LTC2984 datasheet(48/78 Pages) AD

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LTC2984

2984fb

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Figure 23. Sense Resistor with Kelvin Connections Channel Assignment Convention

2984 F23

CHRSENSE–1

CHRSENSE–2

CHRSENSE

RSENSE

TIES TO RTD TERMINAL 4

EXCITATION

CURRENT

FLOW

= CHRSENSE (3 ≤ RSENSE ≤ 20)

CHANNEL

ASSIGNMENT

Figure 24. Sense Resistor with Kelvin Connections Example

2984 F24

SENSE RESISTOR ASSIGNED TO CH6 (CHSENSE=6)

CH4

CHANNEL ASSIGNMENT

MEMORY LOCATIONS 0x214 TO 0x217

0.01µF

CH5

CH6

CH15

CH16

CH17

RTD ASSIGNED TO CH16 (CHRTD=16)

CHANNEL ASSIGNMENT

MEMORY LOCATIONS 0x23C TO 0x23F

RESULTS MEMORY LOCATIONS 0x04C TO 0x04F

RSENSE

0.01µF

4-WIRE PT-10

0.01µF

Example: 4-Wire RTD with Kelvin RSENSE

It is possible to cancel the parasitic lead resistance in

the sense resistors by configuring the 4-wire RTD with a

4-wire (Kelvin connected) sense resistor. This is useful

when using a PT-10 or PT-50 with a small valued RSENSE

or when the sense resistor is remotely located or in ap-

plications requiring extreme precision.

The 4-wire RTD channel assignments follow the general

conventions previously defined (Figures 17 and 19) for

a standard 4-wire RTD. The sense resistor follows the

convention shown in Figure 23.

Figure 24 shows a typical temperature measurement sys-

tem using a 4-wire RTD with a Kelvin connected RSENSE.

In this example, the 4-wire RTD’s terminals tie to CH17,

CH16, CH15, and CH6. The sense resistor ties to CH6, CH5,

and CH4 and excitation current is applied to CH4 and CH17.

The sense resistor’s nominal value is 1kΩ in order to ac-

commodate a 1mA excitation current. The sense resistor

and RTD connect together at CH6. This topology supports

bothrotated,sharedandstandard4-wireRTDtopologies.If

rotated or shared configuration are not used then terminal

1 of the RTD is tied to ground instead of CH17, freeing up

one input channel. Channel assignment data is shown in

Tables 52 and 53.

A conversion is initiated on CH16 by writing 10010000

into memory location 0x000. Once the conver-

sion is complete, the INTERRUPT pin goes HIGH

and memory location 0x000 becomes 01010000

(see Table 6). The resulting temperature in °C can be read

from memory locations 0x04C to 0x04F (corresponding

to CH16).

APPLICATIONS INFORMATION

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