LTC2995 datasheet(26/90 Pages) AD

26 / 90 page

LTM2985

Rev. 0

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THERMOCOUPLE MEASUREMENTS

Channel Assignment – Thermocouples

For each thermocouple tied to the LTM2985, a 32-bit

channel assignment word is programmed into a memory

location corresponding to the channel the sensor is tied to

(see Table 15). This word includes (1) thermocouple type,

(2) cold junction channel pointer, (3) sensor configuration

and (4) custom thermocouple data pointer.

(1) Thermocouple Type

The thermocouple type is determined by the first five input

bits B31 to B27 as shown in Table 16. Standard NIST coef-

ficients for types J, K, E, N, R, S, T and B thermocouples

are stored in the device ROM. If custom thermocouples

are used, the custom thermocouple sensor type can be

selected. In this case, user-specific data can be stored in

the on-chip RAM starting at the address defined in the

custom thermocouple data pointer.

(2) Cold Junction Channel Pointer

The cold junction compensation can be a diode, active

analog temperature sensor, RTD, or thermistor. The cold

junction channel pointer tells the LTM2985 which chan-

nel (1 to 10) the cold junction sensor is assigned to (see

Table 17). When a conversion is performed on a channel

tied to a thermocouple, the cold junction sensor is simul-

taneously and automatically measured. The final output

data uses the embedded coefficients stored in ROM to

automatically compensate the cold junction temperature

and output the thermocouple sensor temperature.

(3) Sensor Configuration

The sensor configuration field (see Table 18) is used

to select single-ended (B21=1) or differential (B21=0)

input and allows selection of open circuit current

if internal open-circuit detect is enabled (bit B20).

Single-ended readings are measured relative to the

COM pin and differential are measured between the

selected CHTC and adjacent CHTC-1 (see Figure 7).

If open-circuit detection is enabled, B20=1, then the user

can select the pulsed current value applied during open-

circuit detect using bits B18 and B19 . The user determines

the value of the open circuit current based on the size of

theexternalprotectionresistorandfiltercapacitor(typically

10µA). This network needs to settle within 50ms to 1µV

or less. The duration of the current pulse is approximately

8msandoccurs50msbeforethenormalconversioncycle.

Thermocouple channel assignments follow the general

convention shown in Figure 7. The thermocouple positive

terminal ties to CHTC (where TC is the selected channel

number) for both the single-ended and differential modes

of operation. For single-ended measurements the thermo-

couple negative terminal and the COM pin are grounded.

The thermocouple negative terminal is tied to CHTC-1

for differential measurements. This node can either be

grounded or tied to a bias voltage.

APPLICATIONS INFORMATION

Table 15. Thermocouple Channel Assignment Word

(1) THERMOCOUPLE

TYPE

(2) COLD JUNCTION

CHANNEL POINTER

(3) SENSOR

CONFIGURATION

(4) CUSTOM THERMOCOUPLE

DATA POINTER

TABLES 4, 16

TABLE 17

TABLE 18

TABLES 86 TO 88

Measurement Type 31 30 29 28 27 26 25 24 23 22

19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Thermocouple

Types 1 to 9

Cold Junction

Channel Assignment

[4:0]

SGL=1

DIFF=0

Check

Current

[1:0]

0 0 0 0 0 0

Custom Address

[5:0]

Custom Length –1

[5:0]

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