HCPL788J datasheet(19/20 Pages) HP

19 / 20 page

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 VOUT (pin 12) and ABSVAL

(pin 13). These op-amps can swing nearly from rail

to rail when there is no load current. The internal

VDD2 is about 100 mV below the external VDD2. 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 VDD2. When VREF is

tied to VDD2, the output can not reach VREF 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 VREF and VDD2 voltages can be used. Since

5.0 V is normally recommended for VDD2, use of

4.5 V or 4.096 V references for VREF allow the out-

puts to swing all the way up to VREF (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 VREF. HP has

accounted for LED degradation in the design of the

product to ensure long life.

Ideally gain would be VREF/512 mV, however, due to

internal settling characteristics, the average effective

value of the internal 256 mV reference is 252 mV.

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 VREF

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 VREF/504 mV, not

VREF/512 mV as expected, based on

Figure 24?

HCPL788J Datasheet(PDF) 19 Page - Agilent(Hewlett-Packard)

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