZZ
Z
vv
+
=
V
N
R
IN
C
IN
R
ISO
C
ISO
SystemGround (GND1)
BusReturn (GND2)
16V
A orB
49
9
INISO
ISO
N
2GND
10x610
10
RR
R
v
v
+
=
+
=
94.0
16
1
1
1
C
C
1
1
C
1
C
1
C
1
v
v
IN
ISO
INISO
ISO
N
2GND
=
+
=
+
=
+
=
ISO1176
SLLS897D –MARCH 2008–REVISED MARCH 2010 www.ti.com
APPLICATION INFORMATION
Transient Voltages
Isolating of a circuit insulates it from other circuits and earth, so that noise voltage develops across the insulation
rather than circuit components. The most common noise threat to data-line circuits is voltage surges or electrical
fast transients that occur after installation. The transient ratings of the ISO1176 standard are sufficient for all but
the most severe installations. However, some equipment manufacturers use ESD generators to test equipment
transient susceptibility. This practice can exceed insulation ratings. ESD generators simulate static discharges
that may occur during device or equipment handling with low-energy but high-voltage transients.
Figure 29 models the ISO1176 bus IO connected to a noise generator. C
IN
and R
IN
is the device, and any other
stray or added capacitance or resistance across the A or B pin to GND2. C
ISO
and R
ISO
is the capacitance and
resistance between GND1 and GND2 of the ISO1176, plus those of any other insulation (transformer, etc.). Stray
inductance is assumed to be negligible.
From this model, the voltage at the isolated bus return
is
(1)
and is always less than 16 V from V
N
. If the ISO1176
is tested as a stand-alone device,
• R
IN
= 6 x 10
4
Ω,
• C
IN
= 16 x 10
–12
F,
• R
ISO
= 10
9
Ω and
• C
ISO
= 10
–12
F.
Notice from Figure 29 that the resistor ratio determines
the voltage ratio at low frequencies, and that the
inverse capacitance ratio determines the voltage ration
at high frequencies. In the stand-alone case and for
low frequencies,
(2)
Figure 29. Device Model For Static Discharge
Testing
or essentially all of the noise appears across the
barrier.
At high frequencies,
(3)
and 94% of V
N
appears across the barrier. As long as
R
ISO
is greater than R
IN
and C
ISO
is less than C
IN
, most
of the transient noise appears across the isolation
barrier, as it should.
Using ESD generators to test equipment transient susceptibility, or considering product claims of ESD ratings
above the barrier transient ratings of an isolated interface is not recommended. ESD is best managed through
recessing or covering connector pins in a conductive connector shell, and by proper installer training.
20 Submit Documentation Feedback Copyright © 2008–2010, Texas Instruments Incorporated
Product Folder Link(s) :ISO1176