LTC2995 datasheet(22/90 Pages) AD

22 / 90 page

LTM2985

Rev. 0

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BitsB4toB0determinewhichinputchanneltheconversion

is performed upon and are simply the binary equivalent

of the channel number (see Table 7). These bits are also

usedforEEPROMreadandwriteoperations(seeTable 12).

Bit B5 should be set to 0.

Bits B7 and B6 serve as start/done bits. In order to start

a conversion, these bits must be set to “10” (B7=1 and

B6=0). When the conversion begins, the INT pin goes

LOW. Once the conversion is complete, bits B7 and B6

will toggle to “01” (B7=0 and B6=1) (Address = 0x000)

and the INT pin will go HIGH, indicating the conversion is

complete and the result is available.

State 4: Conversion

The measurement cycle starts after the Initiate Conversion

command is written into RAM location 0x000 (Table 6).

TheLTM2985simultaneouslymeasurestheselectedinput

sensor, sense resistors (RTDs and thermistors) and cold

junction temperatures if applicable (thermocouples).

Once the conversion is started, the user is locked out of

the RAM, with the exception of reading status data stored

in RAM memory location 0x000.

Once the conversion is started the INT pin goes low.

Depending on the sensor configuration, two or three

82ms cycles are required per temperature result. These

correspond to conversion rates of 167ms and 251ms,

respectively (assuming a filter frequency setting of 55Hz).

Details describing these modes are described in the 2- and

3-cycle Conversion Modes section of the data sheet.

The end of conversion can be monitored either through

the INT pin (LOW to HIGH transition), or by reading the

command status register in RAM memory location 0x000

(start bit, B7, toggles from 1 to 0 and DONE bit, B6, toggles

from 0 to 1).

APPLICATIONS INFORMATION

State 5: Read Results

Once the conversion is complete, the conversion results

can be read from RAM memory locations corresponding

to the input channel (see Table 8).

The conversion result is 32 bits long and contains both

the sensor temperature (D23 to D0) and sensor fault data

(D31 to D24) (see Tables 9A and 9B).

The result is reported in °C for all temperature sensors

with a range of –273.15°C to 8192°C and 1/1024°C

resolution or in °F with a range of –459.67°F to 8192°F

with 1/1024°F resolution. Included with the conversion

result are seven sensor fault bits and a valid bit. These

sensor fault bits are set to a 1 if there was a problem

associated with the corresponding conversion result

(see Table 10). Two types of errors are reported: hard

errors and soft errors. Hard errors indicate the reading is

invalid and the resulting temperature reported is –999°C

or °F. Soft errors indicate operation beyond the normal

temperature range of the sensor or the input range of the

ADC. In this case, the calculated temperature is reported

but the accuracy may be compromised. Details relating

to each fault type are sensor specific and are described

in detail in the sensor specific sections of this data sheet.

Bit D24 is the valid bit and will be set to a 1 for valid data.

Oncethedatareadiscomplete,thedeviceisreadyforanew

InitiateConversioncommand.Incaseswherenewchannel

configuration data is required, the user has access to the

RAM in order to modify existing channel assignment data.

Table 8. Conversion Result Memory Map

CONVERSION

CHANNEL

START

ADDRESS

END ADDRESS SIZE (BYTES)

CH1

0x010

0x013

CH2

0x014

0x017

CH3

0x018

0x01B

CH4

0x01C

0x01F

CH5

0x020

0x023

CH6

0x024

0x027

CH7

0x028

0x02B

CH8

0x02C

0x02F

CH9

0x030

0x033

CH10

0x034

0x037

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