High-Voltage, Fault-Protected

Analog Multiplexers

6

_______________________________________________________________________________________

_______________Typical Applications

Figure 6 shows a typical data acquisition system

using the MAX378 multiplexer. Since the multiplexer

is driving a high-impedance input, its error is a func-

= 2.0k

Ω x (2nA + 30pA)

= 18.0µV maximum error

In most cases, this error is low enough that preamplifi-

cation of input signals is not needed, even with very

low-level signals such as 40µV/°C from type J thermo-

couples.

In systems with fewer than eight inputs, an unused chan-

nel can be connected to the system ground reference

point for software zero correction. A second channel

connected to the system voltage reference allows gain

correction of the entire data acquisition system as well.

A MAX420 precision op amp is connected as a pro-

grammable-gain amplifier, with gains ranging from 1 to

10,000. The guaranteed 5µV unadjusted offset of the

MAX420 maintains high signal accuracy, while program-

mable gain allows the output signal level to be scaled to

the optimum range for the remainder of the data acqui-

sition system, normally a Sample/Hold and A/D. Since

the gain-changing multiplexer is not connected to the

external sensors, it can be either a DG508A multiplexer

or the fault-protected MAX358 or MAX378.

Truth Table—MAX378

A2

A1

A0

EN

ON

SWITCH

X

0

0

0

0

1

1

1

1

X

0

0

1

1

0

0

1

1

X

0

1

0

1

0

1

0

1

0

1

1

1

1

1

1

1

1

NONE

1

2

3

4

5

6

7

8

Truth Table—MAX379

A1

A0

EN

ON

SWITCH

X

0

0

1

1

X

0

1

0

1

0

1

1

1

1

NONE

1

2

3

4

MAX420

+15V

-15V

V+

+15V

1M

100k

10k

OUT

OUT

1k

111

Ω

IN1

IN1

IN2

IN3

IN4

IN5

V+

THERMOCOUPLE

+15V

-15V

V-

GND

DG508A

MAX358

OR

MAX378

MAX378

IN2

STRAIN GUAGE

IN7

+10V

GAIN REFERENCE

IN8

ZERO REFERENCE

IN3

4-20mA LOOP

TRANSMITTER

IN4

IN5

IN6

-15V

V-

GND

Figure 6. Typical Data Acquisition Front End