HCPL788J Datasheet, PDF (18/20 Pages) Agilent(Hewlett-Packard) – Isolation Amplifier with Short Circuit and Overload Detection

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2.4: Do I really need an RC filter on the input?
What is it for? Are other values of R and
C okay?
2.5: How do I ensure that the HCPL-788J is
not destroyed as a result of short circuit
conditions which cause voltage drops
across the sense resistor that exceed the
ratings of the HCPL-788J’s inputs?
This filter prevents damage from input spikes which
may go beyond the absolute maximum ratings of the
HCPL-788J inputs during ESD and other transient
events. The filter also prevents aliasing of high fre-
quency (above 3 MHz) noise at the sampled input.
Other RC values are certainly OK, but should be
chosen to prevent the input voltage (pin 1) from ex-
ceeding ± 5 V for any conceivable current waveform
in the sense resistor. Remember to account for in-
ductance of the sense resistor since it is possible to
momentarily have tens of volts across even a 1 mΩ
resistor if di/dt is quite large.
Select the sense resistor so that it will have less than
5 V drop when short circuits occur. The ony other
requirement is to shut down the drive before the
sense resistor is damaged or its solder joints melt.
This ensures that the input of the HCPL-788J can not
be damaged by sense resistors going open-circuit.
3. Isolation and Insulation
3.1: How many volts will the HCPL-788J
withstand?
3.2: What happens if I don’t use the 470 pF
output capacitors HP recommends?
The momentary (1 minute) withstand voltage is
3500 V rms per UL1577 and CSA Component
Acceptance Notice #5.
These capacitors are to reduce the narrow output
spikes caused by high common mode slew rates. If
your application does not have rapid common mode
voltage changes, these capacitors are not needed.
4. Accuracy
4.1: What is the meaning of the offset errors
and gain errors in terms of the output?
For zero input, the output should ideally be ¹/₂ of
V_REF. The nominal slope of the input/output relation-
ship is V_REF divided by 0.504 V. Offset errors change
only the DC input voltage needed to make the output
equal to ¹/₂ of V_REF. Gain errors change only the
slope of the input/output relationship. For example,
if V_REF is 4.0 V, the gain should be 7.937 V/V. For
zero input, the output should be 2.000 V. Input offset
voltage of ± 3 mV means the output voltage will be
2.000 V ± 0.003*7.937 or 2.000 ± 23.8 mV when the
input is zero. Gain tolerance of ± 5% means that the
slope will be 7.937 ± 0.397. Over the full range of
± 3 mV input offset error and ± 5% gain error, the
output voltage will be 2.000 ± 25.0 mV when the
input is zero.