6

®

ISO254

INPUT COMMON-MODE RANGE

The linear common-mode range of the input circuitry of the

ISO254 is approximately

±12.7V (or 2.3V from the power

supplies). As the output voltage increases, however, the

linear input range will be limited by the output voltage swing

of the internal amplifiers. Thus, the linear common-mode

range is related to the output voltage of the complete input

amplifier—see performance curves “Input Common-Mode

Range vs Output Voltage.”

A combination of common-mode and differential input

voltage can cause the output voltage of the internal amplifi-

ers to saturate. For applications where input common-mode

range must be maximized, limit the output voltage swing by

selecting a lower gain of the programmable-gain input.

Input-overload can produce an output voltage that appears

normal. For example, an input voltage of +20V on one input

and +40V on the other input will exceed the linear common-

mode range of both input amplifiers. Since both input

amplifiers are saturated to nearly the same output voltage

limit, the difference voltage measured by the output ampli-

fier will be near zero. The output of the programmable-gain

amplifier will be near 0V even though both inputs are

overloaded.

BASIC OPERATION

ISO254 is a precision, powered, three-port isolated program-

mable-gain amplifier. The input and output sections are

galvanically isolated by matched and EMI shielded capaci-

tors built into the plastic package. The DC/DC converter

input is also galvanically isolated from both the input and

output supplies.

SIGNAL AND POWER CONNECTIONS

Figure 1 shows proper power and signal connections. The

2.2

µF tantalum capacitor and the outputs V

220nF ceramic capacitors located as close to the amplifier as

possible. All ground connections should be run indepen-

dently to a common point. Signal Common on the output

section provides a low-impedance point for sensing signal

ground in noisy applications. Com1 and Com2 must have a

path to ground for signal current return and should be

maintained within

±1V of GND1 and GND2 respectively.

FIGURE 1. Basic Connections.

220nF

220nF

1

10

100

1000

0

0

1

1

0

1

0

1

GAIN

A1

A0

2.2µF

+V

S1

Com1

–V

IN

+V

IN

–V

S1

GND1

28

26

3

4

27

25

+V

S2

Com2

–V

S2

GND2

12

V

OUT

13

11

10

V

OUT

14

+15V

OUT

–15V

OUT

220nF

220nF

+15V

OUT

–15V

OUT

ISO254

PGA

A0

A1

1

SYNC

GND3

+V

S3

16

17

15

2

SYNC +15V

V

IN

V

CM