LTC2418IGN datasheet(41/48 Pages) LINER

41 / 48 page

LTC2414/LTC2418

241418fa

MULTIPLE CHANNEL USAGE

The LTC2414/LTC2418 have up to sixteen input channels

and this feature provides a very flexible and efficient

solution in applications where more than one variable

need to be measured.

Measurements of a Ladder of Sensors

In industrial process, it is likely that a large group of real

world phenomena need to be monitored where the speed

is not critical. One example is the cracking towers in

petroleum refineries where a group of temperature mea-

surements need to be taken and related. This is done by

passing an excitation current through a ladder of RTDs.

The configuration using a single LTC2418 to monitor up to

eight RTDs in differential mode is shown in Figure 48. A

high accuracy R1 is used to set the excitation current and

the reference voltage. A larger value of 25k is selected to

0.1

µF

15V

–

REF+

REF–

CH0

CH1

GND

VCC

2410 F53

LTC2414/

LTC2418

LT1236-5

RN1

10k

Ω

10V

350

Ω BRIDGE

TWO ELEMENTS

VARYING

RN1

10k

2N3904

1/2

LT1112

0.1

µF

15V

–5V

–15V

–

RN1

10k

RN1 IS CADDOCK T914 10K-010-02

Q2, Q3

2N3906

×2

1/2

LT1112

RN1

10k

Ω

×2

µF

0.1

µF

Ω

Figure 47. Use Resistor Arrays to Provide Precise Matching in Excitation Amplifier

reduce the self-heating effects. R1 can also be broken into

two resistors, one 25k to set the excitation current and the

other a high accuracy 1k resistor to set the reference

voltage, assuming 100

Ω platinum RTDs. This results in a

reduced reference voltage and a reduced common mode

difference between the reference and the input signal,

which improves the conversion linearity and reduces total

error.

Each input should be taken close to the related RTD to

minimize the error caused by parasitic wire resistance.

The interference on a signal transmission line from RTD to

the LTC2418 is rejected due to the excellent common

mode rejection and the digital LPF included in the LTC2418.

It should be noted that the input source resistance of CHO

can have a maximum value of 800

Ω • 8 = 6.4k, so the

parasitic capacitance and resistance of the connection

wires need to be minimized in order not to degrade the

converter performance.

APPLICATIO S I FOR ATIO

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