LTC2995 datasheet(38/90 Pages) AD

38 / 90 page

LTM2985

Rev. 0

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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. This topology,

however, causes 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

removedifsharingisenabled(1RSENSEformultipleRTDs).

Figure 10. 2-Wire RTD Channel Assignment Convention

2985 F10

OPTIONAL GND, REMOVE FOR RSENSE SHARING

2ND TERMINAL TIES TO SENSE RESISTOR (CHRSENSE)

CHRTD-1

CHRTD

EXCITATION

CURRENT

FLOW

= CHRTD (2 ≤ RTD ≤ 10)

CHANNEL

ASSIGNMENT

Figure 11. Sense Resistor Channel Assignment Convention

for 2-Wire RTDs

2985 F11

CHRSENSE-1

CHRSENSE

RSENSE

EXCITATION

CURRENT

FLOW

= CHRSENSE (2 ≤ RSENSE ≤ 10)

CHANNEL

ASSIGNMENT

Example: 2-Wire RTDs with Shared RSENSE

Figure 12showsatypicaltemperaturemeasurementsystem

using multiple 2-wire RTDs. In this example, a PT-1000

RTD ties to CH7 and CH8 and an NI-120 RTD ties to CH9

and CH10. Using this configuration, the LTM2985 can

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

RTD #1 sensor type and configuration data are assigned

to CH8. 32 bits of binary configuration data are mapped

directly into memory locations 0x21C to 0x21F (see

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

assigned to CH10. 32-bits of binary configuration data are

mapped directly into memory locations 0x224 to 0x227

(see Table 41). The sense resistor is assigned to CH6.

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

32 bits of binary configuration data are mapped directly

into memory locations 0x214 to 0x217 (see Table 42).

A conversion is initiated on CH8 by writing 10001000

into memory location 0x000. Once the conversion is

complete, the INT pin goes HIGH and memory location

0x000 becomes 01001000. The resulting temperature in

°C can be read from memory locations 0x02C to 0x02F

(corresponding to CH8). A conversion can be initiated and

read from CH10 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

LTC2995 Datasheet(PDF) 38 Page - Analog Devices

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