LTC2995 datasheet(18/90 Pages) AD

18 / 90 page

LTM2985

Rev. 0

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The LTM2985 combines high accuracy with ease of use.

The basic operation is simple and is composed of five

states (see Figure 3).

APPLICATIONS INFORMATION

Figure 3. Basic Operation

2985 F03

POWER-UP,

SLEEP

(OPTIONAL)

≈ 200ms(MAX)

YES

START-UP

CHANNEL ASSIGNMENT

INITIATE CONVERSION

CONVERSION

READ RESULTS

STATUS CHECK

COMPLETE?

Conversion States Overview

1. Start-Up. After power is applied to the LTM2985

(VCC2 > 2.6V),thereisa200mswakeupperiod.During

this time, the LDO, charge pump, ADCs and reference

arepoweredupandtheinternalRAMisinitialized.Once

start-up is complete, the INT pin goes HIGH and the

command status register will return a value of 0x40

(Start bit = 0, Done bit = 1) when read.

2. Channel Assignment. The device automatically enters

thechannelassignmentstateafterstart-upiscomplete.

While in this state, the user writes sensor specific data

for each input channel into RAM. The user can also load

it from the EEPROM (see the EEPROM section for more

details). The assignment data contains information

aboutthesensortype,pointerstocoldjunctionsensors

or sense resistors and sensor specific parameters.

3. InitiateConversion.Aconversionisinitiatedbywriting

a measurement command into RAM memory location

0x000. This command is a pointer to the channel in

which the conversion will be performed.

4. Conversion. A new conversion begins automatically

followinganInitiateConversioncommand.Inthisstate,

the ADC is running a conversion on the specified chan-

nel and associated cold junction or RSENSE channel (if

applicable). The user is locked out of RAM access while

in the state (except for reading status location 0x000).

The end of conversion is indicated by both the INT pin

going HIGH and a status register START bit going LOW

and DONE bit going HIGH.

5. Read Results. In this state, the user has access to

RAM and can read the completed conversion results

and fault status bits. It is also possible for the user to

modify/append the channel assignment data during the

read results state.

Conversion State Details

State 1: Start-Up

The start-up state automatically occurs when power is ap-

pliedtotheLTM2985.Ifthepowerdropsbelowathreshold

of ≈2.6V and then returns to the normal operating voltage

(2.85V to 5.25V), the LTM2985 resets and enters the

power-up state. Note that the LTM2985 also enters the

start-up state at the conclusion of the sleep state.

In the first phase of the start-up state all critical analog

circuits are powered up. This includes the LDO, reference,

charge pump and ADCs. During this first phase, the com-

mand status register will be inaccessible to the user. This

phase takes a maximum of 100ms to complete. Once this

phase completes, the command status register will be

accessible and return a value of 0x80 until the LTM2985

is completely initialized. Once the LTM2985 is initialized

and ready to use, the INT pin will go high and the com-

mand status register will return a read value of 0x40 (Start

bit = 0, Done bit = 1). At this point the LTM2985 is fully

initialized and is ready to perform a conversion.

State 2: Channel Assignment

The LTM2985 RAM can be programmed with up to 10 sets

of 32-bit (4-byte) channel assignment data. These reside

sequentially in RAM with a one-to-one correspondence

to each of the 10 analog input channels (see Table 3).

Channels that are not used should have their channel

assignment data set to all zeros (default at START-UP).

LTC2995 Datasheet(PDF) 18 Page - Analog Devices

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