LTC2995 datasheet(48/90 Pages) AD

48 / 90 page

LTM2985

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Example: Multiple 4-Wire RTDs with Shared RSENSE

Figure 22 shows a typical temperature measurement

system using two 4-wire RTDs with a shared RSENSE.

The LTM2985 can support up to two 4-wire RTDs with

a single sense resistor. In this example, the first 4-wire

RTD’s terminals tie to CH5, CH4, CH3 and CH2 and the

2nd ties to CH8, CH7, CH6 and CH2. The sense resistor

ties to CH1 and CH2. The sense resistor and both RTDs

connecttogetheratCH2.Thischannelassignmentconven-

tion is identical to that of the rotating RTD. This topology

supports both rotated and non-rotated RTD excitations.

Channel assignment data for each sensor is shown in

Table 49 to Table 51.

A conversion is initiated on CH4 by writing 10000100

into memory location 0x000. Once the conversion is

complete, the INT pin goes HIGH and memory location

0x000 becomes 01000100. The resulting temperature in

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

(corresponding to CH4). A conversion can be initiated and

read from CH7 in a similar fashion.

Table 49. Channel Assignment Data for 4-Wire RTD #1 (PT-100, RSENSE on CH2, 4-Wire, Shared RSENSE, Rotated 100µA Excitation

Current,

α = 0.003926 Curve)

CONFIGURATION

FIELD

DESCRIPTION # BITS

BINARY DATA

MEMORY

ADDRESS 0x20C

MEMORY

ADDRESS 0x20D

MEMORY

ADDRESS 0x20E

MEMORY

ADDRESS 0x20F

(1) RTD TYPE

PT-100

01100 0 1 1 0 0

(2) Sense

Resistor Channel

Pointer

CH2

00010

0 0 0 1 0

(3) Sensor

Configuration

4-Wire

Rotated

1010

1 0 1 0

(4) Excitation

Current

100µA

0101

0 1 0 1

(5) Curve

ITS-90,

α = 0.003926

1 1

(6) Custom RTD

Data Pointer

Not Custom

000000000000

0 0 0 0 0 0 0 0 0 0 0 0

Figure 22. Shared RSENSE 4-Wire RTD Example

RSENSE

10k

0.01µF

2985 F22

0.01µF

SENSE RESISTOR ASSIGNED TO CH2 (CHRSENSE=2)

RTD #1 ASSIGNED TO CH4 (CHRTD=4)

CH2

CH1

CHANNEL ASSIGNMENT

MEMORY LOCATIONS 0x204 TO 0x207

CHANNEL ASSIGNMENT

MEMORY LOCATIONS 0x20C TO 0x20F

RESULT MEMORY LOCATIONS 0x01C TO 0x01F

0.01µF

CH3

CH4

CH5

CH6

CH7

CH8

RTD #2 ASSIGNED TO CH7 (CHRTD=7)

CHANNEL ASSIGNMENT

MEMORY LOCATIONS 0x218 TO 0x21B

RESULT MEMORY LOCATIONS 0x028 TO 0x02B

4-WIRE PT-100

0.01µF

4-WIRE PT-500

0.01µF

LTC2995 Datasheet(PDF) 48 Page - Analog Devices

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