ISO100

7

THEORY OF OPERATION

The ISO100 is fundamentally a unity gain current amplifier

intended to transfer small signals between electrical circuits

separated by high voltages or different references. In most

applications, an output voltage is obtained by passing the

The ISO100 uses a single light emitting diode (LED) and a

pair of photodiode detectors coupled together to isolate the

output signal from the input.

a midscale reference. The LED and photodiodes (D

are arranged such that the same amount of light falls on each

photodiode. Thus, the currents generated by the diodes

match very closely. As a result, the transfer function de-

pends upon optical match rather than absolute performance.

Laser-trimming of the components improves matching and

enhances accuracy, while negative feedback improves lin-

optical path formed by the LED and D

transferred across the isolation barrier by the matched light

path to D

The overall isolation amplifier is noninverting (a positive

going input produces a positive going output).

INSTALLATION AND

OPERATING INSTRUCTIONS

UNIPOLAR OPERATION

voltage at pin 15 to decrease. Because the amplifier is

generating an increasing current. The current increases until

the sum of the currents in and out of the input node (–Input

to A

input current plus the bias current. As a result, no bias

(I

current-to-voltage converter, as described below.

voltage then becomes:

V

(1)

Unipolar operation does have some constraints, however. In

this mode the input current must be negative so as to produce

current more negative than 20nA is necessary to keep the

LED turned on and the loop stabilized. When this condition

is not met, the output may be indeterminant. Many sensors

generate unidirectional signals, e.g., photoconductive and

photodiode devices, as well as some applications of thermo-

couples. However, other applications do require bipolar

operation of the ISO100.

BIPOLAR OPERATION

To activate the bipolar mode, reference currents as shown in

Figure 1 are attached to the input nodes of the op amps. The

input stage stabilizes just as it did in unipolar operation.

FIGURE 1. Simplified Block Diagram of the ISO100.

A

1

16

15

18

D

1

D

2

A

2

8

Isolation

Barrier

7

3

I

REF2

R

F

I

REF1

Input Common

Output Common

17

Optical

Assembly

LED

V

V

OUT

R

IN

–In

I

IN

V

IN

–

+In

Input Circuit

Output Circuit

Connect pins 15 and 16 for bipolar,

and pins 16 and 17 for unipolar.

Connect pins 7 and 8 for bipolar,

and pins 8 and 9 for unipolar.

9

+