6

®

3650/52

DEFINITIONS

The isolation-mode voltage is the voltage which appears

across the isolation barrier, i.e., between the input common

and the output common. (See Figure 1.)

Two isolation voltages are given in the electrical specifica-

tions: “rated continuous” and “test voltage”. Since it is

impractical on a production basis to test a “continuous”

voltage (infinite test time is implied), it is a generally

accepted practice to test at a significantly higher voltage for

some reasonable length of time. For the 3650 and 3652, the

“test voltage” is equal to 1000V plus two times the “rated

continuous” voltage. Thus, for a continuous rating of 2000V,

each unit is tested at 5000V.

The common-mode voltage is the voltage midway between

the two inputs of the amplifier measured with respect to

input common. It is the algebraic average of the voltage

applied at the amplifiers’ input terminals. In the circuit in

involve a large system “common-mode voltage.” Usually in

the system “common-mode voltage” is applied to the ampli-

ISOLATION-MODE REJECTION

The isolation-mode rejection is defined by the equation in

Figure 1. The isolation-mode rejection is not infinite be-

cause there is some leakage across the isolation barrier due

to the isolation resistance and capacitance.

FIGURE 1. Illustration of Isolation-Mode and Common-

Mode Specifications.

NONLINEARITY

Nonlinearity is specified to be the peak deviation from a best

straightline expressed as a percent of peak-to-peak full scale

output (i.e.

±10mV at 20Vp-p ≈ 0.05%).

THEORY OF OPERATION

Prior to the introduction of the 3650 family optical isolation

had not been practical in linear circuits. A single LED and

photodiode combination, while useful in a wide range of

digital isolation applications, has fundamental limitations—

primarily nonlinearity and instability as a function of time

and temperature.

The 3650 and 3652 use a unique technique to overcome the

limitations of the single LED and photodiode isolator.

Figure 2 is an elementary equivalent circuit for the 3650,

which can be used to understand the basic operation without

considering the cluttering details of offset adjustment and

biasing for bipolar operation.

FIGURE 2. Simplified Equivalent Circuit of Linear Isolator.

nonlinearities and time-temperature instabilities. Amplifier

closely matched, and since they receive equal amounts of

the overall transfer function is:

This improved isolator circuit overcomes the primary

limitations of the single LED and photodiode combination.

The transfer function is now virtually independent of any

degradation in the LED output as long as the two photo-

NOTE: (1) The only effect of decreased LED output is a slight decrease in full

scale swing capability. See Typical Performance Curves.

C

(Output)

+

R

G1

R

G2

R

IN

V

ISO

V

CM

V

D

–

+

V+

V–

C

(Input)

I

L

–

V

OUT

Isolation Barrier

System

Ground

10

6

R

V

CM

CMRR

V

V

+

V

ISO

IMRR

V

IN

V

R

K

R

G

A

1

R

G

Input Common

I

IN

+V

–V

I

1

λ1

λ2

CR

3

CR

1

CR

2

R

K

A

2

+V

CC

V

OUT

–V

CC

Output Common

I

2

I

2

I

3

Isolation Barrier

–

+

–

+

R

G