HCPL788J Datasheet, PDF (19/20 Pages) Agilent(Hewlett-Packard) – Isolation Amplifier with Short Circuit and Overload Detection

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4.2: Can the signal to noise ratio be improved?
4.3: I need 1% tolerance on gain. Does HP sell
a more precise version?
4.4: The output doesn’t go all the way to V_REF
when the input is above full scale.
Why not?
4.5: Does the gain change if the internal
LED light output degrades with time?
4.6: Why is gain defined as V_REF/504 mV, not
V_REF/512 mV as expected, based on
Figure 24?
Yes. Some noise energy exists beyond the 30 kHz
bandwidth of the HCPL-788J. An external RC low
pass filter can be used to improve the signal to noise
ratio. For example, a 680 Ω, 4700 pF RC filter will
cut the rms output noise roughly by a factor of 2.
This filter reduces the -3dB signal bandwidth only
by about 10%. In applications needing only a few
kHz bandwidth even better noise performance can
be obtained. The noise spectral density is roughly
400 nV/√ Hz below 15 kHz (input referred). As
an example, a 2 κHz (680 Ω, 0.1 µF) RC low pass
filter reduces output noise to a typical value of
0.08 mVrms.
At present HP does not have a standard product
with tighter gain tolerance. A 100 Ω variable
resistor divider can be used to adjust the input
voltage at pin 1, if needed.
Op-amps are used to drive V_OUT (pin 12) and ABSVAL
(pin 13). These op-amps can swing nearly from rail
to rail when there is no load current. The internal
V_DD2 is about 100 mV below the external V_DD2. In
addition, the pullup and pulldown output transistors
are not identical in capability. The net result is that
the output can typically swing to within 20 mV of
GND₂ and to within 150 mV of V_DD2. When V_REF is
tied to V_DD2, the output can not reach V_REF exactly.
This limitation has no effect on gain — only on
maximum output voltage. The output remains linear
and accurate for all inputs between -200 mV and
+200 mV. For the maximum possible swing range,
separate V_REF and V_DD2 voltages can be used. Since
5.0 V is normally recommended for V_DD2, use of
4.5 V or 4.096 V references for V_REF allow the out-
puts to swing all the way up to V_REF (and down to
typically 20 mV).
No. The LED is used only to transmit a digital
pattern. Gain is determined by a bandgap voltage
reference and the user-provided V_REF. HP has
accounted for LED degradation in the design of the
product to ensure long life.
Ideally gain would be V_REF/512 mV, however, due to
internal settling characteristics, the average effective
value of the internal 256 mV reference is 252 mV.