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Avago, Avago Technologies, and the A logo are trademarks of Avago Technologies in the United States and other countries.

Data subject to change. Copyright © 2005-2012 Avago Technologies. All rights reserved. Obsoletes AV01-0569EN

AV02-1289EN - July 9, 2012

3. ISOLATION AND INSULATION

3.1: How many volts will the HCPL-7840 withstand?

The momentary (1 minute) withstand voltage is 3750 V rms per UL 1577 and CSA Component Acceptance

Notice #5.

4. ACCURACY

4.1: Can the signal to noise ratio be improved?

Yes. Some noise energy exists beyond the 100 kHz bandwidth of the HCPL-7800(A). Additional filtering using dif-

ferent filter R,C values in the post-amplifier application circuit can be used to improve the signal to noise ratio. For

example, by using values of R3 = R4 = 10 kΩ, C5 = C6 = 470 pF in the application circuit the rms output noise will

be cut roughly by a factor of 2. In applications needing only a few kHz bandwidth even better noise performance

can be obtained. The noise spectral density is roughly 500 nV/√ Hz below 20 kHz (input referred).

4.2: I need 1% tolerance on gain. Does Avago sell a more precise version?

The HCPL-7800A is gain-trimmed and matched to within ±1% tolerance (at room temperature.)

4.3: Does the gain change if the internal LED light output degrades with time?

No. The LED is used only to transmit a digital pattern. Avago has accounted for LED degradation in the design

of the product to ensure long life.

5. POWER SUPPLIES AND START-UP

5.1: What are the output voltages before the input side power supply is turned on?

V

O+

is close to 1.29 V and V

O-

is close to 3.80 V. This is equivalent to the output response at the condition that LED

is completely off.

5.2: How long does the HCPL-7840 take to begin working properly after power-up?

Within 1 ms after V

DD1

and V

DD2

powered the device starts to work. But it takes longer time for output to settle down

completely. In case of the offset measurement while both inputs are tied to ground there is initially V

OS

adjustment

(about 60 ms). The output completely settles down in 100 ms after device powering up.

6. MISCELLANEOUS

6.1: How does the HCPL-7840 measure negative signals with only a +5 V supply?

The inputs have a series resistor for protection against large negative inputs. Normal signals are no more than 200

mV in amplitude. Such signals do not forward bias any junctions sufficiently to interfere with accurate operation

of the switched capacitor input circuit.