LTC2984 datasheet(40/78 Pages) AD

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LTC2984

2984fb

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Figure 13. 3-Wire RTD Channel Assignment Convention

Figure 14. 3-Wire Sense Resistor Channel Assignment

Convention for 3-Wire RTDs

2984 F13

3RD TERMINAL TIES TO SENSE RESISTOR

CHRTD-1

CHRSENSE

CHRTD

EXCITATION

CURRENT

FLOW

= CHRTD (2 ≤ RTD ≤ 20)

CHANNEL

ASSIGNMENT

2984 F14

CHRSENSE-1

CHRSENSE

RSENSE

2x EXCITATION

CURRENT

FLOW

= CHRSENSE (2 ≤ RSENSE ≤ 20)

(OPTIONAL GND, REMOVE FOR RSENSE SHARING)

CHANNEL

ASSIGNMENT

Example: 3-Wire RTD

3-wire RTD channel assignments follow the general con-

vention shown in Figure 13. Terminals 1 and 2 tie to the

input/excitation current sources and terminal 3 connects

to the sense resistor. Channel assignment data is mapped

to memory locations corresponding to CHRTD.

Sense resistor channel assignments follow the general

convention shown in Figure 14. The sense resistor is tied

between CHRSENSE and CHRSENSE-1, where CHRSENSE is

tied to the 3rd terminal of the RTD and CHRSENSE-1 is tied

to ground (or left floating for RSENSE sharing). Channel

assignmentdata(seeTable37)ismappedintothememory

location corresponding to CHRSENSE.

Figure 15 shows a typical temperature measurement sys-

tem using a 3-wire RTD. In this example, a 3-wire RTD’s

terminals tie to CH9, CH8, and CH7. The sense resistor

ties to CH7 and CH6. The sense resistor and RTD connect

together at CH7.

The 3-wire RTD reduces the errors associated with para-

sitic lead resistance by applying excitation current to each

RTD input. This first order cancellation removes matched

lead resistance errors. This cancellation does not remove

errors due to thermocouple effects or mismatched lead

resistances. The RTD sensor type and configuration data

are assigned to CH9. 32 bits of binary configuration data

aremappeddirectlyintomemorylocations0x220to0x223

(see Table 43). The sense resistor is assigned to CH7. The

user-programmable value of this resistor is 12150.39Ω.

32 bits of binary configuration data are mapped directly

into memory locations 0x218 to 0x21B (see Table 44).

A conversion is initiated on CH9 by writing 10001001 into

memorylocation0x000.Oncetheconversioniscomplete,

the INTERRUPT pin goes HIGH and memory location

0x000 becomes 01001001. The resulting temperature in

°C can be read from memory locations 0x030 to 0x033

(corresponding to CH9).

APPLICATIONS INFORMATION

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