16

2.3. Do I really need an RC filter on the input?What is it for? Are other

values of R and C okay?

The input anti-aliasing filter (R=39

Ω, C=0.01 μF)

shown in the typical application circuit is recom-

mended for filtering fast switching voltage transients

from the input signal. (This helps to attenuate higher

signal frequencies which could otherwise alias with

the input sampling rate and cause higher input offset

voltage.)

Some issues to keep in mind using different filter

resistors or capacitors are:

1. Filter resistor: Input bias current for pins 2 and 3:

This is on the order of 500 nA. If you are using a

single filter resistor in series with pin 2 but not pin

3 the IxR drop across this resistor will add to the

offset error of the device. As long as this IR drop

is small compared to the input offset voltage

there should not be a problem. If larger-valued

resistors are used in series, it is better to put half

of the resistance in series with pin 2 and half the

resistance in series with pin 3. In this case, the

offset voltage is due mainly to resistor mismatch

(typically less than 1% of the resistance design

value) multiplied by the input bias.

2. Filter resistor: The equivalent input resistance

for ACPL-C78A/C780/C784 is around 500 k

Ω.

It is therefore best to ensure that the filter

resistance is not a significant percentage of

this value; otherwise the offset voltage will be

increased through the resistor divider effect.

will vary with respect to Vin.]

3. The input bandwidth is changed as a result of this

different R-C filter configuration. In fact this is one

of the main reasons for changing the input-filter

R-C time constant.

4. Filter capacitance: The input capacitance of the

ACPL-C78A/C780/C784 is approximately 1.5 pF.

For proper operation the switching input-side

sampling capacitors must be charged from a

relatively fixed (low impedance) voltage source.

Therefore, if a filter capacitor is used it is best for

this capacitor to be a few orders of magnitude

works well.)

2.4. How do I ensure that the ACPL-C78A/C780/C784 is not destroyed

as a result of short circuit conditions which cause voltage drops

across the sense resistor that exceed the ratings of the ACPL-

C78A/C780/C784’s inputs?

Select the sense resistor so that it will have less than 5 V

drop when short circuits occur. The only other require-

ment is to shut down the drive before the sense resistor

is damaged or its solder joints melt. This ensures that

the input of the ACPL-C78A/C780/C784 can not be

damaged by sense resistors going open-circuit.

3. Isolation and insulation

3.1. How many volts will the ACPL-C78A/C780/C784 withstand?

The momentary (1 minute) withstand voltage is 5000

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 ACPL-C78A/C780/C784. Additional

filtering using different filter R,C values in the post-am-

plifier application circuit can be used to improve the

signal to noise ratio. For example, by using values of R3

= R4 = 10 k

Ω, C5=C6=470pFintheapplicationcircuit

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

noisespectraldensityisroughly500nV/sqrt(Hz)below

20 kHz (input referred).

4.2. 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 Technologies has accounted for LED degrada-

tion 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?

is equivalent to the output response at the condition

that LED is completely off.

5.2. How long does the ACPL-C78A/C780/C784 take to begin working

properly after power-up?

starts to work. But it takes longer time for output to

settle down completely. In case of the offset mea-

surement while both inputs are tied to ground there

completely settles down in 100 ms after device

powering up.

6. Miscellaneous

6.1. How does the ACPL-C78A/C780/C784 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.