13

Designing the input resistor divider

1. Choose the sensing current (Isense) for bus voltage. E.g., 1 mA

2. Determine R2,

=

Voltage input range

= 2 kΩ

2 V

1 mA

(600 V – 0 V) • 2 kΩ

2 V

– 2 kΩ = 598 kΩ

(V+ – V-) •

= Voltage input range, or

3. Determine R1 using voltage divider formula:

=

Voltage input range

= 2 kΩ

2 V

1 mA

(600 V – 0 V) • 2 kΩ

2 V

– 2 kΩ = 598 kΩ

(V+ – V-) •

= Voltage input range, or

where (V+ – V-) is the high voltage input , E.g.: 0 to 600 V,

=

Voltage input range

= 2 kΩ

2 V

1 mA

(600 V – 0 V) • 2 kΩ

2 V

– 2 kΩ = 598 kΩ

(V+ – V-) •

= Voltage input range, or

To reduce the voltage stress of a sole resistor, R1 can be a series of several resistors.

Post Amplifier Circuit

The output of ACPL-C87AT/BT is a differential output

to convert the differential output to single ended output

with a reference ground. The post amplifier circuit can

also be configured to establish a desired gain if needed.

It also functions as filter to high frequency chopper

noise. The bandwidth can be adjusted by changing the

feedback resistor and capacitor (R7 and C7). Adjusting this

bandwidth to a minimum level helps minimize the output

noise.

Post op-amp resistive loading (R4, R5) should be equal

or greater than 20 k

W (Figure 31). Resistor values lower

than this can affect the overall system error due to output

impedance of isolation amplifier.

The application circuit in Figure 28 features two op-amps

to improve the linearity at voltage near 0 V caused by the

limited headroom of the amplifier. The second op-amp

can set the reference voltage to above 0 V.

Shutdown Function

ACPL-C87AT/BT has a shutdown function to disable the

of 5V on SHDN pin will shutdown the device producing an

SHDN function (example, from microprocessor), an opto-

coupler (ACPL-M49T) is used.

Total System Error

Total system error is the sum of the resistor divider error,

isolation amplifier error and post amplifier error. The

resistor divider error is due to the accuracy of the resistors

used. It is recommended to use high accuracy resistor of

0.1%. Post Amplifier Error is due to the resistor matching

and the voltage offset characteristic which can be found

on the supplier datasheet.

Isolation Amplifier Error is shown in the table below:

Isolation Amplifier Error Calculation

Typical

3

s distribution or specification *

Fig

ACPL-C87AT

ACPL-C87BT

A Error due to offset voltage (25 °C)

0.015% 0.5%

0.5%

Offset Voltage /Recommended

input voltage range (2.0 V)

specs

B Error due to offset voltage drift

(across temperature)

0.1%

0.4%

0.4%

Offset Voltage /Recommended

input voltage range (2.0 V)

C Error due to gain tolerance (25 °C)

0%

1%

0.5%

specs

D Error due to gain drift (across temperature)

0.25%

0.8%

0.8%

E Error due to Nonlinearity (across temperature) 0.05%

0.12%

0.12%

F

Total uncalibrated error (A+B+C+D+E)

0.415% 2.82%

2.32%

specs

G

Total offset calibrated error (F – A)

0.4%

2.32%

1.82%

H

Total gain and offset calibrated error (G – C)

0.4%

1.32%

1.32%

* 3

s distribution is based on corner wafers.