LTC2984 datasheet(57/78 Pages) AD

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LTC2984

2984fb

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APPLICATIONS INFORMATION

Example: Shared/Rotated Differential Thermistor

The differential thermistor configuration allows separate

internal ground sensing for each sensor. In this configura-

tion, one sense resistor can be used for multiple thermis-

tors. Differential thermistors follow the convention shown

in Figure 31. Terminal 1 ties to CHTHERM and terminal 2

ties to CHTHERM-1 and the sense resistor. Channel assign-

ment data (see Table 54) is mapped to memory locations

corresponding to CHTHERM.

Figure 31. Thermistor with Shared RSENSE Channel

Assignment Convention

2984 F31

CHTHERM–1

CHTHERM

EXCITATION

CURRENT

FLOW

= CHTHERM (2 ≤ THERM ≤ 20)

2ND TERMINAL TIES TO SENSE RESISTOR

CHANNEL

ASSIGNMENT

Sense resistor channel assignments follow the general

convention shown in Figure 32. The sense resistor is tied

between CHRSENSEandCHRSENSE-1,whereCHSENSEistied

to the 2nd terminal of the thermistor. Channel assignment

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

corresponding to CHTHERM.

Figure 32. Sense Resistor Channel Assignment

Convention for Thermistors

2984 F32

CHRSENSE-1

CHRSENSE

RSENSE

EXCITATION

CURRENT

FLOW

= CHRSENSE (2 ≤ RSENSE ≤ 20)

CHANNEL

ASSIGNMENT

Figure 33 shows a typical temperature measurement

system using a shared sense resistor and one rotated/

one non-rotated differential thermistors. In this example

a 30kΩ (44032 Type) thermistor is tied to a 10.0kΩ sense

resistor and configured as rotated/shared. The second

thermistor a 2.25kΩ (44004 Type) is configured as a

non-rotated/shared. Channel assignment data are shown

in Tables 64 to 66.

A conversion is initiated on CH18bywriting10010010into

memory location 0x000. Once the conversion is complete,

the INTERRUPT pin goes HIGH and memory location

0x000 becomes 01010010. The resulting temperature in

°C can be read from memory locations 0x054 to 0x057

(corresponding to CH16). A conversion can be initiated

and read from CH20 in a similar fashion.

Figure 33. Rotated and Shared Thermistor Example

2984 F33

THERMISTOR #1 ASSIGNED TO CH18 (CHTHERM=18)

CHANNEL ASSIGNMENT

MEMORY LOCATIONS 0x244 TO 0x247

RESULT MEMORY LOCATIONS 0x054 TO 0x057

100pF

CH17

CH16

CH15

CH18

CH19

CH20

THERMISTOR #2 ASSIGNED TO CH20 (CHTHERM=20)

CHANNEL ASSIGNMENT

MEMORY LOCATIONS 0x24C TO 0x24F

RESULT MEMORY LOCATIONS 0x05C TO 0x05F

SENSE RESISTOR ASSIGNED TO CH16 (CHSENSE=16)

CHANNEL ASSIGNMENT

MEMORY LOCATIONS 0x23C TO 0x23F

TYPE 44032

RSENSE

10k

100pF

TYPE 44033

100pF

LTC2984 Datasheet(PDF) 57 Page - Analog Devices

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