
ISO212P
7
The bulk inception voltage, on the other hand, varies with
the insulation system and the number of ionization defects.
This directly establishes the absolute maximum voltage
(transient) that can be applied across the test device before
destructive partial discharge can begin.
Measuring the bulk extinction
voltage
provides
a lower,
more conservative, voltage from which to derive a safe
continuous rating. In production, it’s acceptable to measure
at a level somewhat below the expected inception voltage
and then de-rate by a factor related to expectations about the
system transients. The isolation amplifier has been exten-
sively evaluated under a combination of high temperatures
and high voltage to confirm its performance in this respect.
The
ISO212P
is free of partial discharges at rated voltages.
PARTIAL DISCHARGE TESTING IN PRODUCTION
Not only does this test method provide far more qualitative
information about stress withstand levels than did previous
stress tests, but it also provides quantitative measurements
from which quality assurance and control measures can be
based. Tests similar to this test have been used by some
manufacturers such as those of high voltage power distribu-
tion equipment for some time, but they employed a simple
measurement of RF noise to detect ionization. This method
was not quantitative with regard to energy of the discharge
and was not sensitive enough for small components such as
isolation amplifiers. Now, however, manufacturers of HV
test equipment have developed means to measure partial
discharge, and VDE, the German standards group, has adopted
use of this method for the testing of opto-couplers. To
accommodate poorly defined transients, the part under test is
exposed to a voltage that is 1.6 times the continuous rated
voltage and must display < 5pC partial discharge level in a
100% production test.
INSTALLATION AND
OPERATING INSTRUCTIONS
POWER SUPPLY AND SIGNAL CONNECTIONS
As with any mixed analog and digital signal component,
correct decoupling and signal routing precautions must be
used to optimize performance. Figure 1 shows the proper
power supply and signal connections. V
CC
should be by-
passed to Com 2 with a 0.1
μ
F ceramic capacitor as close to
the device as possible. Short leads will minimize lead
inductance. A ground plane will also reduce noise problems.
If a low impedance ground plane is not used, signal common
lines, and either O/P High or O/P Low pin should be tied
directly to the ground at the supply and Com 2 returned via
a separate trace to the supply ground.
To avoid gain and isolation mode (IMR) errors introduced
by the external circuit, connect grounds as indicated in
Figure 3. Layout practices associated with isolation amplifi-
ers are very important. In particular, the capacitance associ-
ated with the barrier, and series resistance in the signal and
reference leads, must be minimized. Any capacitance across
the barrier will increase AC leakage and, in conjunction with
ground line resistance, may degrade high frequency IMR.
VOLTAGE GAIN MODIFICATIONS
The uncommitted operational amplifier at the input can be
used to provide gain, signal inversion, active filtering or
current to voltage conversion. The standard design approach
for any op-amp stage can be used, provided that the full scale
voltage appearing on f
B
does not exceed
±
5V.
If the input op-amp is overdriven, ripple at the output will
result. To prevent this, the feedback resistor should have a
minimum value of 10k
.
Also, it should be noted that the current required to drive the
equivalent impedance of the feedback network is supplied
by the internal DC/DC converter and must be taken into
account when calculating the loading added to
±
V
SS1
.
Since gain inversion can be incorporated in either the input
or output stage of the ISO212P, it is possible to use the input
amplifier in a non-inverting configuration and preserve the
high impedance this configuration offers. Signal inversion at
the output is easily accomplished by connecting O/P High to
Com 2 instead of O/P Low.
ISOLATED POWER OUTPUT DRIVE CAPABILITY
On the input side of the ISO212P, there are two power
supplies capable of delivering 5mA at
±
8V to power exter-
nal circuitry. When using these supplies with external loads,
it is recommended that additional decoupling in the form of
10
μ
F tantalum bead capacitors be added to improve the
voltage regulation. Loss of linearity will result if additional
filtering is not used with an output load. Again, power
dissipated in the feedback loop around the input op amp
must be subtracted from the available power output at
±
V
SS1.
If the ISO212P is to be used in multiple applications, care
should be taken in the design of the power distribution
FIGURE 3. Technique for Connecting Com 1 and Com 2.
V
ISO
CC
+V
CC
–V
Power
Supply
Load
Circuit
R
O/P Low
Com 2
Com 1
Input
Common
C
INT
C
EXT 2
C
EXT 1
O/P High
C and R have a direct effect.
C has minimal effect on total IMR.
f
B
–
+