
7
ISO176
ISO176 can also be synchronized to a 400kHz to 700kHz
Square-Wave External Clock since an internal clamp and
filter provide signal conditioning. A square-wave signal of
25% to 75% duty cycle, and
±
3V to
±
20V level can be used
to directly drive the ISO176.
With the addition of the signal conditioning circuit shown in
Figure 2, any 10% to 90% duty-cycle square-wave signal
can be used to drive the ISO176 Ext Osc pin. With the values
shown, the circuit can be driven by a 4Vp-p TTL signal. For
a higher or lower voltage input, increase or decrease the 1k
resistor, R
X
, proportionally, e.g. for a
±
4V square-wave
(8Vp-p) R
X
should be increased to 2k
. The value of C
X
used in the Figure 2 circuit depends on the frequency of the
external clock signal. C
X
should be 30pF for ISO176.
When periodic noise from external sources such as system
clocks and DC/DC converters are a problem, ISO176 can be
used to reject this noise. The amplifier can be synchronized
to an external frequency source, f
EXT
, placing the amplifier
response curve at one of the frequency and amplitude nulls
indicated in the “Signal Response vs Carrier Frequency”
performance curve.
ISOLATION MODE VOLTAGE
Isolation Mode Voltage (IMV) is the voltage appearing
between isolated grounds GND1 and GND2. The IMV can
induce error at the output as indicated by the plots of IMV
versus Frequency. It should be noted that if the IMV fre-
quency exceeds f
C
/2, the output will display spurious out-
puts in a manner similar to that described above, and the
amplifier response will be identical to that shown in the
“Signal Response vs Carrier Frequency” performance curve.
This occurs because IMV-induced errors behave like input-
referred error signals. To predict the total IMR, divide the
isolation voltage by the IMR shown in “IMR vs Frequency”
performance curve and compute the amplifier response to
this input-referred error signal from the data given in the
“Signal Response vs Carrier Frequency” performance curve.
Due to effects of very high-frequency signals, typical IMV
performance can be achieved only when dV/dT of the
isolation mode voltage falls below 1000V/
μ
s. For conve-
nience, this is plotted in the typical performance curves for
the as a function of voltage and frequency for sinusoidal
voltages. When dV/dT exceeds 1000V/
μ
s but falls below
20kV/
μ
s, performance may be degraded. At rates of change
above 20kV/
μ
s, the amplifier may be damaged, but the
barrier retains its full integrity. Lowering the power supply
voltages below
±
15V may decrease the dV/dT to 500V/
μ
s
for typical performance, but the maximum dV/dT of 20kV/
μ
s remains unchanged.
Leakage current is determined solely by the impedance of
the barrier capacitance and is plotted in the “Isolation Leak-
age Current vs Frequency” curve.
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 higher voltage for some shorter time.
The relationship between actual test voltage and the continu-
ous derated maximum specification is an important one.
Historically, Burr-Brown has chosen a deliberately conser-
vative one: VTEST = (2 x ACrms continuous rating) +
1000V for 10 seconds, followed by a test at rated ACrms
voltage for one minute. This choice was appropriate for
conditions where system transients are 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 ISO176.
CARRIER FREQUENCY CONSIDERATIONS
ISO176 amplifier transmits the signal across the ISO-barrier
by a duty-cycle modulation technique. This system works
like any linear amplifier for input signals having frequencies
below one half the carrier frequency, f
C
. For signal frequen-
cies above f
C
/2, the behavior becomes more complex. The
Signal Response versus Carrier Frequency performance curve
describes this behavior graphically. The upper curve illus-
trates the response for input signals varying from DC to f
C
/
2. At input frequencies at or above f
C
/2, the device generates
an output signal component that varies in both amplitude
and frequency, as shown by the lower curve. The lower
horizontal scale shows the periodic variation in the fre-
quency of the output component. Note that at the carrier
frequency and its harmonics, both the frequency and ampli-
tude of the response go to zero. These characteristics can be
exploited in certain applications.
It should be noted that for the ISO176, the carrier frequency
is nominally 500kHz and the –3dB point of the amplifier is
60kHz. Spurious signals at the output are not significant
under these circumstances unless the input signal contains
significant components above 250kHz.
10k
C
X
OPA602
R
X
1k
1μF
Square-Wave In
Triangle Out
to ISO166/176
Ext Osc
FIGURE 2. Square-Wave to Triangle Wave Signal Condi-
tioner for Driving ISO176 Ext Osc Pin.