®

ISC300

4

THEORY OF OPERATION

The ISC300 has no galvanic connection between the input

and output sections. The differential input signal is multi-

plied by the programmable gain amplifier and accurately

transferred across the isolation barrier to the output. The

output section demodulates the signal transferred from the

input section and transfers power to the input section.

ISC300 DESIGN

The ISC300 consists of:

• A filtered differential high impedance input.

• Precision matched current sources.

• Fault detect bias resistors.

• Digitally selectable internal calibration references.

• Digitally selectable gain.

• Isolation of all digital and analog signals.

• Isolated DC/DC converter.

• Synchronizable internal oscillator.

• Two isolated power supplies available for external cir-

cuitry.

• Externally available 10V reference.

INPUT SECTION

Filter

Since the ISC300 is designed to measure slowly changing

processes, the input filter is set for a cut off frequency of

2Hz. This gives good noise rejection at power frequencies of

50Hz and 60Hz.

Sense Lines

The two sense lines can be configured to detect short or open

circuits e.g. transducer burn out. This would be indicated by

an out of range output (see Input Configuration in Applica-

tions section).

Multiplexer

The multiplexer is used to route either the measurement

channel or the precision voltage references (used in system

calibration) to the programmable gain amplifier.

Isolation Barrier

The isolation barrier consists of two transformers and three

opto couplers. One transformer transmits the signal from the

input side to the output side. The other transmits power from

the output side to the input side. The opto-couplers are used

to isolate the logic used for mux select, gain and reference

voltage control.

Voltage Reference

The voltage reference provides 10V, 0.1V and 0V references

for channel calibration. The 10V reference is also available

externally.

Current References

Two matched 200

µA current references are available for the

excitation of RTDs or for use in external signal conditioning

circuitry.

Channel 1 Out

MUX

PGA

Channel 4

A

1

A

0

Channel 3

Channel 2

Ref

0.100V

Com

1

10.0V

99R

R

Current

Reference

20M

Filter

Modulate

Demod

Power

Rectify

Com 1

Com 2

O

P

T

O

C

O

U

P

L

E

L

A

T

C

H

Out

V

CKI

Com 2

V

CLK

G

A

0

A

1

RST

DCom 2

28

25

26

27

18

22

19

20

21

23

24

20M

+V

ISO

–V

ISO

+V

ISO

–V

ISO

Sense 2

+I

P

–I

P

Sense 1

V

REF

I

REF1

I

REF2

Com 1

–V

ISO

2

8

5

6

3

1

4

7

10

9

Barrier

Output Side

Input Side

FIGURE 1. ISC300 Block Diagram.