LTC2995 datasheet(59/90 Pages) AD

59 / 90 page

LTM2985

Rev. 0

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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

2985 F31

CHTHERM–1

CHTHERM

EXCITATION

CURRENT

FLOW

= CHTHERM (2 ≤ THERM ≤ 10)

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 CHRSENSE and CHRSENSE-1, where CHRSENSE is

tied to the 2nd terminal of the thermistor. Channel as-

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

location corresponding to CHTHERM.

Figure 32. Sense Resistor Channel Assignment

Convention for Thermistors

2985 F32

CHRSENSE-1

CHRSENSE

RSENSE

EXCITATION

CURRENT

FLOW

= CHRSENSE (2 ≤ RSENSE ≤ 10)

CHANNEL

ASSIGNMENT

Figure 33 shows a typical temperature measurement

system using a shared sense resistor and one rotated/

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 (44033 type) is configured as a

non-rotated/shared. Channel assignment data are shown

in Tables 64 to 66.

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.

Figure 33. Rotated and Shared Thermistor Example

2985 F33

THERMISTOR #1 ASSIGNED TO CH8 (CHTHERM=8)

CHANNEL ASSIGNMENT

MEMORY LOCATIONS 0x21C TO 0x21F

RESULT MEMORY LOCATIONS 0x02C TO 0x02F

100pF

CH7

CH6

CH5

CH8

CH9

CH10

THERMISTOR #2 ASSIGNED TO CH10 (CHTHERM=10)

CHANNEL ASSIGNMENT

MEMORY LOCATIONS 0x224 TO 0x227

RESULT MEMORY LOCATIONS 0x034 TO 0x037

SENSE RESISTOR ASSIGNED TO CH6 (CHRSENSE=6)

CHANNEL ASSIGNMENT

MEMORY LOCATIONS 0x214 TO 0x217

TYPE 44032

RSENSE

10k

100pF

TYPE 44033

100pF

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