LTC2492

18

2492fb

APPLICATIONS INFORMATION

conversion automatically begins at power up using this

default configuration. Once the conversion is complete,

a new word may be written into the device.

The first three bits shifted into the device consist of two

preamble bits and an enable bit. These bits are used to

enable the device configuration and input channel selection.

Valid settings for these three bits are 000, 100 and 101.

Other combinations should be avoided. If the first three bits

are 000 or 100, the following data is ignored (don’t care)

and the previously selected input channel and configuration

remain valid for the next conversion.

If the first three bits shifted into the device are 101, then

the next five bits select the input channel for the next

conversion cycle (see Table 3).

The first input bit (SGL) following the 101 sequence

determines if the input selection is differential (SGL =

0) or single-ended (SGL = 1). For SGL = 0, two adjacent

channels can be selected to form a differential input. For

SGL = 1, one of four channels is selected as the positive

input. The negative input is COM for all single-ended

operations. The remaining four bits (ODD, A2, A1, A0)

determine which channel(s) is/are selected and the polarity

(for a differential input).

The next serial input bit immediately following the input

channel selection is the enable bit for the conversion

configuration (EN2). If this bit is set to 0, then the next

conversion is performed using the previously selected

converter configuration.

A new configuration can be loaded into the device by

setting EN2 = 1 (see Table 4). The first bit (IM) is used

to select the internal temperature sensor. If IM = 1, the

following conversion will be performed on the internal

temperature sensor rather than the selected input channel.

The next two bits (FA and FB) are used to set the rejection

frequency. The final bit (SPD) is used to select either the

1x output rate if SPD = 0 (auto-calibration is enabled and

the offset is continuously calibrated and removed from

the final conversion result) or the 2x output rate if SPD

= 1 (offset calibration disabled, multiplexing output rates

up to 15Hz with no latency). When IM = 1 (temperature

measurement) SPD will be ignored and the device will

operate in 1

× mode. The configuration remains valid until

Table 3 Channel Selection

MUX ADDRESS

CHANNEL SELECTION

SGL

ODD/

SIGN

A2

A1

A0

0

1

2

3

COM

*0

0000

00001

01000

01001

10000

10001

11000

11001

*Default at power up

Table 2. Output Data Format

Differential Input Voltage

Bit 31

`E`O`C

Bit 30

DMY

Bit 29

SIG

Bit 28

MSB

Bit 27

Bit 26

Bit 25

…

Bit 5

LSB

Bits 4 to 0

SUB LSBs

0

0

1

1

0

0

0

…

0

00000

0

0

1

0

1

1

1

…

1

XXXXX

0

0

1

0

1

0

0

…

0

XXXXX

0

0

1

0

0

1

1

…

1

XXXXX

0

0

0

1

0

0

0

0

…

0

XXXXX

–1LSB

0

0

0

1

1

1

1

…

1

XXXXX

0

0

0

1

1

0

0

…

0

XXXXX

0

0

0

1

0

1

1

…

1

XXXXX

0

0

0

1

0

0

0

…

0

XXXXX

0

0

0

0

1

1

1

…

1

11111

** Sub LSBs are below the 24-bit level. They may be included in averaging, or discarded without loss of resolution.