performance curve shows this behavior graphically; at input
frequencies above 250kHz the device generates an output
signal component of reduced magnitude at a frequency
below 250kHz. This is the aliasing effect of sampling at
frequencies less than 2 times the signal frequency (the
Nyquist frequency). Note that at the carrier frequency and its
harmonics, both the frequency and amplitude of the aliasing
go to zero.
ISOLATION MODE VOLTAGE INDUCED ERRORS
IMV can induce errors at the output as indicated by the plots
of IMV vs Frequency. It should be noted that if the IMV
frequency exceeds 250kHz, the output also will display
spurious outputs (aliasing), in a manner similar to that for
V
IN
> 250kHz and the amplifier response will be identical to
that shown in the Signal Response to Inputs Greater Than
250kHz performance curve. This occurs because IMV-
induced errors behave like input-referred error signals. To
predict the total error, divide the isolation voltage by the
IMR shown in the IMR vs Frequency curve and compute the
amplifier response to this input-referred error signal from
the data given in the Signal Response to Inputs Greater than
250kHz performance curve. For example, if a 800kHz
1000Vrms IMR is present, then a total of [(–60dB) +
(–30dB)] x (1000V) = 32mV error signal at 200kHz plus a
1V, 800kHz error signal will be present at the output.
HIGH IMV dV/dt ERRORS
As the IMV frequency increases and the dV/dt exceeds
1000V/µs, the sense amp may start to false trigger, and the
output will display spurious errors. The common mode
current being sent across the barrier by the high slew rate is
the cause of the false triggering of the sense amplifier.
Lowering the power supply voltages below
±15V
may
decrease the dV/dt to 500V/µs for typical performance.
Isolation Barrier
HIGH VOLTAGE TESTING
Burr-Brown Corporation has adopted a partial discharge test
criterion that conforms to the German VDE0884 Optocou-
pler Standards. This method requires the measurement of
minute current pulses (<5pC) while applying 2400Vrms,
60Hz high voltage stress across every ISO122 isolation
barrier. No partial discharge may be initiated to pass this
test. This criterion confirms transient overvoltage (1.6
x
1500Vrms) protection without damage to the ISO122. Lifetest
results verify the absence of failure under continuous rated
voltage and maximum temperature.
This new test method represents the “state of the art” for
non-destructive high voltage reliability testing. It is based on
the effects of non-uniform fields that exist in heterogeneous
dielectric material during barrier degradation. In the case of
void non-uniformities, electric field stress begins to ionize
the void region before bridging the entire high voltage
barrier. The transient conduction of charge during and after
the ionization can be detected externally as a burst of 0.01-
0.1µs current pulses that repeat on each AC voltage cycle.
The minimum AC barrier voltage that initiates partial dis-
charge is defined as the “inception voltage.” Decreasing the
barrier voltage to a lower level is required before partial
discharge ceases and is defined as the “extinction voltage.”
We have characterized and developed the package insulation
processes to yield an inception voltage in excess of 2400Vrms
so that transient overvoltages below this level will not
damage the ISO122. The extinction voltage is above
1500Vrms so that even overvoltage induced partial dis-
charge will cease once the barrier voltage is reduced to the
1500Vrms (rated) level. Older high voltage test methods
relied on applying a large enough overvoltage (above rating)
to break down marginal parts, but not so high as to damage
good ones. Our new partial discharge testing gives us more
confidence in barrier reliability than breakdown/no break-
down criteria.
A
0
ISO150
A
1
+15V –15V
V
IN
–V
S2
Gnd
Gnd
+V
S2
–
V
S1
±V
S1
1µF
1µF
1µF
±V
S2
1µF
V
IN
V
OUT
1
6
2
7
PGA
8
102
45
3
1
2
+15V –15V
9
15 15
10
8
7
V
OUT
ISO122P
16
+V
S1
FIGURE 2. Basic Signal and Power Connections.
FIGURE 3. Programmable-Gain Isolation Channel with
Gains of 1, 10, and 100.
®
7
ISO122
TI [ TEXAS INSTRUMENTS ]
