Typical Applications

DATA ACQUISITION SYSTEM

A SIMPLIFIED SYSTEM DISCUSSION

Analog multiplexers (MUX) are usually used for multi-chan-

nel Data Acquisition Units (DAU)

Figure 5 shows a system

in which 8 different analog inputs are sampled and convert-

ed into digital words for further processing The sample and

hold circuit is optional depending on input speed require-

ments and on AD converter speed

Parameters characterizing the system are

System Channels

The number of multiplexer channels

Accuracy

The conversion accuracy of each individual sam-

ple with the system operating at the throughput rate

Speed or Throughput Rate

Number of samplessecond

channel the system can handle

For a discussion on system structure addressing mode and

processor interfacing see application note AN-159

A ACCURACY CONSIDERATIONS

1 Multiplexer’s Influence on System Accuracy

(Figure 6)

a The error (E) caused by the finite ‘‘ON’’ resist-

by

100

where

D

negligible for JFET switches like the LF11508

e

to 12 LSB in a 12-bit system

min

e

E

e

45 MX

Note that if temperature effects are included some

gain (or full scale) drift will occur but effects on linearity

are small

settle within a predetermined accuracy

as

shown in Table I

pacitance at this node

TABLE I

ERROR %

BITS

TO 12 LSB

02

8

62t

005

10

76t

001

12

9t

00008

16

118t

a tolerable error The ‘‘OFF’’ settling time should

be taken into account for bipolar inputs where its

effects will appear as a worse case of doubling

2 Sample and Hold Influence on System Accuracy

The sample and hold if used also introduces errors into

the system accuracy due to

digital Hold command and the actual Hold occurance

analog input and settle within a predetermined error

band

mode caused by an undesirable charge injected into

For more details on sample and hold errors see the

LF198LF298LF398 data sheet

3 AD Converter Influence on System Accuracy

The ‘‘accuracy’’ of the AD converter is the best possible

system accuracy In most data acquisition systems the

AD converter is the most expensive single component

so its error will often dominate system error Care should

be taken that MUX SH and input source errors do not

exceed system error requirements when

added to AD

errors For instance if an 8-bit accuracy system is desired

and an 8-bit AD converter is used the accuracy of the

MUX and SH should be far better than 8 bits

For details on AD converter specifications see AN-156

TLH5668 – 13

FIGURE 5 Random-Addressed Multiplexed DAU

FIGURE 6 8-Channel MUX

7