LTC2995 datasheet(44/90 Pages) AD

44 / 90 page

LTM2985

Rev. 0

For more information www.analog.com

Example: Standard 4-Wire RTD (No Rotation or RSENSE

Sharing)

Standard 4-wire RTD channel assignments follow the

general convention shown in Figure 16. Terminal 1 is

tied to ground, 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 17. The sense resistor is tied

between CHRSENSE and CHRSENSE-1, where CHRSENSE is

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

data (see Table 37) is mapped into a memory location

corresponding to CHRSENSE.

Figure 18 shows a typical temperature measurement sys-

tem using a 4-wire RTD. In this example, a 4-wire RTD’s

APPLICATIONS INFORMATION

Figure 16. 4-Wire RTD Channel Assignment Convention

Figure 17. Sense Resistor Channel Assignment Convention for

4-Wire RTDs

2985 F16

CHRTD-1

CHRTD

CHRSENSE

4TH TERMINAL TIES TO SENSE RESISTOR (CHRSENSE)

EXCITATION

CURRENT

FLOW

= CHRTD (2 ≤ RTD ≤ 10)

CHANNEL

ASSIGNMENT

2985 F17

CHRSENSE-1

CHRSENSE

RSENSE

EXCITATION

CURRENT

FLOW

= CHRSENSE (2 ≤ RSENSE ≤ 10)

CHANNEL

ASSIGNMENT

terminalstietoGND,CH4,CH3andCH2.Thesenseresistor

ties to CH2 and CH1. The sense resistor and RTD share

a common connection at CH2. The RTD sensor type and

configuration data are assigned to CH4. 32 bits of binary

configuration data are mapped directly into memory loca-

tions 0x20C to 0x20F (see Table 45). The sense resistor

is assigned to CH2. The user-programmable value of this

resistor is 5000.2Ω. 32 bits of binary configuration data

aremappeddirectlyintomemorylocations0x204to0x207

(see Table 46).

A conversion is initiated on CH4 by writing 10000100 into

the data byte at memory location 0x000. Once the conver-

sion is complete, the INT pin goes HIGH and memory loca-

tion0x000becomes01000100.Theresultingtemperature

in °C can be read from memory locations 0x01C to 0x01F

(corresponding to CH4).

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