LTC2984 datasheet(36/78 Pages) AD

36 / 78 page

LTC2984

2984fb

For more information www.linear.com/LTC2984

Fault Reporting – RTD

Each sensor type has unique fault reporting mechanism

indicated in the most significant byte of the data output

word. Table 35 shows faults reported in the measurement

of RTDs.

BitD31indicatestheRTDorRSENSEisopen,shorted,ornot

pluggedin.Thisisahardfaultand–999°Cor°Fisreported.

Bit D30 indicates a bad ADC reading. This can be a result

of either a broken (open) sensor or an excessive noise

event (ESD or static discharge into the sensor path). This

is a hard error and –999°C or °F is reported. In the case of

an excessive noise event, the device should recover and

the following conversions will be valid if the noise was a

random infrequent event. Bits D29 and D28 are not used

for RTDs. Bits D27 and D26 indicate over or under tem-

perature limits (see Table 36). The calculated temperature

is reported, but the accuracy may be compromised. Bit

D25 indicates the absolute voltage measured by the ADC

is beyond its normal operating range. If an RTD is used

as the cold junction element, any hard or soft error is also

flagged in the thermocouple result.

Sense Resistor

Channel Assignment

For each sense resistor tied to the LTC2984, a 32-bit

channel assignment word is programmed into a memory

location corresponding to the channel the sensor is tied

to (see Table 37). This word includes (1) sense resistor

selection and (2) sense resistor value.

Table 36. Voltage and Resistance Ranges

RTD TYPE

MIN Ω

MAX Ω

LOW TEMP LIMIT °C

HIGH TEMP LIMIT °C

PT-10

1.95

34.5

–200

850

PT-50

9.75

172.5

–200

850

PT-100

19.5

345

–200

850

PT-200

690

–200

850

PT-500

97.5

1725

–200

850

PT-1000

195

3450

–200

850

NI-120

66.6

380.3

–80

260

Custom Table

Lowest Table Entry

Highest Table Entry

Lowest Table Entry

Highest Table Entry

Table 35. RTD Fault Reporting

BIT

FAULT

ERROR TYPE

DESCRIPTION

OUTPUT RESULT

D31

Sensor Hard Fault

Hard

Open or Short RTD or RSENSE

–999°C or °F

D30

Hard ADC-Out-of-Range

Hard

Bad ADC Reading (Could Be Large External Noise Event)

–999°C or °F

D29

Not Used for RTDs

N/A

Always 0

Valid Reading

D28

Not Used for RTDs

N/A

Always 0

Valid Reading

D27

Sensor Over Range

Soft

T > High Temp Limit (See Table 36)

Suspect Reading

D26

Sensor Under Range

Soft

T < Low Temp Limit (See Table 36)

Suspect Reading

D25

ADC Out-of-Range

Soft

ADC Absolute Input Voltage Is Beyond ±1.125 • VREF/2

Suspect Reading

D24

Valid

N/A

Result Valid (Should Be 1) Discard Results if 0

Valid Reading

APPLICATIONS INFORMATION

Table 37. Sense Resistor Channel Assignment Word

(1) SENSOR TYPE

(2) SENSE RESISTOR VALUE (Ω)

FIGURE 39

FIGURE 43

Measurement Class

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Sense Resistor

Type = 29

Sense Resistor Value (17, 10) Up to ≈ 131,072Ω with 1/1024Ω Resolution

LTC2984 Datasheet(PDF) 36 Page - Analog Devices

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