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ISO212P
6
works. The uncommitted operational amplifier may be con-
figured for signal buffering or amplification, depending on
the application.
The modulator converts the input signal to an amplitude-
modulated AC signal that is magnetically coupled to the
demodulator by a miniature transformer providing the sig-
nal-path isolation. The demodulator recovers the input sig-
nal from the modulated signal using a synchronous tech-
nique to minimize noise and interference.
ABOUT THE BARRIER
For any isolation product, barrier integrity is of paramount
importance in achieving high reliability. The ISO212P uses
miniature toroidal transformers designed to give maximum
isolation performance when encapsulated with a high-die-
lectric-strength material. The internal component layout is
designed so that circuitry associated with each side of the
barrier is positioned at opposite ends of the package. Areas
where high electric fields can exist are positioned in the
center of the package. The result is that the dielectric
strength of the barrier typically exceeds 3kVrms.
ISOLATION VOLTAGE RATINGS
Because a long term test is impractical in a manufacturing
situation, the generally accepted practice is to perform a
production test at a high voltage for some shorter time. The
relationship between actual test voltage and the continuous
derated maximum specification is an important one. Histori-
cally, Burr-Brown has chosen a deliberately conservative
one: V
TEST
= (2
x
ACrms continuous rating) + 1000V for ten
seconds, followed by a test at rated ACrms voltage for one
minute. This choice was appropriate for conditions where
system transients were not well defined.
Recent improvements in high voltage stress testing have
produced a more meaningful test for determining maximum
permissible voltage ratings, and Burr-Brown has chosen to
apply this new technology in the manufacture and testing of
the ISO212P.
PARTIAL DISCHARGE
When an insulation defect such as a void occurs within an
insulation system, the defect will display localized corona or
ionization during exposure to high voltage stress. This ioni-
zation requires a higher applied voltage to start the discharge
and a lower voltage to extinguish it once started. The higher
start voltage is known as the inception voltage and the lower
voltage is called the extinction voltage. Just as the total
insulation system has an inception voltage, so do the individ-
ual voids. A voltage will build up across a void until its
inception voltage is reached. At this point, the void will
ionize, effectively shorting itself out. This action redistrib-
utes electrical charge within the dielectric and is known as
partial discharge. If the applied voltage gradient across the
device continues to rise, another partial discharge cycle
begins. The importance of this phenomenon is that if the
discharge does not occur, the insulation system retains its
integrity. If the discharge begins and is allowed to continue,
the action of the ions and electrons within the defect will
eventually degrade any organic insulation system in which
they occur. The measurement of partial discharge is both
useful in rating the devices and in providing quality control
of the manufacturing process. The inception voltage of these
voids tends to be constant, so that the measurement of total
charge being re-distributed within the dielectric is a very
good indicator of the size of the voids and their likelihood of
becoming an incipient failure.
FIGURE 2. Simplified Diagram of Isolation Amplifier.
–
+
4
f
B
7
Off Adjust
3
–I/P
1
+I/P
8
Off Adjust
6
+V O/P
5
–V O/P
2
Com 1
0.47μF
+8V
–8V
0.47μF
Rectifier
Modulator
Signal
Power
25kHz
Oscillator
Demodulator
50kHz
38
O/P High
37
O/P Low
31
+V
CC
34
Clock Out
35
Clock In
32
Com 2