LTC2984 datasheet(44/78 Pages) AD

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LTC2984

2984fb

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Example: 4-Wire RTD with Rotation

One method to improve the accuracy of an RTD over the

standard 4-wire implementation is by rotating the excita-

tion current source. Parasitic thermocouple effects are

automatically removed through autorotation. In order to

perform autorotation, the 1st terminal of the RTD ties to

CHRTD+1 instead of GND, as in the standard case. This

allows the LTC2984 to automatically change the direc-

tion of the current source without the need for additional

external components.

4-wire RTD with rotation channel assignments follow

the general convention shown in Figure 19. Terminal 1 is

tied to CHRTD+1, terminals 2 and 3 (Kelvin sensed signal)

tie to CHRTD and CHRTD-1, and the 4th terminal ties to the

sense resistor. Channel assignment data (see Table 29)

is mapped to memory locations corresponding to CHRTD.

Sense resistor channel assignments follow the general

convention shown in Figure 20. The sense resistor is tied

between CHRSENSE and CHRSENSE-1, where CHRSENSE is

tied to the 4th terminal of the RTD. Channel assignment

APPLICATIONS INFORMATION

data is mapped into a memory location corresponding to

CHRSENSE.

Figure 21 shows a typical temperature measurement

system using a rotating 4-wire RTD. In this example

a 4-wire RTD’s terminals tie to CH17, CH16, CH15,

and CH6. The sense resistor is tied to CH6 and CH5.

The sense resistor and RTD connect together at CH6.

The RTD sensor type and configuration data are as-

signed to CH16. 32 bits of binary configuration data are

mapped directly into memory locations 0x23C to 0x23F

(see Table 47). The sense resistor is assigned to CH6.

Theuserprogrammablevalueofthisresistoris10.0102kΩ.

32 bits of binary configuration data are mapped directly

into memory locations 0x214 to 0x217 (see Table 48).

A conversion is initiated on CH16bywriting10010000into

memory location 0x000. Once the conversion is complete,

the INTERRUPT pin goes HIGH and memory location

0x000 becomes 01010000. The resulting temperature in

°C can be read from memory locations 0x04C to 0x04F

(corresponding to CH16).

Figure 19. 4-Wire RTD Channel Assignment Convention

2984 F19

CHRTD–1

CHRTD

CHRTD+1

EXCITATION

CURRENT

FLOW

= CHRTD (2 ≤ RTD ≤ 19)

CHRSENSE 4TH TERMINAL TIES TO SENSE RESISTOR

CHANNEL

ASSIGNMENT

Figure 20. Sense Resistor Channel Assignment Convention for

4-Wire RTDs with Rotation

2984 F20

CHRSENSE-1

CHRSENSE

RSENSE

EXCITATION

CURRENT

FLOW

= CHRSENSE (2 ≤ RSENSE ≤ 20)

CHANNEL

ASSIGNMENT

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