LTC2984 datasheet(37/78 Pages) AD

37 / 78 page

LTC2984

2984fb

For more information www.linear.com/LTC2984

APPLICATIONS INFORMATION

(1) Sensor Type

The sense resistor is selected by setting the first 5 input

bits, B31 to B27, to 11101 (see Table 38).

Sense resistor channel assignments follow the general

convention shown in Figure 11. The sense resistor is tied

between CHRSENSE and CHRSENSE-1, where CHRSENSE is

tied to the 2nd terminal of the RTD. Channel assignment

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

corresponding to CHRSENSE.

Table 38. Sense Resistor Selection

(1) SENSOR TYPE

B31

B30

B29

B28

B27

SENSOR TYPE

Sense Resistor

(2) Sense Resistor Value

The last field in the channel assignment word (B26 to B0)

sets the value of the sense resistor within the range 0 to

131,072Ω with 1/1024Ω precision (see Table 39). The top

17 bits (B26 to B10) create the integer and bits B9 to B0

create the fraction of the sense resistor value.

Example: 2-Wire RTD

The simplest RTD configuration is the 2-wire configura-

tion, 2-wire RTDs follow the general convention shown in

Figure 10. They require only two connections per RTD and

can be tied directly to 2-lead RTD elements. The disad-

vantages of this topology are errors due to parasitic lead

resistance. If sharing is not selected (1 RSENSE per RTD),

then CHRTD should be grounded. The ground connection

should be removed if sharing is enabled (1 RSENSE for

multiple RTDs).

Figure 10. 2-Wire RTD Channel Assignment Convention

2984 F10

OPTIONAL GND, REMOVE FOR RSENSE SHARING

2ND TERMINAL TIES TO SENSE RESISTOR (CHRSENSE)

CHRTD-1

CHRTD

EXCITATION

CURRENT

FLOW

= CHRTD (2 ≤ RTD ≤ 20)

CHANNEL

ASSIGNMENT

Figure 11. Sense Resistor Channel Assignment Convention

for 2-Wire RTDs

2984 F11

CHRSENSE-1

CHRSENSE

RSENSE

EXCITATION

CURRENT

FLOW

= CHRSENSE (2 ≤ RSENSE ≤ 20)

CHANNEL

ASSIGNMENT

Example: 2-Wire RTDs with Shared RSENSE

Figure12showsatypicaltemperaturemeasurementsystem

usingmultiple2-wireRTDs.Inthisexample,aPT-1000RTD

ties to CH17 and CH18 and an NI-120 RTD ties to CH19 and

CH20. Using this configuration, the LTC2984 can digitize

up to nine 2-wire RTDs with a single sense resistor.

RTD #1 sensor type and configuration data are as-

signed to CH18. 32 bits of binary configuration data are

mapped directly into memory locations 0x244 to 0x247

(see Table 40). RTD #2 sensor type and configuration data

are assigned to CH20. 32-bits of binary configuration data

aremappeddirectlyintomemorylocations0x24Cto0x24F

(see Table 41). The sense resistor is assigned to CH16.

The user-programmable value of this resistor is 5001.5Ω.

32 bits of binary configuration data are mapped directly

into memory locations 0x23C to 0x23F (see Table 42).

A conversion is initiated on CH18 by writing 10010010 into

memory location 0x000. Once the conversion is complete,

theINTERRUPTpingoesHIGHandmemorylocation0x000

becomes01010010.Theresultingtemperaturein°Ccanbe

read from memory locations 0x054 to 0x057 (correspond-

ing to CH18). A conversion can be initiated and read from

CH20 in a similar fashion.

Table 39. Example Sense Resistor Values

(2) SENSE RESISTOR VALUE (Ω)

B26 B25 B24 B23 B22 B21 B20 B19 B18 B17 B16 B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0

Example R

216 215 214 213 212 211 210 29 28 27 26 25 24 23 22 21 20 2–1 2–2 2–3 2–4 2–5 2–6 2–7 2–8 2–9 2–10

10,000.2Ω

99.99521kΩ

1.0023kΩ

LTC2984 Datasheet(PDF) 37 Page - Analog Devices

LTC2984 Datasheet(HTML) 37 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78

Datasheet Download

Link URL