LTC2485 datasheet(14/40 Pages) LINER

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LTC2485

2485fa

Accessing the Special Features of the LTC2485

The LTC2485 combines a high resolution, low noise

∆Σ

analog-to-digital converter with an on-chip selectable tem-

perature sensor, programmable digital filter and output

rate control. These special features are selected through

a single 8-bit serial input word during the data input/out-

put cycle (see Figure 3).

The LTC2485 powers up in a default mode commonly

used for most measurements. The device will remain in

this mode until a valid write cycle is performed. In this

default mode, the measured input is external, the digital

filter simultaneously rejects 50Hz and 60Hz line frequency

noise, and the speed mode is 1x (offset automatically,

continuously calibrated).

The I2C serial interface grants access to any or all special

functions contained within the LTC2485. In order to

change the mode of operation, a valid write address

followed by 8 bits of data are shifted into the device (see

Table 1). The first 4 bits are reserved and should be low.

The 5th bit (IM) is used to select the internal temperature

sensor as the conversion input, while the 6th and 7th bits

(FA, FB) combine to determine the line frequency rejec-

tion mode. The 8th bit (SPD) is used to double the output

rate by disabling the offset auto calibration.

Temperature Sensor (IM)

The LTC2485 includes an on-chip temperature sensor. The

temperature sensor is selected by setting IM = 1 in the serial

input data stream. Conversions are performed directly on

the temperature sensor by the converter. While operating

in this mode, the device behaves as a temperature to bits

converter. The digital reading is proportional to the abso-

lute temperature of the device. This feature allows the

converter to linearize temperature sensors or continuously

remove temperature effects from external sensors. Several

applications leveraging this feature are presented in more

detail in the applications section. While operating in this

mode, the speed is set to normal independent of the con-

trol bit (SPD).

Table 1. Selecting Special Modes

FB SPD

COMMENTS

External Input, 50Hz and 60Hz Rejection,

Autocalibration

010

External Input, 50Hz Rejection,

Autocalibration

100

External Input, 60Hz Rejection,

Autocalibration

External Input, 50Hz and 60Hz Rejection,

2x Speed

External Input, 50Hz Rejection, 2x Speed

External Input, 60Hz Rejection, 2x Speed

Temperature Input, 50Hz and 60Hz Rejection,

Autocalibration

Temperature Input, 50Hz Rejection,

Autocalibration

Temperature Input, 60Hz Rejection,

Autocalibration

Reserved, Do Not Use

Figure 3. Timing Diagram for Writing to the LTC2485

SLEEP

DATA INPUT

ACK BY

LTC2485

START BY

MASTER

ACK BY

LTC2485

SPD

…

7-BIT ADDRESS

2485 F03

SDA

SCL

APPLICATIO S I FOR ATIO

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