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LTC2424/LTC2428

The LTC2428’s Resolution and Accuracy Allows You

to Measure Points in a Ladder of Sensors

In many industrial processes, for example, cracking tow-

ers in petroleum refineries, a group of temperature mea-

surements must be related to one another. A series of

platinum RTDs that sense slow changing temperatures

can be configured into a resistive ladder, using the LTC2428

to sense each node. This approach allows a single excita-

tion current passed through the entire ladder, reducing

total supply current consumption. In addition, this ap-

proach requires only one high precision resistor, thereby

reducing cost. A group of up to seven temperatures can be

measured as a group by a single LTC2428 in a loop-pow-

ered remote acquisition unit. In the example shown in

Figure 33, the excitation current is 240µA at 0°C. The

LTC2428 requires 300µA, leaving nearly 3.5mA for the

remainder of the remote transmitter.

The resistance of any of the RTDs (PT1 to PT7) is deter-

mined from the voltage across it, as compared to the

voltage drop across the reference resistor (R1). This is a

ratiometric implementation where the voltage drop across

R1 is given by V

REF

– V

CH1

. Channel 7 is used to measure

the voltage on a representative length of wire. If the same

type and length of wire is used for all connections, then

errors associated with the voltage drops across all wiring

can be removed in software. The contribution of wiring

drop can be scaled if wire lengths are not equal.

Gain can be added to this circuit as the total voltage drop

across all the RTDs is small compared to ADC full-scale

range. The maximum recommended gain is 50, as limited

by both amplifier noise contribution, as well as the maxi-

mum voltage developed at CH0 when all sensors are at the

maximum temperature specified for platinum RTDs.

24248 F33

CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

8-CHANNEL

MUX

5ZS

SET

GND

1, 6, 16, 18, 22, 27, 28

CSADC

CSMUX

SCK

CLK

SDO

LTC2428

ADCINMUXOUT

0.1µF

47µF

R2300µA

OPTIONAL

GAIN

BLOCK

TO PT3-PT6

PT1

100Ω

PLATINUM

RTD

OPTIONAL

PROTECTION

RESISTORS

5k MAX

UP TO SEVERAL

HUNDRED FEET.

ALL SAME

WIRE TYPE

PT2

PT7

20.1k

0.1%

3 2, 8

1µF

SET

–

LTC1050

LTC1634-2.5

20-BIT

∆Σ ADC

–

Figure 33. Measuring Up to Seven RTD Temperatures with One Reference Resistor and One Reference Current

APPLICATIONS INFORMATION

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