LTC2995 datasheet(31/90 Pages) AD

31 / 90 page

LTM2985

Rev. 0

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an excessive noise event, the device should recover and

the following conversions will be valid if the noise event

was a random, infrequent event. Bits D29 and D28 are not

used for diodes. Bits D27 and D26 indicate over or under

temperaturelimits(definedasT> 130°CorT<–60°C).The

calculated temperature is reported, but the accuracy may

be compromised. Bit D25 indicates the absolute voltage

measuredbytheADCisbeyonditsnormaloperatingrange.

If a diode is used as the cold junction element, any hard

or soft error is flagged in the corresponding thermocouple

result (bits D28 and D29 in Table 19).

Example: Single-Ended Type K and Differential Type T

Thermocouples with Shared Diode Cold Junction

Compensation

Figure 9showsatypicaltemperaturemeasurementsystem

where two thermocouples share a single cold junction

diode. In this example, a Type K thermocouple is tied to

CH1 and a Type T thermocouple is tied to CH3 and CH4.

They both share a single cold junction diode with ideal-

ity factor of

η = 1.003 tied to CH2. Channel assignment

data for both thermocouples and the diode are shown in

Figure 9. Dual Thermocouple with Diode Cold Junction Example

TYPE K

0.1µF

TYPE T

η = 1.003

2985 F09

TYPE K THERMOCOUPLE ASSIGNED TO CH1 (CHTC=1)

DIODE COLD JUNCTION ASSIGNED TO CH2 (CHD=2)

TYPE T THERMOCOUPLE JUNCTION ASSIGNED TO CH4 (CHTC=4)

CH4

CH3

CH2

CH1

0.1µF

COM

CHANNEL ASSIGNMENT

MEMORY LOCATIONS 0x200 TO 0x203

RESULT MEMORY LOCATIONS 0x010 TO 0x013

CHANNEL ASSIGNMENT

MEMORY LOCATIONS 0x204 TO 0x207

RESULT MEMORY LOCATIONS 0x014 TO 0x017

CHANNEL ASSIGNMENT

MEMORY LOCATIONS 0x20C TO 0x20F

RESULT MEMORY LOCATIONS 0x01C TO 0x01F

Tables 26to28.Thermocouple#1(TypeK)sensortypeand

configuration data are assigned to CH1. 32-bits of binary

configuration data are mapped directly into memory loca-

tions0x200to0x203(seeTable 26).Thecoldjunctiondiode

sensor type and configuration data are assigned to CH2.

32-bits of binary configuration data are mapped directly

into memory locations 0x204 to 0x207 (see Table 27).

Thermocouple #2 (Type T) sensor type and configuration

data are assigned to CH4. 32-bits of binary configuration

data are mapped directly into memory locations 0x20C

to 0x20F (see Table 28). A conversion is initiated on CH1

by writing 10000001 into memory location 0x000. Both

the Type K thermocouple and the diode are measured

simultaneously. The LTM2985 calculates the cold junction

compensationanddeterminesthetemperatureoftheType

K thermocouple. Once the conversion is complete, the

INT pin goes HIGH and memory location 0x000 becomes

01000001. Similarly, a conversion can be initiated on CH4

by writing 10000100 into memory location 0x000. The

results (in °C) can be read from memory locations 0x010

to 0x013 for CH1 and 0x01C to 0x01F for CH4.

APPLICATIONS INFORMATION

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