LTC2995 datasheet(68/90 Pages) AD

68 / 90 page

LTM2985

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

The error status byte is located in the upper byte of the

data output word. Table 68 shows format of the error

status byte for the analog sensor type. Bit D31 and D30

indicate 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). Either of these

are 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 event

was a random, infrequent event. Bits D27 and D26 indicate

over or under temperature limits have been exceeded the

table limits as described in Table 68. Bit D25 indicates

the absolute voltage measured by the ADC is beyond its

normal operating range.

Example: Differential Active Analog

Temperature Sensor

In this example, a simplified temperature curve is imple-

mented (see Figure 54). Points P1 to P9 represent the

normaloperatingrangeofthecustomdevice.Voltageread-

ings above point P9 result in a soft fault and the reported

result is a linear extrapolation using a slope determined

by points P8 and P9 (the final two table entries). Voltage

readings below point P1 are also reported as soft faults.

The reported result is the extrapolation between point P1

and P0, where P0 is typically the lowest possible sensor

output voltage. Sensor output voltages below P0 (in mV)

will report P0 output.

In order to program the LTM2985 with an active analog

temperature sensor table, both the mV data and the Kelvin

data are converted to 24-bit binary values (represented as

two, 3-byte table entries, see Table 69). Since some analog

sensors generate negative output voltages, the table mV

valuesare2’scompliment.Thesensoroutputvoltage(units

= mV) follows the convention shown in Table 71, where

the first bit is the sign, the next 11 bits are the integer part

and the remaining 12 bits are the fractional part.

Table 68. Active Analog Temperature Sensor Fault Data Byte

BIT

FAULT

ERROR TYPE

DESCRIPTION

OUTPUT RESULT

D31

Hard Fault

Hard

VADC < –1.75 • VREF/2 or VADC > 1.75 • VREF/2

–999

D30

Range Hard Fault

Hard

VADC < –1.75 • VREF/2 or VADC > 1.75 • VREF/2

–999

D29

Not Used

N/A

D28

Not Used

N/A

D27

Soft Above

Soft

VADC > Last Table Point Voltage

Suspect Reading

D26

Soft Below

Soft

VADC < Second Table Point (P1) Voltage

Suspect Reading

D25

Soft Range

Soft

VADC < –1.125 • VREF/2 or VADC > 1.125 • VREF/2

Suspect Reading

D24

Result Valid (Always 1)

Active Analog Temperature Sensor Table Format

Table 69. Active Analog Temperature Sensor Table Format

ADDRESS

BYTE 0

BYTE 1

BYTE 2

BYTE 3

BYTE 4

BYTE 5

0x250 + 6 • Start Address

Table Entry #1 (mV)

Table Entry #1 (Kelvin)

0x250 + 6 • Start Address + 6

Table Entry #2 (mV)

Table Entry #2 (Kelvin)

0x250 + 6 • Start Address + 12

Table Entry #3 (mV)

Table Entry #3 (Kelvin)

•

Max Address = 0x3CA

Table Entry #64 (mV)

Table Entry #64 (Kelvin)

LTC2995 Datasheet(PDF) 68 Page - Analog Devices

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